json.hpp

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/*
    __ _____ _____ _____
 __|  |   __|     |   | |  JSON for Modern C++
|  |  |__   |  |  | | | |  version 2.1.1
|_____|_____|_____|_|___|  https://github.com/nlohmann/json

Licensed under the MIT License <http://opensource.org/licenses/MIT>.
Copyright (c) 2013-2017 Niels Lohmann <http://nlohmann.me>.

Permission is hereby  granted, free of charge, to any  person obtaining a copy
of this software and associated  documentation files (the "Software"), to deal
in the Software  without restriction, including without  limitation the rights
to  use, copy,  modify, merge,  publish, distribute,  sublicense, and/or  sell
copies  of  the Software,  and  to  permit persons  to  whom  the Software  is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE  IS PROVIDED "AS  IS", WITHOUT WARRANTY  OF ANY KIND,  EXPRESS OR
IMPLIED,  INCLUDING BUT  NOT  LIMITED TO  THE  WARRANTIES OF  MERCHANTABILITY,
FITNESS FOR  A PARTICULAR PURPOSE AND  NONINFRINGEMENT. IN NO EVENT  SHALL THE
AUTHORS  OR COPYRIGHT  HOLDERS  BE  LIABLE FOR  ANY  CLAIM,  DAMAGES OR  OTHER
LIABILITY, WHETHER IN AN ACTION OF  CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE  OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.
*/

#ifndef NLOHMANN_JSON_HPP
#define NLOHMANN_JSON_HPP

#include <algorithm> // all_of, copy, fill, find, for_each, none_of, remove, reverse, transform
#include <array> // array
#include <cassert> // assert
#include <cctype> // isdigit
#include <ciso646> // and, not, or
#include <cmath> // isfinite, labs, ldexp, signbit
#include <cstddef> // nullptr_t, ptrdiff_t, size_t
#include <cstdint> // int64_t, uint64_t
#include <cstdlib> // abort, strtod, strtof, strtold, strtoul, strtoll, strtoull
#include <cstring> // strlen
#include <forward_list> // forward_list
#include <functional> // function, hash, less
#include <initializer_list> // initializer_list
#include <iomanip> // setw
#include <iostream> // istream, ostream
#include <iterator> // advance, begin, back_inserter, bidirectional_iterator_tag, distance, end, inserter, iterator, iterator_traits, next, random_access_iterator_tag, reverse_iterator
#include <limits> // numeric_limits
#include <locale> // locale
#include <map> // map
#include <memory> // addressof, allocator, allocator_traits, unique_ptr
#include <numeric> // accumulate
#include <sstream> // stringstream
#include <stdexcept> // domain_error, invalid_argument, out_of_range
#include <string> // getline, stoi, string, to_string
#include <type_traits> // add_pointer, conditional, decay, enable_if, false_type, integral_constant, is_arithmetic, is_base_of, is_const, is_constructible, is_convertible, is_default_constructible, is_enum, is_floating_point, is_integral, is_nothrow_move_assignable, is_nothrow_move_constructible, is_pointer, is_reference, is_same, is_scalar, is_signed, remove_const, remove_cv, remove_pointer, remove_reference, true_type, underlying_type
#include <utility> // declval, forward, make_pair, move, pair, swap
#include <vector> // vector

// exclude unsupported compilers
#if defined(__clang__)
    #if (__clang_major__ * 10000 + __clang_minor__ * 100 + __clang_patchlevel__) < 30400
        #error "unsupported Clang version - see https://github.com/nlohmann/json#supported-compilers"
    #endif
#elif defined(__GNUC__)
    #if (__GNUC__ * 10000 + __GNUC_MINOR__ * 100 + __GNUC_PATCHLEVEL__) < 40900
//        #error "unsupported GCC version - see https://github.com/nlohmann/json#supported-compilers"
    #endif
#endif

// disable float-equal warnings on GCC/clang
#if defined(__clang__) || defined(__GNUC__) || defined(__GNUG__)
    #pragma GCC diagnostic push
    #pragma GCC diagnostic ignored "-Wfloat-equal"
#endif

// disable documentation warnings on clang
#if defined(__clang__)
    #pragma GCC diagnostic push
    #pragma GCC diagnostic ignored "-Wdocumentation"
#endif

// allow for portable deprecation warnings
#if defined(__clang__) || defined(__GNUC__) || defined(__GNUG__)
    #define JSON_DEPRECATED __attribute__((deprecated))
#elif defined(_MSC_VER)
    #define JSON_DEPRECATED __declspec(deprecated)
#else
    #define JSON_DEPRECATED
#endif

// allow to disable exceptions
#if not defined(JSON_NOEXCEPTION) || defined(__EXCEPTIONS)
    #define JSON_THROW(exception) throw exception
    #define JSON_TRY try
    #define JSON_CATCH(exception) catch(exception)
#else
    #define JSON_THROW(exception) std::abort()
    #define JSON_TRY if(true)
    #define JSON_CATCH(exception) if(false)
#endif

/*!
@brief namespace for Niels Lohmann
@see https://github.com/nlohmann
@since version 1.0.0
*/
namespace nlohmann
{

/*!
@brief unnamed namespace with internal helper functions

This namespace collects some functions that could not be defined inside the
@ref basic_json class.

@since version 2.1.0
*/
namespace detail
{
///////////////////////////
// JSON type enumeration //
///////////////////////////

/*!
@brief the JSON type enumeration

This enumeration collects the different JSON types. It is internally used to
distinguish the stored values, and the functions @ref basic_json::is_null(),
@ref basic_json::is_object(), @ref basic_json::is_array(),
@ref basic_json::is_string(), @ref basic_json::is_boolean(),
@ref basic_json::is_number() (with @ref basic_json::is_number_integer(),
@ref basic_json::is_number_unsigned(), and @ref basic_json::is_number_float()),
@ref basic_json::is_discarded(), @ref basic_json::is_primitive(), and
@ref basic_json::is_structured() rely on it.

@note There are three enumeration entries (number_integer, number_unsigned, and
number_float), because the library distinguishes these three types for numbers:
@ref basic_json::number_unsigned_t is used for unsigned integers,
@ref basic_json::number_integer_t is used for signed integers, and
@ref basic_json::number_float_t is used for floating-point numbers or to
approximate integers which do not fit in the limits of their respective type.

@sa @ref basic_json::basic_json(const value_t value_type) -- create a JSON
value with the default value for a given type

@since version 1.0.0
*/
enum class value_t : uint8_t
{
    null,            ///< null value
    object,          ///< object (unordered set of name/value pairs)
    array,           ///< array (ordered collection of values)
    string,          ///< string value
    boolean,         ///< boolean value
    number_integer,  ///< number value (signed integer)
    number_unsigned, ///< number value (unsigned integer)
    number_float,    ///< number value (floating-point)
    discarded        ///< discarded by the the parser callback function
};

/*!
@brief comparison operator for JSON types

Returns an ordering that is similar to Python:
- order: null < boolean < number < object < array < string
- furthermore, each type is not smaller than itself

@since version 1.0.0
*/
inline bool operator<(const value_t lhs, const value_t rhs) noexcept
{
    static constexpr std::array<uint8_t, 8> order = {{
            0, // null
            3, // object
            4, // array
            5, // string
            1, // boolean
            2, // integer
            2, // unsigned
            2, // float
        }
    };

    // discarded values are not comparable
    if (lhs == value_t::discarded or rhs == value_t::discarded)
    {
        return false;
    }

    return order[static_cast<std::size_t>(lhs)] <
           order[static_cast<std::size_t>(rhs)];
}


/////////////
// helpers //
/////////////

// alias templates to reduce boilerplate
template<bool B, typename T = void>
using enable_if_t = typename std::enable_if<B, T>::type;

template<typename T>
using uncvref_t = typename std::remove_cv<typename std::remove_reference<T>::type>::type;

// taken from http://stackoverflow.com/a/26936864/266378
template<typename T>
using is_unscoped_enum =
    std::integral_constant<bool, std::is_convertible<T, int>::value and
    std::is_enum<T>::value>;

/*
Implementation of two C++17 constructs: conjunction, negation. This is needed
to avoid evaluating all the traits in a condition

For example: not std::is_same<void, T>::value and has_value_type<T>::value
will not compile when T = void (on MSVC at least). Whereas
conjunction<negation<std::is_same<void, T>>, has_value_type<T>>::value will
stop evaluating if negation<...>::value == false

Please note that those constructs must be used with caution, since symbols can
become very long quickly (which can slow down compilation and cause MSVC
internal compiler errors). Only use it when you have to (see example ahead).
*/
template<class...> struct conjunction : std::true_type {};
template<class B1> struct conjunction<B1> : B1 {};
template<class B1, class... Bn>
struct conjunction<B1, Bn...> : std::conditional<bool(B1::value), conjunction<Bn...>, B1>::type {};

template<class B> struct negation : std::integral_constant < bool, !B::value > {};

// dispatch utility (taken from ranges-v3)
template<unsigned N> struct priority_tag : priority_tag < N - 1 > {};
template<> struct priority_tag<0> {};


//////////////////
// constructors //
//////////////////

template<value_t> struct external_constructor;

template<>
struct external_constructor<value_t::boolean>
{
    template<typename BasicJsonType>
    static void construct(BasicJsonType& j, typename BasicJsonType::boolean_t b) noexcept
    {
        j.m_type = value_t::boolean;
        j.m_value = b;
        j.assert_invariant();
    }
};

template<>
struct external_constructor<value_t::string>
{
    template<typename BasicJsonType>
    static void construct(BasicJsonType& j, const typename BasicJsonType::string_t& s)
    {
        j.m_type = value_t::string;
        j.m_value = s;
        j.assert_invariant();
    }
};

template<>
struct external_constructor<value_t::number_float>
{
    template<typename BasicJsonType>
    static void construct(BasicJsonType& j, typename BasicJsonType::number_float_t val) noexcept
    {
        // replace infinity and NAN by null
        if (not std::isfinite(val))
        {
            j = BasicJsonType{};
        }
        else
        {
            j.m_type = value_t::number_float;
            j.m_value = val;
        }
        j.assert_invariant();
    }
};

template<>
struct external_constructor<value_t::number_unsigned>
{
    template<typename BasicJsonType>
    static void construct(BasicJsonType& j, typename BasicJsonType::number_unsigned_t val) noexcept
    {
        j.m_type = value_t::number_unsigned;
        j.m_value = val;
        j.assert_invariant();
    }
};

template<>
struct external_constructor<value_t::number_integer>
{
    template<typename BasicJsonType>
    static void construct(BasicJsonType& j, typename BasicJsonType::number_integer_t val) noexcept
    {
        j.m_type = value_t::number_integer;
        j.m_value = val;
        j.assert_invariant();
    }
};

template<>
struct external_constructor<value_t::array>
{
    template<typename BasicJsonType>
    static void construct(BasicJsonType& j, const typename BasicJsonType::array_t& arr)
    {
        j.m_type = value_t::array;
        j.m_value = arr;
        j.assert_invariant();
    }

    template<typename BasicJsonType, typename CompatibleArrayType,
             enable_if_t<not std::is_same<CompatibleArrayType,
                                          typename BasicJsonType::array_t>::value,
                         int> = 0>
    static void construct(BasicJsonType& j, const CompatibleArrayType& arr)
    {
        using std::begin;
        using std::end;
        j.m_type = value_t::array;
        j.m_value.array = j.template create<typename BasicJsonType::array_t>(begin(arr), end(arr));
        j.assert_invariant();
    }
};

template<>
struct external_constructor<value_t::object>
{
    template<typename BasicJsonType>
    static void construct(BasicJsonType& j, const typename BasicJsonType::object_t& obj)
    {
        j.m_type = value_t::object;
        j.m_value = obj;
        j.assert_invariant();
    }

    template<typename BasicJsonType, typename CompatibleObjectType,
             enable_if_t<not std::is_same<CompatibleObjectType,
                                          typename BasicJsonType::object_t>::value,
                         int> = 0>
    static void construct(BasicJsonType& j, const CompatibleObjectType& obj)
    {
        using std::begin;
        using std::end;

        j.m_type = value_t::object;
        j.m_value.object = j.template create<typename BasicJsonType::object_t>(begin(obj), end(obj));
        j.assert_invariant();
    }
};


////////////////////////
// has_/is_ functions //
////////////////////////

/*!
@brief Helper to determine whether there's a key_type for T.

This helper is used to tell associative containers apart from other containers
such as sequence containers. For instance, `std::map` passes the test as it
contains a `mapped_type`, whereas `std::vector` fails the test.

@sa http://stackoverflow.com/a/7728728/266378
@since version 1.0.0, overworked in version 2.0.6
*/
#define NLOHMANN_JSON_HAS_HELPER(type)                                        \
    template<typename T> struct has_##type {                                  \
    private:                                                                  \
        template<typename U, typename = typename U::type>                     \
        static int detect(U &&);                                              \
        static void detect(...);                                              \
    public:                                                                   \
        static constexpr bool value =                                         \
                std::is_integral<decltype(detect(std::declval<T>()))>::value; \
    }

NLOHMANN_JSON_HAS_HELPER(mapped_type);
NLOHMANN_JSON_HAS_HELPER(key_type);
NLOHMANN_JSON_HAS_HELPER(value_type);
NLOHMANN_JSON_HAS_HELPER(iterator);

#undef NLOHMANN_JSON_HAS_HELPER


template<bool B, class RealType, class CompatibleObjectType>
struct is_compatible_object_type_impl : std::false_type {};

template<class RealType, class CompatibleObjectType>
struct is_compatible_object_type_impl<true, RealType, CompatibleObjectType>
{
    static constexpr auto value =
        std::is_constructible<typename RealType::key_type,
        typename CompatibleObjectType::key_type>::value and
        std::is_constructible<typename RealType::mapped_type,
        typename CompatibleObjectType::mapped_type>::value;
};

template<class BasicJsonType, class CompatibleObjectType>
struct is_compatible_object_type
{
    static auto constexpr value = is_compatible_object_type_impl <
                                  conjunction<negation<std::is_same<void, CompatibleObjectType>>,
                                  has_mapped_type<CompatibleObjectType>,
                                  has_key_type<CompatibleObjectType>>::value,
                                  typename BasicJsonType::object_t, CompatibleObjectType >::value;
};

template<typename BasicJsonType, typename T>
struct is_basic_json_nested_type
{
    static auto constexpr value = std::is_same<T, typename BasicJsonType::iterator>::value or
                                  std::is_same<T, typename BasicJsonType::const_iterator>::value or
                                  std::is_same<T, typename BasicJsonType::reverse_iterator>::value or
                                  std::is_same<T, typename BasicJsonType::const_reverse_iterator>::value or
                                  std::is_same<T, typename BasicJsonType::json_pointer>::value;
};

template<class BasicJsonType, class CompatibleArrayType>
struct is_compatible_array_type
{
    static auto constexpr value =
        conjunction<negation<std::is_same<void, CompatibleArrayType>>,
        negation<is_compatible_object_type<
        BasicJsonType, CompatibleArrayType>>,
        negation<std::is_constructible<typename BasicJsonType::string_t,
        CompatibleArrayType>>,
        negation<is_basic_json_nested_type<BasicJsonType, CompatibleArrayType>>,
        has_value_type<CompatibleArrayType>,
        has_iterator<CompatibleArrayType>>::value;
};

template<bool, typename, typename>
struct is_compatible_integer_type_impl : std::false_type {};

template<typename RealIntegerType, typename CompatibleNumberIntegerType>
struct is_compatible_integer_type_impl<true, RealIntegerType, CompatibleNumberIntegerType>
{
    // is there an assert somewhere on overflows?
    using RealLimits = std::numeric_limits<RealIntegerType>;
    using CompatibleLimits = std::numeric_limits<CompatibleNumberIntegerType>;

    static constexpr auto value =
        std::is_constructible<RealIntegerType,
        CompatibleNumberIntegerType>::value and
        CompatibleLimits::is_integer and
        RealLimits::is_signed == CompatibleLimits::is_signed;
};

template<typename RealIntegerType, typename CompatibleNumberIntegerType>
struct is_compatible_integer_type
{
    static constexpr auto value =
        is_compatible_integer_type_impl <
        std::is_integral<CompatibleNumberIntegerType>::value and
        not std::is_same<bool, CompatibleNumberIntegerType>::value,
        RealIntegerType, CompatibleNumberIntegerType > ::value;
};


// trait checking if JSONSerializer<T>::from_json(json const&, udt&) exists
template<typename BasicJsonType, typename T>
struct has_from_json
{
  private:
    // also check the return type of from_json
    template<typename U, typename = enable_if_t<std::is_same<void, decltype(uncvref_t<U>::from_json(
                 std::declval<BasicJsonType>(), std::declval<T&>()))>::value>>
    static int detect(U&&);
    static void detect(...);

  public:
    static constexpr bool value = std::is_integral<decltype(
                                      detect(std::declval<typename BasicJsonType::template json_serializer<T, void>>()))>::value;
};

// This trait checks if JSONSerializer<T>::from_json(json const&) exists
// this overload is used for non-default-constructible user-defined-types
template<typename BasicJsonType, typename T>
struct has_non_default_from_json
{
  private:
    template <
        typename U,
        typename = enable_if_t<std::is_same<
                                   T, decltype(uncvref_t<U>::from_json(std::declval<BasicJsonType>()))>::value >>
    static int detect(U&&);
    static void detect(...);

  public:
    static constexpr bool value = std::is_integral<decltype(detect(
                                      std::declval<typename BasicJsonType::template json_serializer<T, void>>()))>::value;
};

// This trait checks if BasicJsonType::json_serializer<T>::to_json exists
template<typename BasicJsonType, typename T>
struct has_to_json
{
  private:
    template<typename U, typename = decltype(uncvref_t<U>::to_json(
                 std::declval<BasicJsonType&>(), std::declval<T>()))>
    static int detect(U&&);
    static void detect(...);

  public:
    static constexpr bool value = std::is_integral<decltype(detect(
                                      std::declval<typename BasicJsonType::template json_serializer<T, void>>()))>::value;
};


/////////////
// to_json //
/////////////

template<typename BasicJsonType, typename T, enable_if_t<
             std::is_same<T, typename BasicJsonType::boolean_t>::value, int> = 0>
void to_json(BasicJsonType& j, T b) noexcept
{
    external_constructor<value_t::boolean>::construct(j, b);
}

template<typename BasicJsonType, typename CompatibleString,
         enable_if_t<std::is_constructible<typename BasicJsonType::string_t,
                     CompatibleString>::value, int> = 0>
void to_json(BasicJsonType& j, const CompatibleString& s)
{
    external_constructor<value_t::string>::construct(j, s);
}

template<typename BasicJsonType, typename FloatType,
         enable_if_t<std::is_floating_point<FloatType>::value, int> = 0>
void to_json(BasicJsonType& j, FloatType val) noexcept
{
    external_constructor<value_t::number_float>::construct(j, static_cast<typename BasicJsonType::number_float_t>(val));
}

template <
    typename BasicJsonType, typename CompatibleNumberUnsignedType,
    enable_if_t<is_compatible_integer_type<typename BasicJsonType::number_unsigned_t,
                CompatibleNumberUnsignedType>::value, int> = 0 >
void to_json(BasicJsonType& j, CompatibleNumberUnsignedType val) noexcept
{
    external_constructor<value_t::number_unsigned>::construct(j, static_cast<typename BasicJsonType::number_unsigned_t>(val));
}

template <
    typename BasicJsonType, typename CompatibleNumberIntegerType,
    enable_if_t<is_compatible_integer_type<typename BasicJsonType::number_integer_t,
                CompatibleNumberIntegerType>::value, int> = 0 >
void to_json(BasicJsonType& j, CompatibleNumberIntegerType val) noexcept
{
    external_constructor<value_t::number_integer>::construct(j, static_cast<typename BasicJsonType::number_integer_t>(val));
}

template<typename BasicJsonType, typename UnscopedEnumType,
         enable_if_t<is_unscoped_enum<UnscopedEnumType>::value, int> = 0>
void to_json(BasicJsonType& j, UnscopedEnumType e) noexcept
{
    external_constructor<value_t::number_integer>::construct(j, e);
}

template <
    typename BasicJsonType, typename CompatibleArrayType,
    enable_if_t <
        is_compatible_array_type<BasicJsonType, CompatibleArrayType>::value or
        std::is_same<typename BasicJsonType::array_t, CompatibleArrayType>::value,
        int > = 0 >
void to_json(BasicJsonType& j, const  CompatibleArrayType& arr)
{
    external_constructor<value_t::array>::construct(j, arr);
}

template <
    typename BasicJsonType, typename CompatibleObjectType,
    enable_if_t<is_compatible_object_type<BasicJsonType, CompatibleObjectType>::value,
                int> = 0 >
void to_json(BasicJsonType& j, const  CompatibleObjectType& arr)
{
    external_constructor<value_t::object>::construct(j, arr);
}


///////////////
// from_json //
///////////////

// overloads for basic_json template parameters
template<typename BasicJsonType, typename ArithmeticType,
         enable_if_t<std::is_arithmetic<ArithmeticType>::value and
                     not std::is_same<ArithmeticType,
                                      typename BasicJsonType::boolean_t>::value,
                     int> = 0>
void get_arithmetic_value(const BasicJsonType& j, ArithmeticType& val)
{
    switch (static_cast<value_t>(j))
    {
        case value_t::number_unsigned:
        {
            val = static_cast<ArithmeticType>(
                      *j.template get_ptr<const typename BasicJsonType::number_unsigned_t*>());
            break;
        }
        case value_t::number_integer:
        {
            val = static_cast<ArithmeticType>(
                      *j.template get_ptr<const typename BasicJsonType::number_integer_t*>());
            break;
        }
        case value_t::number_float:
        {
            val = static_cast<ArithmeticType>(
                      *j.template get_ptr<const typename BasicJsonType::number_float_t*>());
            break;
        }
        default:
        {
            JSON_THROW(
                std::domain_error("type must be number, but is " + j.type_name()));
        }
    }
}

template<typename BasicJsonType>
void from_json(const BasicJsonType& j, typename BasicJsonType::boolean_t& b)
{
    if (not j.is_boolean())
    {
        JSON_THROW(std::domain_error("type must be boolean, but is " + j.type_name()));
    }
    b = *j.template get_ptr<const typename BasicJsonType::boolean_t*>();
}

template<typename BasicJsonType>
void from_json(const BasicJsonType& j, typename BasicJsonType::string_t& s)
{
    if (not j.is_string())
    {
        JSON_THROW(std::domain_error("type must be string, but is " + j.type_name()));
    }
    s = *j.template get_ptr<const typename BasicJsonType::string_t*>();
}

template<typename BasicJsonType>
void from_json(const BasicJsonType& j, typename BasicJsonType::number_float_t& val)
{
    get_arithmetic_value(j, val);
}

template<typename BasicJsonType>
void from_json(const BasicJsonType& j, typename BasicJsonType::number_unsigned_t& val)
{
    get_arithmetic_value(j, val);
}

template<typename BasicJsonType>
void from_json(const BasicJsonType& j, typename BasicJsonType::number_integer_t& val)
{
    get_arithmetic_value(j, val);
}

template<typename BasicJsonType, typename UnscopedEnumType,
         enable_if_t<is_unscoped_enum<UnscopedEnumType>::value, int> = 0>
void from_json(const BasicJsonType& j, UnscopedEnumType& e)
{
    typename std::underlying_type<UnscopedEnumType>::type val;
    get_arithmetic_value(j, val);
    e = static_cast<UnscopedEnumType>(val);
}

template<typename BasicJsonType>
void from_json(const BasicJsonType& j, typename BasicJsonType::array_t& arr)
{
    if (not j.is_array())
    {
        JSON_THROW(std::domain_error("type must be array, but is " + j.type_name()));
    }
    arr = *j.template get_ptr<const typename BasicJsonType::array_t*>();
}

// forward_list doesn't have an insert method
template<typename BasicJsonType, typename T, typename Allocator>
void from_json(const BasicJsonType& j, std::forward_list<T, Allocator>& l)
{
    // do not perform the check when user wants to retrieve jsons
    // (except when it's null.. ?)
    if (j.is_null())
    {
        JSON_THROW(std::domain_error("type must be array, but is " + j.type_name()));
    }
    if (not std::is_same<T, BasicJsonType>::value)
    {
        if (not j.is_array())
        {
            JSON_THROW(std::domain_error("type must be array, but is " + j.type_name()));
        }
    }
    for (auto it = j.rbegin(), end = j.rend(); it != end; ++it)
    {
        l.push_front(it->template get<T>());
    }
}

template<typename BasicJsonType, typename CompatibleArrayType>
void from_json_array_impl(const BasicJsonType& j, CompatibleArrayType& arr, priority_tag<0>)
{
    using std::begin;
    using std::end;

    std::transform(j.begin(), j.end(),
                   std::inserter(arr, end(arr)), [](const BasicJsonType & i)
    {
        // get<BasicJsonType>() returns *this, this won't call a from_json
        // method when value_type is BasicJsonType
        return i.template get<typename CompatibleArrayType::value_type>();
    });
}

template<typename BasicJsonType, typename CompatibleArrayType>
auto from_json_array_impl(const BasicJsonType& j, CompatibleArrayType& arr, priority_tag<1>)
-> decltype(
    arr.reserve(std::declval<typename CompatibleArrayType::size_type>()),
    void())
{
    using std::begin;
    using std::end;

    arr.reserve(j.size());
    std::transform(
        j.begin(), j.end(), std::inserter(arr, end(arr)), [](const BasicJsonType & i)
    {
        // get<BasicJsonType>() returns *this, this won't call a from_json
        // method when value_type is BasicJsonType
        return i.template get<typename CompatibleArrayType::value_type>();
    });
}

template<typename BasicJsonType, typename CompatibleArrayType,
         enable_if_t<is_compatible_array_type<BasicJsonType, CompatibleArrayType>::value and
                     not std::is_same<typename BasicJsonType::array_t, CompatibleArrayType>::value, int> = 0>
void from_json(const BasicJsonType& j, CompatibleArrayType& arr)
{
    if (j.is_null())
    {
        JSON_THROW(std::domain_error("type must be array, but is " + j.type_name()));
    }

    // when T == BasicJsonType, do not check if value_t is correct
    if (not std::is_same<typename CompatibleArrayType::value_type, BasicJsonType>::value)
    {
        if (not j.is_array())
        {
            JSON_THROW(std::domain_error("type must be array, but is " + j.type_name()));
        }
    }
    from_json_array_impl(j, arr, priority_tag<1> {});
}

template<typename BasicJsonType, typename CompatibleObjectType,
         enable_if_t<is_compatible_object_type<BasicJsonType, CompatibleObjectType>::value, int> = 0>
void from_json(const BasicJsonType& j, CompatibleObjectType& obj)
{
    if (not j.is_object())
    {
        JSON_THROW(std::domain_error("type must be object, but is " + j.type_name()));
    }

    auto inner_object = j.template get_ptr<const typename BasicJsonType::object_t*>();
    using std::begin;
    using std::end;
    // we could avoid the assignment, but this might require a for loop, which
    // might be less efficient than the container constructor for some
    // containers (would it?)
    obj = CompatibleObjectType(begin(*inner_object), end(*inner_object));
}

// overload for arithmetic types, not chosen for basic_json template arguments
// (BooleanType, etc..); note: Is it really necessary to provide explicit
// overloads for boolean_t etc. in case of a custom BooleanType which is not
// an arithmetic type?
template<typename BasicJsonType, typename ArithmeticType,
         enable_if_t <
             std::is_arithmetic<ArithmeticType>::value and
             not std::is_same<ArithmeticType, typename BasicJsonType::number_unsigned_t>::value and
             not std::is_same<ArithmeticType, typename BasicJsonType::number_integer_t>::value and
             not std::is_same<ArithmeticType, typename BasicJsonType::number_float_t>::value and
             not std::is_same<ArithmeticType, typename BasicJsonType::boolean_t>::value,
             int> = 0>
void from_json(const BasicJsonType& j, ArithmeticType& val)
{
    switch (static_cast<value_t>(j))
    {
        case value_t::number_unsigned:
        {
            val = static_cast<ArithmeticType>(*j.template get_ptr<const typename BasicJsonType::number_unsigned_t*>());
            break;
        }
        case value_t::number_integer:
        {
            val = static_cast<ArithmeticType>(*j.template get_ptr<const typename BasicJsonType::number_integer_t*>());
            break;
        }
        case value_t::number_float:
        {
            val = static_cast<ArithmeticType>(*j.template get_ptr<const typename BasicJsonType::number_float_t*>());
            break;
        }
        case value_t::boolean:
        {
            val = static_cast<ArithmeticType>(*j.template get_ptr<const typename BasicJsonType::boolean_t*>());
            break;
        }
        default:
        {
            JSON_THROW(std::domain_error("type must be number, but is " + j.type_name()));
        }
    }
}

struct to_json_fn
{
  private:
    template<typename BasicJsonType, typename T>
    auto call(BasicJsonType& j, T&& val, priority_tag<1>) const noexcept(noexcept(to_json(j, std::forward<T>(val))))
    -> decltype(to_json(j, std::forward<T>(val)), void())
    {
        return to_json(j, std::forward<T>(val));
    }

    template<typename BasicJsonType, typename T>
    void call(BasicJsonType&, T&&, priority_tag<0>) const noexcept
    {
        static_assert(sizeof(BasicJsonType) == 0,
                      "could not find to_json() method in T's namespace");
    }

  public:
    template<typename BasicJsonType, typename T>
    void operator()(BasicJsonType& j, T&& val) const
    noexcept(noexcept(std::declval<to_json_fn>().call(j, std::forward<T>(val), priority_tag<1> {})))
    {
        return call(j, std::forward<T>(val), priority_tag<1> {});
    }
};

struct from_json_fn
{
  private:
    template<typename BasicJsonType, typename T>
    auto call(const BasicJsonType& j, T& val, priority_tag<1>) const
    noexcept(noexcept(from_json(j, val)))
    -> decltype(from_json(j, val), void())
    {
        return from_json(j, val);
    }

    template<typename BasicJsonType, typename T>
    void call(const BasicJsonType&, T&, priority_tag<0>) const noexcept
    {
        static_assert(sizeof(BasicJsonType) == 0,
                      "could not find from_json() method in T's namespace");
    }

  public:
    template<typename BasicJsonType, typename T>
    void operator()(const BasicJsonType& j, T& val) const
    noexcept(noexcept(std::declval<from_json_fn>().call(j, val, priority_tag<1> {})))
    {
        return call(j, val, priority_tag<1> {});
    }
};

// taken from ranges-v3
template<typename T>
struct static_const
{
    static constexpr T value{};
};

template<typename T>
constexpr T static_const<T>::value;
} // namespace detail


/// namespace to hold default `to_json` / `from_json` functions
namespace
{
constexpr const auto& to_json = detail::static_const<detail::to_json_fn>::value;
constexpr const auto& from_json = detail::static_const<detail::from_json_fn>::value;
}


/*!
@brief default JSONSerializer template argument

This serializer ignores the template arguments and uses ADL
([argument-dependent lookup](http://en.cppreference.com/w/cpp/language/adl))
for serialization.
*/
template<typename = void, typename = void>
struct adl_serializer
{
    /*!
    @brief convert a JSON value to any value type

    This function is usually called by the `get()` function of the
    @ref basic_json class (either explicit or via conversion operators).

    @param[in] j         JSON value to read from
    @param[in,out] val  value to write to
    */
    template<typename BasicJsonType, typename ValueType>
    static void from_json(BasicJsonType&& j, ValueType& val) noexcept(
        noexcept(::nlohmann::from_json(std::forward<BasicJsonType>(j), val)))
    {
        ::nlohmann::from_json(std::forward<BasicJsonType>(j), val);
    }

    /*!
    @brief convert any value type to a JSON value

    This function is usually called by the constructors of the @ref basic_json
    class.

    @param[in,out] j  JSON value to write to
    @param[in] val     value to read from
    */
    template<typename BasicJsonType, typename ValueType>
    static void to_json(BasicJsonType& j, ValueType&& val) noexcept(
        noexcept(::nlohmann::to_json(j, std::forward<ValueType>(val))))
    {
        ::nlohmann::to_json(j, std::forward<ValueType>(val));
    }
};


/*!
@brief a class to store JSON values

@tparam ObjectType type for JSON objects (`std::map` by default; will be used
in @ref object_t)
@tparam ArrayType type for JSON arrays (`std::vector` by default; will be used
in @ref array_t)
@tparam StringType type for JSON strings and object keys (`std::string` by
default; will be used in @ref string_t)
@tparam BooleanType type for JSON booleans (`bool` by default; will be used
in @ref boolean_t)
@tparam NumberIntegerType type for JSON integer numbers (`int64_t` by
default; will be used in @ref number_integer_t)
@tparam NumberUnsignedType type for JSON unsigned integer numbers (@c
`uint64_t` by default; will be used in @ref number_unsigned_t)
@tparam NumberFloatType type for JSON floating-point numbers (`double` by
default; will be used in @ref number_float_t)
@tparam AllocatorType type of the allocator to use (`std::allocator` by
default)
@tparam JSONSerializer the serializer to resolve internal calls to `to_json()`
and `from_json()` (@ref adl_serializer by default)

@requirement The class satisfies the following concept requirements:
- Basic
 - [DefaultConstructible](http://en.cppreference.com/w/cpp/concept/DefaultConstructible):
   JSON values can be default constructed. The result will be a JSON null
   value.
 - [MoveConstructible](http://en.cppreference.com/w/cpp/concept/MoveConstructible):
   A JSON value can be constructed from an rvalue argument.
 - [CopyConstructible](http://en.cppreference.com/w/cpp/concept/CopyConstructible):
   A JSON value can be copy-constructed from an lvalue expression.
 - [MoveAssignable](http://en.cppreference.com/w/cpp/concept/MoveAssignable):
   A JSON value van be assigned from an rvalue argument.
 - [CopyAssignable](http://en.cppreference.com/w/cpp/concept/CopyAssignable):
   A JSON value can be copy-assigned from an lvalue expression.
 - [Destructible](http://en.cppreference.com/w/cpp/concept/Destructible):
   JSON values can be destructed.
- Layout
 - [StandardLayoutType](http://en.cppreference.com/w/cpp/concept/StandardLayoutType):
   JSON values have
   [standard layout](http://en.cppreference.com/w/cpp/language/data_members#Standard_layout):
   All non-static data members are private and standard layout types, the
   class has no virtual functions or (virtual) base classes.
- Library-wide
 - [EqualityComparable](http://en.cppreference.com/w/cpp/concept/EqualityComparable):
   JSON values can be compared with `==`, see @ref
   operator==(const_reference,const_reference).
 - [LessThanComparable](http://en.cppreference.com/w/cpp/concept/LessThanComparable):
   JSON values can be compared with `<`, see @ref
   operator<(const_reference,const_reference).
 - [Swappable](http://en.cppreference.com/w/cpp/concept/Swappable):
   Any JSON lvalue or rvalue of can be swapped with any lvalue or rvalue of
   other compatible types, using unqualified function call @ref swap().
 - [NullablePointer](http://en.cppreference.com/w/cpp/concept/NullablePointer):
   JSON values can be compared against `std::nullptr_t` objects which are used
   to model the `null` value.
- Container
 - [Container](http://en.cppreference.com/w/cpp/concept/Container):
   JSON values can be used like STL containers and provide iterator access.
 - [ReversibleContainer](http://en.cppreference.com/w/cpp/concept/ReversibleContainer);
   JSON values can be used like STL containers and provide reverse iterator
   access.

@invariant The member variables @a m_value and @a m_type have the following
relationship:
- If `m_type == value_t::object`, then `m_value.object != nullptr`.
- If `m_type == value_t::array`, then `m_value.array != nullptr`.
- If `m_type == value_t::string`, then `m_value.string != nullptr`.
The invariants are checked by member function assert_invariant().

@internal
@note ObjectType trick from http://stackoverflow.com/a/9860911
@endinternal

@see [RFC 7159: The JavaScript Object Notation (JSON) Data Interchange
Format](http://rfc7159.net/rfc7159)

@since version 1.0.0

@nosubgrouping
*/
template <
    template<typename U, typename V, typename... Args> class ObjectType = std::map,
    template<typename U, typename... Args> class ArrayType = std::vector,
    class StringType = std::string,
    class BooleanType = bool,
    class NumberIntegerType = std::int64_t,
    class NumberUnsignedType = std::uint64_t,
    class NumberFloatType = double,
    template<typename U> class AllocatorType = std::allocator,
    template<typename T, typename SFINAE = void> class JSONSerializer = adl_serializer
    >
class basic_json
{
  private:
    template<detail::value_t> friend struct detail::external_constructor;
    /// workaround type for MSVC
    using basic_json_t = basic_json<ObjectType, ArrayType, StringType,
          BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType,
          AllocatorType, JSONSerializer>;

  public:
    using value_t = detail::value_t;
    // forward declarations
    template<typename U> class iter_impl;
    template<typename Base> class json_reverse_iterator;
    class json_pointer;
    template<typename T, typename SFINAE>
    using json_serializer = JSONSerializer<T, SFINAE>;

    /////////////////////
    // container types //
    /////////////////////

    /// @name container types
    /// The canonic container types to use @ref basic_json like any other STL
    /// container.
    /// @{

    /// the type of elements in a basic_json container
    using value_type = basic_json;

    /// the type of an element reference
    using reference = value_type&;
    /// the type of an element const reference
    using const_reference = const value_type&;

    /// a type to represent differences between iterators
    using difference_type = std::ptrdiff_t;
    /// a type to represent container sizes
    using size_type = std::size_t;

    /// the allocator type
    using allocator_type = AllocatorType<basic_json>;

    /// the type of an element pointer
    using pointer = typename std::allocator_traits<allocator_type>::pointer;
    /// the type of an element const pointer
    using const_pointer = typename std::allocator_traits<allocator_type>::const_pointer;

    /// an iterator for a basic_json container
    using iterator = iter_impl<basic_json>;
    /// a const iterator for a basic_json container
    using const_iterator = iter_impl<const basic_json>;
    /// a reverse iterator for a basic_json container
    using reverse_iterator = json_reverse_iterator<typename basic_json::iterator>;
    /// a const reverse iterator for a basic_json container
    using const_reverse_iterator = json_reverse_iterator<typename basic_json::const_iterator>;

    /// @}


    /*!
    @brief returns the allocator associated with the container
    */
    static allocator_type get_allocator()
    {
        return allocator_type();
    }

    /*!
    @brief returns version information on the library

    This function returns a JSON object with information about the library,
    including the version number and information on the platform and compiler.

    @return JSON object holding version information
    key         | description
    ----------- | ---------------
    `compiler`  | Information on the used compiler. It is an object with the following keys: `c++` (the used C++ standard), `family` (the compiler family; possible values are `clang`, `icc`, `gcc`, `ilecpp`, `msvc`, `pgcpp`, `sunpro`, and `unknown`), and `version` (the compiler version).
    `copyright` | The copyright line for the library as string.
    `name`      | The name of the library as string.
    `platform`  | The used platform as string. Possible values are `win32`, `linux`, `apple`, `unix`, and `unknown`.
    `url`       | The URL of the project as string.
    `version`   | The version of the library. It is an object with the following keys: `major`, `minor`, and `patch` as defined by [Semantic Versioning](http://semver.org), and `string` (the version string).

    @liveexample{The following code shows an example output of the `meta()`
    function.,meta}

    @complexity Constant.

    @since 2.1.0
    */
    static basic_json meta()
    {
        basic_json result;

        result["copyright"] = "(C) 2013-2017 Niels Lohmann";
        result["name"] = "JSON for Modern C++";
        result["url"] = "https://github.com/nlohmann/json";
        result["version"] =
        {
            {"string", "2.1.1"},
            {"major", 2},
            {"minor", 1},
            {"patch", 1}
        };

#ifdef _WIN32
        result["platform"] = "win32";
#elif defined __linux__
        result["platform"] = "linux";
#elif defined __APPLE__
        result["platform"] = "apple";
#elif defined __unix__
        result["platform"] = "unix";
#else
        result["platform"] = "unknown";
#endif

#if defined(__clang__)
        result["compiler"] = {{"family", "clang"}, {"version", __clang_version__}};
#elif defined(__ICC) || defined(__INTEL_COMPILER)
        result["compiler"] = {{"family", "icc"}, {"version", __INTEL_COMPILER}};
#elif defined(__GNUC__) || defined(__GNUG__)
        result["compiler"] = {{"family", "gcc"}, {"version", std::to_string(__GNUC__) + "." + std::to_string(__GNUC_MINOR__) + "." + std::to_string(__GNUC_PATCHLEVEL__)}};
#elif defined(__HP_cc) || defined(__HP_aCC)
        result["compiler"] = "hp"
#elif defined(__IBMCPP__)
        result["compiler"] = {{"family", "ilecpp"}, {"version", __IBMCPP__}};
#elif defined(_MSC_VER)
        result["compiler"] = {{"family", "msvc"}, {"version", _MSC_VER}};
#elif defined(__PGI)
        result["compiler"] = {{"family", "pgcpp"}, {"version", __PGI}};
#elif defined(__SUNPRO_CC)
        result["compiler"] = {{"family", "sunpro"}, {"version", __SUNPRO_CC}};
#else
        result["compiler"] = {{"family", "unknown"}, {"version", "unknown"}};
#endif

#ifdef __cplusplus
        result["compiler"]["c++"] = std::to_string(__cplusplus);
#else
        result["compiler"]["c++"] = "unknown";
#endif
        return result;
    }


    ///////////////////////////
    // JSON value data types //
    ///////////////////////////

    /// @name JSON value data types
    /// The data types to store a JSON value. These types are derived from
    /// the template arguments passed to class @ref basic_json.
    /// @{

    /*!
    @brief a type for an object

    [RFC 7159](http://rfc7159.net/rfc7159) describes JSON objects as follows:
    > An object is an unordered collection of zero or more name/value pairs,
    > where a name is a string and a value is a string, number, boolean, null,
    > object, or array.

    To store objects in C++, a type is defined by the template parameters
    described below.

    @tparam ObjectType  the container to store objects (e.g., `std::map` or
    `std::unordered_map`)
    @tparam StringType the type of the keys or names (e.g., `std::string`).
    The comparison function `std::less<StringType>` is used to order elements
    inside the container.
    @tparam AllocatorType the allocator to use for objects (e.g.,
    `std::allocator`)

    #### Default type

    With the default values for @a ObjectType (`std::map`), @a StringType
    (`std::string`), and @a AllocatorType (`std::allocator`), the default
    value for @a object_t is:

    @code {.cpp}
    std::map<
      std::string, // key_type
      basic_json, // value_type
      std::less<std::string>, // key_compare
      std::allocator<std::pair<const std::string, basic_json>> // allocator_type
    >
    @endcode

    #### Behavior

    The choice of @a object_t influences the behavior of the JSON class. With
    the default type, objects have the following behavior:

    - When all names are unique, objects will be interoperable in the sense
      that all software implementations receiving that object will agree on
      the name-value mappings.
    - When the names within an object are not unique, later stored name/value
      pairs overwrite previously stored name/value pairs, leaving the used
      names unique. For instance, `{"key": 1}` and `{"key": 2, "key": 1}` will
      be treated as equal and both stored as `{"key": 1}`.
    - Internally, name/value pairs are stored in lexicographical order of the
      names. Objects will also be serialized (see @ref dump) in this order.
      For instance, `{"b": 1, "a": 2}` and `{"a": 2, "b": 1}` will be stored
      and serialized as `{"a": 2, "b": 1}`.
    - When comparing objects, the order of the name/value pairs is irrelevant.
      This makes objects interoperable in the sense that they will not be
      affected by these differences. For instance, `{"b": 1, "a": 2}` and
      `{"a": 2, "b": 1}` will be treated as equal.

    #### Limits

    [RFC 7159](http://rfc7159.net/rfc7159) specifies:
    > An implementation may set limits on the maximum depth of nesting.

    In this class, the object's limit of nesting is not constraint explicitly.
    However, a maximum depth of nesting may be introduced by the compiler or
    runtime environment. A theoretical limit can be queried by calling the
    @ref max_size function of a JSON object.

    #### Storage

    Objects are stored as pointers in a @ref basic_json type. That is, for any
    access to object values, a pointer of type `object_t*` must be
    dereferenced.

    @sa @ref array_t -- type for an array value

    @since version 1.0.0

    @note The order name/value pairs are added to the object is *not*
    preserved by the library. Therefore, iterating an object may return
    name/value pairs in a different order than they were originally stored. In
    fact, keys will be traversed in alphabetical order as `std::map` with
    `std::less` is used by default. Please note this behavior conforms to [RFC
    7159](http://rfc7159.net/rfc7159), because any order implements the
    specified "unordered" nature of JSON objects.
    */
    using object_t = ObjectType<StringType,
          basic_json,
          std::less<StringType>,
          AllocatorType<std::pair<const StringType,
          basic_json>>>;

    /*!
    @brief a type for an array

    [RFC 7159](http://rfc7159.net/rfc7159) describes JSON arrays as follows:
    > An array is an ordered sequence of zero or more values.

    To store objects in C++, a type is defined by the template parameters
    explained below.

    @tparam ArrayType  container type to store arrays (e.g., `std::vector` or
    `std::list`)
    @tparam AllocatorType allocator to use for arrays (e.g., `std::allocator`)

    #### Default type

    With the default values for @a ArrayType (`std::vector`) and @a
    AllocatorType (`std::allocator`), the default value for @a array_t is:

    @code {.cpp}
    std::vector<
      basic_json, // value_type
      std::allocator<basic_json> // allocator_type
    >
    @endcode

    #### Limits

    [RFC 7159](http://rfc7159.net/rfc7159) specifies:
    > An implementation may set limits on the maximum depth of nesting.

    In this class, the array's limit of nesting is not constraint explicitly.
    However, a maximum depth of nesting may be introduced by the compiler or
    runtime environment. A theoretical limit can be queried by calling the
    @ref max_size function of a JSON array.

    #### Storage

    Arrays are stored as pointers in a @ref basic_json type. That is, for any
    access to array values, a pointer of type `array_t*` must be dereferenced.

    @sa @ref object_t -- type for an object value

    @since version 1.0.0
    */
    using array_t = ArrayType<basic_json, AllocatorType<basic_json>>;

    /*!
    @brief a type for a string

    [RFC 7159](http://rfc7159.net/rfc7159) describes JSON strings as follows:
    > A string is a sequence of zero or more Unicode characters.

    To store objects in C++, a type is defined by the template parameter
    described below. Unicode values are split by the JSON class into
    byte-sized characters during deserialization.

    @tparam StringType  the container to store strings (e.g., `std::string`).
    Note this container is used for keys/names in objects, see @ref object_t.

    #### Default type

    With the default values for @a StringType (`std::string`), the default
    value for @a string_t is:

    @code {.cpp}
    std::string
    @endcode

    #### Encoding

    Strings are stored in UTF-8 encoding. Therefore, functions like
    `std::string::size()` or `std::string::length()` return the number of
    bytes in the string rather than the number of characters or glyphs.

    #### String comparison

    [RFC 7159](http://rfc7159.net/rfc7159) states:
    > Software implementations are typically required to test names of object
    > members for equality. Implementations that transform the textual
    > representation into sequences of Unicode code units and then perform the
    > comparison numerically, code unit by code unit, are interoperable in the
    > sense that implementations will agree in all cases on equality or
    > inequality of two strings. For example, implementations that compare
    > strings with escaped characters unconverted may incorrectly find that
    > `"a\\b"` and `"a\u005Cb"` are not equal.

    This implementation is interoperable as it does compare strings code unit
    by code unit.

    #### Storage

    String values are stored as pointers in a @ref basic_json type. That is,
    for any access to string values, a pointer of type `string_t*` must be
    dereferenced.

    @since version 1.0.0
    */
    using string_t = StringType;

    /*!
    @brief a type for a boolean

    [RFC 7159](http://rfc7159.net/rfc7159) implicitly describes a boolean as a
    type which differentiates the two literals `true` and `false`.

    To store objects in C++, a type is defined by the template parameter @a
    BooleanType which chooses the type to use.

    #### Default type

    With the default values for @a BooleanType (`bool`), the default value for
    @a boolean_t is:

    @code {.cpp}
    bool
    @endcode

    #### Storage

    Boolean values are stored directly inside a @ref basic_json type.

    @since version 1.0.0
    */
    using boolean_t = BooleanType;

    /*!
    @brief a type for a number (integer)

    [RFC 7159](http://rfc7159.net/rfc7159) describes numbers as follows:
    > The representation of numbers is similar to that used in most
    > programming languages. A number is represented in base 10 using decimal
    > digits. It contains an integer component that may be prefixed with an
    > optional minus sign, which may be followed by a fraction part and/or an
    > exponent part. Leading zeros are not allowed. (...) Numeric values that
    > cannot be represented in the grammar below (such as Infinity and NaN)
    > are not permitted.

    This description includes both integer and floating-point numbers.
    However, C++ allows more precise storage if it is known whether the number
    is a signed integer, an unsigned integer or a floating-point number.
    Therefore, three different types, @ref number_integer_t, @ref
    number_unsigned_t and @ref number_float_t are used.

    To store integer numbers in C++, a type is defined by the template
    parameter @a NumberIntegerType which chooses the type to use.

    #### Default type

    With the default values for @a NumberIntegerType (`int64_t`), the default
    value for @a number_integer_t is:

    @code {.cpp}
    int64_t
    @endcode

    #### Default behavior

    - The restrictions about leading zeros is not enforced in C++. Instead,
      leading zeros in integer literals lead to an interpretation as octal
      number. Internally, the value will be stored as decimal number. For
      instance, the C++ integer literal `010` will be serialized to `8`.
      During deserialization, leading zeros yield an error.
    - Not-a-number (NaN) values will be serialized to `null`.

    #### Limits

    [RFC 7159](http://rfc7159.net/rfc7159) specifies:
    > An implementation may set limits on the range and precision of numbers.

    When the default type is used, the maximal integer number that can be
    stored is `9223372036854775807` (INT64_MAX) and the minimal integer number
    that can be stored is `-9223372036854775808` (INT64_MIN). Integer numbers
    that are out of range will yield over/underflow when used in a
    constructor. During deserialization, too large or small integer numbers
    will be automatically be stored as @ref number_unsigned_t or @ref
    number_float_t.

    [RFC 7159](http://rfc7159.net/rfc7159) further states:
    > Note that when such software is used, numbers that are integers and are
    > in the range \f$[-2^{53}+1, 2^{53}-1]\f$ are interoperable in the sense
    > that implementations will agree exactly on their numeric values.

    As this range is a subrange of the exactly supported range [INT64_MIN,
    INT64_MAX], this class's integer type is interoperable.

    #### Storage

    Integer number values are stored directly inside a @ref basic_json type.

    @sa @ref number_float_t -- type for number values (floating-point)

    @sa @ref number_unsigned_t -- type for number values (unsigned integer)

    @since version 1.0.0
    */
    using number_integer_t = NumberIntegerType;

    /*!
    @brief a type for a number (unsigned)

    [RFC 7159](http://rfc7159.net/rfc7159) describes numbers as follows:
    > The representation of numbers is similar to that used in most
    > programming languages. A number is represented in base 10 using decimal
    > digits. It contains an integer component that may be prefixed with an
    > optional minus sign, which may be followed by a fraction part and/or an
    > exponent part. Leading zeros are not allowed. (...) Numeric values that
    > cannot be represented in the grammar below (such as Infinity and NaN)
    > are not permitted.

    This description includes both integer and floating-point numbers.
    However, C++ allows more precise storage if it is known whether the number
    is a signed integer, an unsigned integer or a floating-point number.
    Therefore, three different types, @ref number_integer_t, @ref
    number_unsigned_t and @ref number_float_t are used.

    To store unsigned integer numbers in C++, a type is defined by the
    template parameter @a NumberUnsignedType which chooses the type to use.

    #### Default type

    With the default values for @a NumberUnsignedType (`uint64_t`), the
    default value for @a number_unsigned_t is:

    @code {.cpp}
    uint64_t
    @endcode

    #### Default behavior

    - The restrictions about leading zeros is not enforced in C++. Instead,
      leading zeros in integer literals lead to an interpretation as octal
      number. Internally, the value will be stored as decimal number. For
      instance, the C++ integer literal `010` will be serialized to `8`.
      During deserialization, leading zeros yield an error.
    - Not-a-number (NaN) values will be serialized to `null`.

    #### Limits

    [RFC 7159](http://rfc7159.net/rfc7159) specifies:
    > An implementation may set limits on the range and precision of numbers.

    When the default type is used, the maximal integer number that can be
    stored is `18446744073709551615` (UINT64_MAX) and the minimal integer
    number that can be stored is `0`. Integer numbers that are out of range
    will yield over/underflow when used in a constructor. During
    deserialization, too large or small integer numbers will be automatically
    be stored as @ref number_integer_t or @ref number_float_t.

    [RFC 7159](http://rfc7159.net/rfc7159) further states:
    > Note that when such software is used, numbers that are integers and are
    > in the range \f$[-2^{53}+1, 2^{53}-1]\f$ are interoperable in the sense
    > that implementations will agree exactly on their numeric values.

    As this range is a subrange (when considered in conjunction with the
    number_integer_t type) of the exactly supported range [0, UINT64_MAX],
    this class's integer type is interoperable.

    #### Storage

    Integer number values are stored directly inside a @ref basic_json type.

    @sa @ref number_float_t -- type for number values (floating-point)
    @sa @ref number_integer_t -- type for number values (integer)

    @since version 2.0.0
    */
    using number_unsigned_t = NumberUnsignedType;

    /*!
    @brief a type for a number (floating-point)

    [RFC 7159](http://rfc7159.net/rfc7159) describes numbers as follows:
    > The representation of numbers is similar to that used in most
    > programming languages. A number is represented in base 10 using decimal
    > digits. It contains an integer component that may be prefixed with an
    > optional minus sign, which may be followed by a fraction part and/or an
    > exponent part. Leading zeros are not allowed. (...) Numeric values that
    > cannot be represented in the grammar below (such as Infinity and NaN)
    > are not permitted.

    This description includes both integer and floating-point numbers.
    However, C++ allows more precise storage if it is known whether the number
    is a signed integer, an unsigned integer or a floating-point number.
    Therefore, three different types, @ref number_integer_t, @ref
    number_unsigned_t and @ref number_float_t are used.

    To store floating-point numbers in C++, a type is defined by the template
    parameter @a NumberFloatType which chooses the type to use.

    #### Default type

    With the default values for @a NumberFloatType (`double`), the default
    value for @a number_float_t is:

    @code {.cpp}
    double
    @endcode

    #### Default behavior

    - The restrictions about leading zeros is not enforced in C++. Instead,
      leading zeros in floating-point literals will be ignored. Internally,
      the value will be stored as decimal number. For instance, the C++
      floating-point literal `01.2` will be serialized to `1.2`. During
      deserialization, leading zeros yield an error.
    - Not-a-number (NaN) values will be serialized to `null`.

    #### Limits

    [RFC 7159](http://rfc7159.net/rfc7159) states:
    > This specification allows implementations to set limits on the range and
    > precision of numbers accepted. Since software that implements IEEE
    > 754-2008 binary64 (double precision) numbers is generally available and
    > widely used, good interoperability can be achieved by implementations
    > that expect no more precision or range than these provide, in the sense
    > that implementations will approximate JSON numbers within the expected
    > precision.

    This implementation does exactly follow this approach, as it uses double
    precision floating-point numbers. Note values smaller than
    `-1.79769313486232e+308` and values greater than `1.79769313486232e+308`
    will be stored as NaN internally and be serialized to `null`.

    #### Storage

    Floating-point number values are stored directly inside a @ref basic_json
    type.

    @sa @ref number_integer_t -- type for number values (integer)

    @sa @ref number_unsigned_t -- type for number values (unsigned integer)

    @since version 1.0.0
    */
    using number_float_t = NumberFloatType;

    /// @}

  private:

    /// helper for exception-safe object creation
    template<typename T, typename... Args>
    static T* create(Args&& ... args)
    {
        AllocatorType<T> alloc;
        auto deleter = [&](T * object)
        {
            alloc.deallocate(object, 1);
        };
        std::unique_ptr<T, decltype(deleter)> object(alloc.allocate(1), deleter);
        alloc.construct(object.get(), std::forward<Args>(args)...);
        assert(object != nullptr);
        return object.release();
    }

    ////////////////////////
    // JSON value storage //
    ////////////////////////

    /*!
    @brief a JSON value

    The actual storage for a JSON value of the @ref basic_json class. This
    union combines the different storage types for the JSON value types
    defined in @ref value_t.

    JSON type | value_t type    | used type
    --------- | --------------- | ------------------------
    object    | object          | pointer to @ref object_t
    array     | array           | pointer to @ref array_t
    string    | string          | pointer to @ref string_t
    boolean   | boolean         | @ref boolean_t
    number    | number_integer  | @ref number_integer_t
    number    | number_unsigned | @ref number_unsigned_t
    number    | number_float    | @ref number_float_t
    null      | null            | *no value is stored*

    @note Variable-length types (objects, arrays, and strings) are stored as
    pointers. The size of the union should not exceed 64 bits if the default
    value types are used.

    @since version 1.0.0
    */
    union json_value
    {
        /// object (stored with pointer to save storage)
        object_t* object;
        /// array (stored with pointer to save storage)
        array_t* array;
        /// string (stored with pointer to save storage)
        string_t* string;
        /// boolean
        boolean_t boolean;
        /// number (integer)
        number_integer_t number_integer;
        /// number (unsigned integer)
        number_unsigned_t number_unsigned;
        /// number (floating-point)
        number_float_t number_float;

        /// default constructor (for null values)
        json_value() = default;
        /// constructor for booleans
        json_value(boolean_t v) noexcept : boolean(v) {}
        /// constructor for numbers (integer)
        json_value(number_integer_t v) noexcept : number_integer(v) {}
        /// constructor for numbers (unsigned)
        json_value(number_unsigned_t v) noexcept : number_unsigned(v) {}
        /// constructor for numbers (floating-point)
        json_value(number_float_t v) noexcept : number_float(v) {}
        /// constructor for empty values of a given type
        json_value(value_t t)
        {
            switch (t)
            {
                case value_t::object:
                {
                    object = create<object_t>();
                    break;
                }

                case value_t::array:
                {
                    array = create<array_t>();
                    break;
                }

                case value_t::string:
                {
                    string = create<string_t>("");
                    break;
                }

                case value_t::boolean:
                {
                    boolean = boolean_t(false);
                    break;
                }

                case value_t::number_integer:
                {
                    number_integer = number_integer_t(0);
                    break;
                }

                case value_t::number_unsigned:
                {
                    number_unsigned = number_unsigned_t(0);
                    break;
                }

                case value_t::number_float:
                {
                    number_float = number_float_t(0.0);
                    break;
                }

                case value_t::null:
                {
                    break;
                }

                default:
                {
                    if (t == value_t::null)
                    {
                        JSON_THROW(std::domain_error("961c151d2e87f2686a955a9be24d316f1362bf21 2.1.1")); // LCOV_EXCL_LINE
                    }
                    break;
                }
            }
        }

        /// constructor for strings
        json_value(const string_t& value)
        {
            string = create<string_t>(value);
        }

        /// constructor for objects
        json_value(const object_t& value)
        {
            object = create<object_t>(value);
        }

        /// constructor for arrays
        json_value(const array_t& value)
        {
            array = create<array_t>(value);
        }
    };

    /*!
    @brief checks the class invariants

    This function asserts the class invariants. It needs to be called at the
    end of every constructor to make sure that created objects respect the
    invariant. Furthermore, it has to be called each time the type of a JSON
    value is changed, because the invariant expresses a relationship between
    @a m_type and @a m_value.
    */
    void assert_invariant() const
    {
        assert(m_type != value_t::object or m_value.object != nullptr);
        assert(m_type != value_t::array or m_value.array != nullptr);
        assert(m_type != value_t::string or m_value.string != nullptr);
    }

  public:
    //////////////////////////
    // JSON parser callback //
    //////////////////////////

    /*!
    @brief JSON callback events

    This enumeration lists the parser events that can trigger calling a
    callback function of type @ref parser_callback_t during parsing.

    @image html callback_events.png "Example when certain parse events are triggered"

    @since version 1.0.0
    */
    enum class parse_event_t : uint8_t
    {
        /// the parser read `{` and started to process a JSON object
        object_start,
        /// the parser read `}` and finished processing a JSON object
        object_end,
        /// the parser read `[` and started to process a JSON array
        array_start,
        /// the parser read `]` and finished processing a JSON array
        array_end,
        /// the parser read a key of a value in an object
        key,
        /// the parser finished reading a JSON value
        value
    };

    /*!
    @brief per-element parser callback type

    With a parser callback function, the result of parsing a JSON text can be
    influenced. When passed to @ref parse(std::istream&, const
    parser_callback_t) or @ref parse(const CharT, const parser_callback_t),
    it is called on certain events (passed as @ref parse_event_t via parameter
    @a event) with a set recursion depth @a depth and context JSON value
    @a parsed. The return value of the callback function is a boolean
    indicating whether the element that emitted the callback shall be kept or
    not.

    We distinguish six scenarios (determined by the event type) in which the
    callback function can be called. The following table describes the values
    of the parameters @a depth, @a event, and @a parsed.

    parameter @a event | description | parameter @a depth | parameter @a parsed
    ------------------ | ----------- | ------------------ | -------------------
    parse_event_t::object_start | the parser read `{` and started to process a JSON object | depth of the parent of the JSON object | a JSON value with type discarded
    parse_event_t::key | the parser read a key of a value in an object | depth of the currently parsed JSON object | a JSON string containing the key
    parse_event_t::object_end | the parser read `}` and finished processing a JSON object | depth of the parent of the JSON object | the parsed JSON object
    parse_event_t::array_start | the parser read `[` and started to process a JSON array | depth of the parent of the JSON array | a JSON value with type discarded
    parse_event_t::array_end | the parser read `]` and finished processing a JSON array | depth of the parent of the JSON array | the parsed JSON array
    parse_event_t::value | the parser finished reading a JSON value | depth of the value | the parsed JSON value

    @image html callback_events.png "Example when certain parse events are triggered"

    Discarding a value (i.e., returning `false`) has different effects
    depending on the context in which function was called:

    - Discarded values in structured types are skipped. That is, the parser
      will behave as if the discarded value was never read.
    - In case a value outside a structured type is skipped, it is replaced
      with `null`. This case happens if the top-level element is skipped.

    @param[in] depth  the depth of the recursion during parsing

    @param[in] event  an event of type parse_event_t indicating the context in
    the callback function has been called

    @param[in,out] parsed  the current intermediate parse result; note that
    writing to this value has no effect for parse_event_t::key events

    @return Whether the JSON value which called the function during parsing
    should be kept (`true`) or not (`false`). In the latter case, it is either
    skipped completely or replaced by an empty discarded object.

    @sa @ref parse(std::istream&, parser_callback_t) or
    @ref parse(const CharT, const parser_callback_t) for examples

    @since version 1.0.0
    */
    using parser_callback_t = std::function<bool(int depth,
                              parse_event_t event,
                              basic_json& parsed)>;


    //////////////////
    // constructors //
    //////////////////

    /// @name constructors and destructors
    /// Constructors of class @ref basic_json, copy/move constructor, copy
    /// assignment, static functions creating objects, and the destructor.
    /// @{

    /*!
    @brief create an empty value with a given type

    Create an empty JSON value with a given type. The value will be default
    initialized with an empty value which depends on the type:

    Value type  | initial value
    ----------- | -------------
    null        | `null`
    boolean     | `false`
    string      | `""`
    number      | `0`
    object      | `{}`
    array       | `[]`

    @param[in] value_type  the type of the value to create

    @complexity Constant.

    @throw std::bad_alloc if allocation for object, array, or string value
    fails

    @liveexample{The following code shows the constructor for different @ref
    value_t values,basic_json__value_t}

    @since version 1.0.0
    */
    basic_json(const value_t value_type)
        : m_type(value_type), m_value(value_type)
    {
        assert_invariant();
    }

    /*!
    @brief create a null object

    Create a `null` JSON value. It either takes a null pointer as parameter
    (explicitly creating `null`) or no parameter (implicitly creating `null`).
    The passed null pointer itself is not read -- it is only used to choose
    the right constructor.

    @complexity Constant.

    @exceptionsafety No-throw guarantee: this constructor never throws
    exceptions.

    @liveexample{The following code shows the constructor with and without a
    null pointer parameter.,basic_json__nullptr_t}

    @since version 1.0.0
    */
    basic_json(std::nullptr_t = nullptr) noexcept
        : basic_json(value_t::null)
    {
        assert_invariant();
    }

    /*!
    @brief create a JSON value

    This is a "catch all" constructor for all compatible JSON types; that is,
    types for which a `to_json()` method exsits. The constructor forwards the
    parameter @a val to that method (to `json_serializer<U>::to_json` method
    with `U = uncvref_t<CompatibleType>`, to be exact).

    Template type @a CompatibleType includes, but is not limited to, the
    following types:
    - **arrays**: @ref array_t and all kinds of compatible containers such as
      `std::vector`, `std::deque`, `std::list`, `std::forward_list`,
      `std::array`, `std::set`, `std::unordered_set`, `std::multiset`, and
      `unordered_multiset` with a `value_type` from which a @ref basic_json
      value can be constructed.
    - **objects**: @ref object_t and all kinds of compatible associative
      containers such as `std::map`, `std::unordered_map`, `std::multimap`,
      and `std::unordered_multimap` with a `key_type` compatible to
      @ref string_t and a `value_type` from which a @ref basic_json value can
      be constructed.
    - **strings**: @ref string_t, string literals, and all compatible string
      containers can be used.
    - **numbers**: @ref number_integer_t, @ref number_unsigned_t,
      @ref number_float_t, and all convertible number types such as `int`,
      `size_t`, `int64_t`, `float` or `double` can be used.
    - **boolean**: @ref boolean_t / `bool` can be used.

    See the examples below.

    @tparam CompatibleType a type such that:
    - @a CompatibleType is not derived from `std::istream`,
    - @a CompatibleType is not @ref basic_json (to avoid hijacking copy/move
         constructors),
    - @a CompatibleType is not a @ref basic_json nested type (e.g.,
         @ref json_pointer, @ref iterator, etc ...)
    - @ref @ref json_serializer<U> has a
         `to_json(basic_json_t&, CompatibleType&&)` method

    @tparam U = `uncvref_t<CompatibleType>`

    @param[in] val the value to be forwarded

    @complexity Usually linear in the size of the passed @a val, also
                depending on the implementation of the called `to_json()`
                method.

    @throw what `json_serializer<U>::to_json()` throws

    @liveexample{The following code shows the constructor with several
    compatible types.,basic_json__CompatibleType}

    @since version 2.1.0
    */
    template<typename CompatibleType, typename U = detail::uncvref_t<CompatibleType>,
             detail::enable_if_t<not std::is_base_of<std::istream, U>::value and
                                 not std::is_same<U, basic_json_t>::value and
                                 not detail::is_basic_json_nested_type<
                                     basic_json_t, U>::value and
                                 detail::has_to_json<basic_json, U>::value,
                                 int> = 0>
    basic_json(CompatibleType && val) noexcept(noexcept(JSONSerializer<U>::to_json(
                std::declval<basic_json_t&>(), std::forward<CompatibleType>(val))))
    {
        JSONSerializer<U>::to_json(*this, std::forward<CompatibleType>(val));
        assert_invariant();
    }

    /*!
    @brief create a container (array or object) from an initializer list

    Creates a JSON value of type array or object from the passed initializer
    list @a init. In case @a type_deduction is `true` (default), the type of
    the JSON value to be created is deducted from the initializer list @a init
    according to the following rules:

    1. If the list is empty, an empty JSON object value `{}` is created.
    2. If the list consists of pairs whose first element is a string, a JSON
       object value is created where the first elements of the pairs are
       treated as keys and the second elements are as values.
    3. In all other cases, an array is created.

    The rules aim to create the best fit between a C++ initializer list and
    JSON values. The rationale is as follows:

    1. The empty initializer list is written as `{}` which is exactly an empty
       JSON object.
    2. C++ has now way of describing mapped types other than to list a list of
       pairs. As JSON requires that keys must be of type string, rule 2 is the
       weakest constraint one can pose on initializer lists to interpret them
       as an object.
    3. In all other cases, the initializer list could not be interpreted as
       JSON object type, so interpreting it as JSON array type is safe.

    With the rules described above, the following JSON values cannot be
    expressed by an initializer list:

    - the empty array (`[]`): use @ref array(std::initializer_list<basic_json>)
      with an empty initializer list in this case
    - arrays whose elements satisfy rule 2: use @ref
      array(std::initializer_list<basic_json>) with the same initializer list
      in this case

    @note When used without parentheses around an empty initializer list, @ref
    basic_json() is called instead of this function, yielding the JSON null
    value.

    @param[in] init  initializer list with JSON values

    @param[in] type_deduction internal parameter; when set to `true`, the type
    of the JSON value is deducted from the initializer list @a init; when set
    to `false`, the type provided via @a manual_type is forced. This mode is
    used by the functions @ref array(std::initializer_list<basic_json>) and
    @ref object(std::initializer_list<basic_json>).

    @param[in] manual_type internal parameter; when @a type_deduction is set
    to `false`, the created JSON value will use the provided type (only @ref
    value_t::array and @ref value_t::object are valid); when @a type_deduction
    is set to `true`, this parameter has no effect

    @throw std::domain_error if @a type_deduction is `false`, @a manual_type
    is `value_t::object`, but @a init contains an element which is not a pair
    whose first element is a string; example: `"cannot create object from
    initializer list"`

    @complexity Linear in the size of the initializer list @a init.

    @liveexample{The example below shows how JSON values are created from
    initializer lists.,basic_json__list_init_t}

    @sa @ref array(std::initializer_list<basic_json>) -- create a JSON array
    value from an initializer list
    @sa @ref object(std::initializer_list<basic_json>) -- create a JSON object
    value from an initializer list

    @since version 1.0.0
    */
    basic_json(std::initializer_list<basic_json> init,
               bool type_deduction = true,
               value_t manual_type = value_t::array)
    {
        // check if each element is an array with two elements whose first
        // element is a string
        bool is_an_object = std::all_of(init.begin(), init.end(),
                                        [](const basic_json & element)
        {
            return element.is_array() and element.size() == 2 and element[0].is_string();
        });

        // adjust type if type deduction is not wanted
        if (not type_deduction)
        {
            // if array is wanted, do not create an object though possible
            if (manual_type == value_t::array)
            {
                is_an_object = false;
            }

            // if object is wanted but impossible, throw an exception
            if (manual_type == value_t::object and not is_an_object)
            {
                JSON_THROW(std::domain_error("cannot create object from initializer list"));
            }
        }

        if (is_an_object)
        {
            // the initializer list is a list of pairs -> create object
            m_type = value_t::object;
            m_value = value_t::object;

            std::for_each(init.begin(), init.end(), [this](const basic_json & element)
            {
                m_value.object->emplace(*(element[0].m_value.string), element[1]);
            });
        }
        else
        {
            // the initializer list describes an array -> create array
            m_type = value_t::array;
            m_value.array = create<array_t>(init);
        }

        assert_invariant();
    }

    /*!
    @brief explicitly create an array from an initializer list

    Creates a JSON array value from a given initializer list. That is, given a
    list of values `a, b, c`, creates the JSON value `[a, b, c]`. If the
    initializer list is empty, the empty array `[]` is created.

    @note This function is only needed to express two edge cases that cannot
    be realized with the initializer list constructor (@ref
    basic_json(std::initializer_list<basic_json>, bool, value_t)). These cases
    are:
    1. creating an array whose elements are all pairs whose first element is a
    string -- in this case, the initializer list constructor would create an
    object, taking the first elements as keys
    2. creating an empty array -- passing the empty initializer list to the
    initializer list constructor yields an empty object

    @param[in] init  initializer list with JSON values to create an array from
    (optional)

    @return JSON array value

    @complexity Linear in the size of @a init.

    @liveexample{The following code shows an example for the `array`
    function.,array}

    @sa @ref basic_json(std::initializer_list<basic_json>, bool, value_t) --
    create a JSON value from an initializer list
    @sa @ref object(std::initializer_list<basic_json>) -- create a JSON object
    value from an initializer list

    @since version 1.0.0
    */
    static basic_json array(std::initializer_list<basic_json> init =
                                std::initializer_list<basic_json>())
    {
        return basic_json(init, false, value_t::array);
    }

    /*!
    @brief explicitly create an object from an initializer list

    Creates a JSON object value from a given initializer list. The initializer
    lists elements must be pairs, and their first elements must be strings. If
    the initializer list is empty, the empty object `{}` is created.

    @note This function is only added for symmetry reasons. In contrast to the
    related function @ref array(std::initializer_list<basic_json>), there are
    no cases which can only be expressed by this function. That is, any
    initializer list @a init can also be passed to the initializer list
    constructor @ref basic_json(std::initializer_list<basic_json>, bool,
    value_t).

    @param[in] init  initializer list to create an object from (optional)

    @return JSON object value

    @throw std::domain_error if @a init is not a pair whose first elements are
    strings; thrown by
    @ref basic_json(std::initializer_list<basic_json>, bool, value_t)

    @complexity Linear in the size of @a init.

    @liveexample{The following code shows an example for the `object`
    function.,object}

    @sa @ref basic_json(std::initializer_list<basic_json>, bool, value_t) --
    create a JSON value from an initializer list
    @sa @ref array(std::initializer_list<basic_json>) -- create a JSON array
    value from an initializer list

    @since version 1.0.0
    */
    static basic_json object(std::initializer_list<basic_json> init =
                                 std::initializer_list<basic_json>())
    {
        return basic_json(init, false, value_t::object);
    }

    /*!
    @brief construct an array with count copies of given value

    Constructs a JSON array value by creating @a cnt copies of a passed value.
    In case @a cnt is `0`, an empty array is created. As postcondition,
    `std::distance(begin(),end()) == cnt` holds.

    @param[in] cnt  the number of JSON copies of @a val to create
    @param[in] val  the JSON value to copy

    @complexity Linear in @a cnt.

    @liveexample{The following code shows examples for the @ref
    basic_json(size_type\, const basic_json&)
    constructor.,basic_json__size_type_basic_json}

    @since version 1.0.0
    */
    basic_json(size_type cnt, const basic_json& val)
        : m_type(value_t::array)
    {
        m_value.array = create<array_t>(cnt, val);
        assert_invariant();
    }

    /*!
    @brief construct a JSON container given an iterator range

    Constructs the JSON value with the contents of the range `[first, last)`.
    The semantics depends on the different types a JSON value can have:
    - In case of primitive types (number, boolean, or string), @a first must
      be `begin()` and @a last must be `end()`. In this case, the value is
      copied. Otherwise, std::out_of_range is thrown.
    - In case of structured types (array, object), the constructor behaves as
      similar versions for `std::vector`.
    - In case of a null type, std::domain_error is thrown.

    @tparam InputIT an input iterator type (@ref iterator or @ref
    const_iterator)

    @param[in] first begin of the range to copy from (included)
    @param[in] last end of the range to copy from (excluded)

    @pre Iterators @a first and @a last must be initialized. **This
         precondition is enforced with an assertion.**

    @throw std::domain_error if iterators are not compatible; that is, do not
    belong to the same JSON value; example: `"iterators are not compatible"`
    @throw std::out_of_range if iterators are for a primitive type (number,
    boolean, or string) where an out of range error can be detected easily;
    example: `"iterators out of range"`
    @throw std::bad_alloc if allocation for object, array, or string fails
    @throw std::domain_error if called with a null value; example: `"cannot
    use construct with iterators from null"`

    @complexity Linear in distance between @a first and @a last.

    @liveexample{The example below shows several ways to create JSON values by
    specifying a subrange with iterators.,basic_json__InputIt_InputIt}

    @since version 1.0.0
    */
    template<class InputIT, typename std::enable_if<
                 std::is_same<InputIT, typename basic_json_t::iterator>::value or
                 std::is_same<InputIT, typename basic_json_t::const_iterator>::value, int>::type = 0>
    basic_json(InputIT first, InputIT last)
    {
        assert(first.m_object != nullptr);
        assert(last.m_object != nullptr);

        // make sure iterator fits the current value
        if (first.m_object != last.m_object)
        {
            JSON_THROW(std::domain_error("iterators are not compatible"));
        }

        // copy type from first iterator
        m_type = first.m_object->m_type;

        // check if iterator range is complete for primitive values
        switch (m_type)
        {
            case value_t::boolean:
            case value_t::number_float:
            case value_t::number_integer:
            case value_t::number_unsigned:
            case value_t::string:
            {
                if (not first.m_it.primitive_iterator.is_begin() or not last.m_it.primitive_iterator.is_end())
                {
                    JSON_THROW(std::out_of_range("iterators out of range"));
                }
                break;
            }

            default:
            {
                break;
            }
        }

        switch (m_type)
        {
            case value_t::number_integer:
            {
                m_value.number_integer = first.m_object->m_value.number_integer;
                break;
            }

            case value_t::number_unsigned:
            {
                m_value.number_unsigned = first.m_object->m_value.number_unsigned;
                break;
            }

            case value_t::number_float:
            {
                m_value.number_float = first.m_object->m_value.number_float;
                break;
            }

            case value_t::boolean:
            {
                m_value.boolean = first.m_object->m_value.boolean;
                break;
            }

            case value_t::string:
            {
                m_value = *first.m_object->m_value.string;
                break;
            }

            case value_t::object:
            {
                m_value.object = create<object_t>(first.m_it.object_iterator,
                                                  last.m_it.object_iterator);
                break;
            }

            case value_t::array:
            {
                m_value.array = create<array_t>(first.m_it.array_iterator,
                                                last.m_it.array_iterator);
                break;
            }

            default:
            {
                JSON_THROW(std::domain_error("cannot use construct with iterators from " + first.m_object->type_name()));
            }
        }

        assert_invariant();
    }

    /*!
    @brief construct a JSON value given an input stream

    @param[in,out] i  stream to read a serialized JSON value from
    @param[in] cb a parser callback function of type @ref parser_callback_t
    which is used to control the deserialization by filtering unwanted values
    (optional)

    @complexity Linear in the length of the input. The parser is a predictive
    LL(1) parser. The complexity can be higher if the parser callback function
    @a cb has a super-linear complexity.

    @note A UTF-8 byte order mark is silently ignored.

    @deprecated This constructor is deprecated and will be removed in version
      3.0.0 to unify the interface of the library. Deserialization will be
      done by stream operators or by calling one of the `parse` functions,
      e.g. @ref parse(std::istream&, const parser_callback_t). That is, calls
      like `json j(i);` for an input stream @a i need to be replaced by
      `json j = json::parse(i);`. See the example below.

    @liveexample{The example below demonstrates constructing a JSON value from
    a `std::stringstream` with and without callback
    function.,basic_json__istream}

    @since version 2.0.0, deprecated in version 2.0.3, to be removed in
           version 3.0.0
    */
    JSON_DEPRECATED
    explicit basic_json(std::istream& i, const parser_callback_t cb = nullptr)
    {
        *this = parser(i, cb).parse();
        assert_invariant();
    }

    ///////////////////////////////////////
    // other constructors and destructor //
    ///////////////////////////////////////

    /*!
    @brief copy constructor

    Creates a copy of a given JSON value.

    @param[in] other  the JSON value to copy

    @complexity Linear in the size of @a other.

    @requirement This function helps `basic_json` satisfying the
    [Container](http://en.cppreference.com/w/cpp/concept/Container)
    requirements:
    - The complexity is linear.
    - As postcondition, it holds: `other == basic_json(other)`.

    @throw std::bad_alloc if allocation for object, array, or string fails.

    @liveexample{The following code shows an example for the copy
    constructor.,basic_json__basic_json}

    @since version 1.0.0
    */
    basic_json(const basic_json& other)
        : m_type(other.m_type)
    {
        // check of passed value is valid
        other.assert_invariant();

        switch (m_type)
        {
            case value_t::object:
            {
                m_value = *other.m_value.object;
                break;
            }

            case value_t::array:
            {
                m_value = *other.m_value.array;
                break;
            }

            case value_t::string:
            {
                m_value = *other.m_value.string;
                break;
            }

            case value_t::boolean:
            {
                m_value = other.m_value.boolean;
                break;
            }

            case value_t::number_integer:
            {
                m_value = other.m_value.number_integer;
                break;
            }

            case value_t::number_unsigned:
            {
                m_value = other.m_value.number_unsigned;
                break;
            }

            case value_t::number_float:
            {
                m_value = other.m_value.number_float;
                break;
            }

            default:
            {
                break;
            }
        }

        assert_invariant();
    }

    /*!
    @brief move constructor

    Move constructor. Constructs a JSON value with the contents of the given
    value @a other using move semantics. It "steals" the resources from @a
    other and leaves it as JSON null value.

    @param[in,out] other  value to move to this object

    @post @a other is a JSON null value

    @complexity Constant.

    @liveexample{The code below shows the move constructor explicitly called
    via std::move.,basic_json__moveconstructor}

    @since version 1.0.0
    */
    basic_json(basic_json&& other) noexcept
        : m_type(std::move(other.m_type)),
          m_value(std::move(other.m_value))
    {
        // check that passed value is valid
        other.assert_invariant();

        // invalidate payload
        other.m_type = value_t::null;
        other.m_value = {};

        assert_invariant();
    }

    /*!
    @brief copy assignment

    Copy assignment operator. Copies a JSON value via the "copy and swap"
    strategy: It is expressed in terms of the copy constructor, destructor,
    and the swap() member function.

    @param[in] other  value to copy from

    @complexity Linear.

    @requirement This function helps `basic_json` satisfying the
    [Container](http://en.cppreference.com/w/cpp/concept/Container)
    requirements:
    - The complexity is linear.

    @liveexample{The code below shows and example for the copy assignment. It
    creates a copy of value `a` which is then swapped with `b`. Finally\, the
    copy of `a` (which is the null value after the swap) is
    destroyed.,basic_json__copyassignment}

    @since version 1.0.0
    */
    reference& operator=(basic_json other) noexcept (
        std::is_nothrow_move_constructible<value_t>::value and
        std::is_nothrow_move_assignable<value_t>::value and
        std::is_nothrow_move_constructible<json_value>::value and
        std::is_nothrow_move_assignable<json_value>::value
    )
    {
        // check that passed value is valid
        other.assert_invariant();

        using std::swap;
        swap(m_type, other.m_type);
        swap(m_value, other.m_value);

        assert_invariant();
        return *this;
    }

    /*!
    @brief destructor

    Destroys the JSON value and frees all allocated memory.

    @complexity Linear.

    @requirement This function helps `basic_json` satisfying the
    [Container](http://en.cppreference.com/w/cpp/concept/Container)
    requirements:
    - The complexity is linear.
    - All stored elements are destroyed and all memory is freed.

    @since version 1.0.0
    */
    ~basic_json()
    {
        assert_invariant();

        switch (m_type)
        {
            case value_t::object:
            {
                AllocatorType<object_t> alloc;
                alloc.destroy(m_value.object);
                alloc.deallocate(m_value.object, 1);
                break;
            }

            case value_t::array:
            {
                AllocatorType<array_t> alloc;
                alloc.destroy(m_value.array);
                alloc.deallocate(m_value.array, 1);
                break;
            }

            case value_t::string:
            {
                AllocatorType<string_t> alloc;
                alloc.destroy(m_value.string);
                alloc.deallocate(m_value.string, 1);
                break;
            }

            default:
            {
                // all other types need no specific destructor
                break;
            }
        }
    }

    /// @}

  public:
    ///////////////////////
    // object inspection //
    ///////////////////////

    /// @name object inspection
    /// Functions to inspect the type of a JSON value.
    /// @{

    /*!
    @brief serialization

    Serialization function for JSON values. The function tries to mimic
    Python's `json.dumps()` function, and currently supports its @a indent
    parameter.

    @param[in] indent If indent is nonnegative, then array elements and object
    members will be pretty-printed with that indent level. An indent level of
    `0` will only insert newlines. `-1` (the default) selects the most compact
    representation.

    @return string containing the serialization of the JSON value

    @complexity Linear.

    @liveexample{The following example shows the effect of different @a indent
    parameters to the result of the serialization.,dump}

    @see https://docs.python.org/2/library/json.html#json.dump

    @since version 1.0.0
    */
    string_t dump(const int indent = -1) const
    {
        std::stringstream ss;

        if (indent >= 0)
        {
            dump(ss, true, static_cast<unsigned int>(indent));
        }
        else
        {
            dump(ss, false, 0);
        }

        return ss.str();
    }

    /*!
    @brief return the type of the JSON value (explicit)

    Return the type of the JSON value as a value from the @ref value_t
    enumeration.

    @return the type of the JSON value

    @complexity Constant.

    @exceptionsafety No-throw guarantee: this member function never throws
    exceptions.

    @liveexample{The following code exemplifies `type()` for all JSON
    types.,type}

    @since version 1.0.0
    */
    constexpr value_t type() const noexcept
    {
        return m_type;
    }

    /*!
    @brief return whether type is primitive

    This function returns true iff the JSON type is primitive (string, number,
    boolean, or null).

    @return `true` if type is primitive (string, number, boolean, or null),
    `false` otherwise.

    @complexity Constant.

    @exceptionsafety No-throw guarantee: this member function never throws
    exceptions.

    @liveexample{The following code exemplifies `is_primitive()` for all JSON
    types.,is_primitive}

    @sa @ref is_structured() -- returns whether JSON value is structured
    @sa @ref is_null() -- returns whether JSON value is `null`
    @sa @ref is_string() -- returns whether JSON value is a string
    @sa @ref is_boolean() -- returns whether JSON value is a boolean
    @sa @ref is_number() -- returns whether JSON value is a number

    @since version 1.0.0
    */
    constexpr bool is_primitive() const noexcept
    {
        return is_null() or is_string() or is_boolean() or is_number();
    }

    /*!
    @brief return whether type is structured

    This function returns true iff the JSON type is structured (array or
    object).

    @return `true` if type is structured (array or object), `false` otherwise.

    @complexity Constant.

    @exceptionsafety No-throw guarantee: this member function never throws
    exceptions.

    @liveexample{The following code exemplifies `is_structured()` for all JSON
    types.,is_structured}

    @sa @ref is_primitive() -- returns whether value is primitive
    @sa @ref is_array() -- returns whether value is an array
    @sa @ref is_object() -- returns whether value is an object

    @since version 1.0.0
    */
    constexpr bool is_structured() const noexcept
    {
        return is_array() or is_object();
    }

    /*!
    @brief return whether value is null

    This function returns true iff the JSON value is null.

    @return `true` if type is null, `false` otherwise.

    @complexity Constant.

    @exceptionsafety No-throw guarantee: this member function never throws
    exceptions.

    @liveexample{The following code exemplifies `is_null()` for all JSON
    types.,is_null}

    @since version 1.0.0
    */
    constexpr bool is_null() const noexcept
    {
        return m_type == value_t::null;
    }

    /*!
    @brief return whether value is a boolean

    This function returns true iff the JSON value is a boolean.

    @return `true` if type is boolean, `false` otherwise.

    @complexity Constant.

    @exceptionsafety No-throw guarantee: this member function never throws
    exceptions.

    @liveexample{The following code exemplifies `is_boolean()` for all JSON
    types.,is_boolean}

    @since version 1.0.0
    */
    constexpr bool is_boolean() const noexcept
    {
        return m_type == value_t::boolean;
    }

    /*!
    @brief return whether value is a number

    This function returns true iff the JSON value is a number. This includes
    both integer and floating-point values.

    @return `true` if type is number (regardless whether integer, unsigned
    integer or floating-type), `false` otherwise.

    @complexity Constant.

    @exceptionsafety No-throw guarantee: this member function never throws
    exceptions.

    @liveexample{The following code exemplifies `is_number()` for all JSON
    types.,is_number}

    @sa @ref is_number_integer() -- check if value is an integer or unsigned
    integer number
    @sa @ref is_number_unsigned() -- check if value is an unsigned integer
    number
    @sa @ref is_number_float() -- check if value is a floating-point number

    @since version 1.0.0
    */
    constexpr bool is_number() const noexcept
    {
        return is_number_integer() or is_number_float();
    }

    /*!
    @brief return whether value is an integer number

    This function returns true iff the JSON value is an integer or unsigned
    integer number. This excludes floating-point values.

    @return `true` if type is an integer or unsigned integer number, `false`
    otherwise.

    @complexity Constant.

    @exceptionsafety No-throw guarantee: this member function never throws
    exceptions.

    @liveexample{The following code exemplifies `is_number_integer()` for all
    JSON types.,is_number_integer}

    @sa @ref is_number() -- check if value is a number
    @sa @ref is_number_unsigned() -- check if value is an unsigned integer
    number
    @sa @ref is_number_float() -- check if value is a floating-point number

    @since version 1.0.0
    */
    constexpr bool is_number_integer() const noexcept
    {
        return m_type == value_t::number_integer or m_type == value_t::number_unsigned;
    }

    /*!
    @brief return whether value is an unsigned integer number

    This function returns true iff the JSON value is an unsigned integer
    number. This excludes floating-point and (signed) integer values.

    @return `true` if type is an unsigned integer number, `false` otherwise.

    @complexity Constant.

    @exceptionsafety No-throw guarantee: this member function never throws
    exceptions.

    @liveexample{The following code exemplifies `is_number_unsigned()` for all
    JSON types.,is_number_unsigned}

    @sa @ref is_number() -- check if value is a number
    @sa @ref is_number_integer() -- check if value is an integer or unsigned
    integer number
    @sa @ref is_number_float() -- check if value is a floating-point number

    @since version 2.0.0
    */
    constexpr bool is_number_unsigned() const noexcept
    {
        return m_type == value_t::number_unsigned;
    }

    /*!
    @brief return whether value is a floating-point number

    This function returns true iff the JSON value is a floating-point number.
    This excludes integer and unsigned integer values.

    @return `true` if type is a floating-point number, `false` otherwise.

    @complexity Constant.

    @exceptionsafety No-throw guarantee: this member function never throws
    exceptions.

    @liveexample{The following code exemplifies `is_number_float()` for all
    JSON types.,is_number_float}

    @sa @ref is_number() -- check if value is number
    @sa @ref is_number_integer() -- check if value is an integer number
    @sa @ref is_number_unsigned() -- check if value is an unsigned integer
    number

    @since version 1.0.0
    */
    constexpr bool is_number_float() const noexcept
    {
        return m_type == value_t::number_float;
    }

    /*!
    @brief return whether value is an object

    This function returns true iff the JSON value is an object.

    @return `true` if type is object, `false` otherwise.

    @complexity Constant.

    @exceptionsafety No-throw guarantee: this member function never throws
    exceptions.

    @liveexample{The following code exemplifies `is_object()` for all JSON
    types.,is_object}

    @since version 1.0.0
    */
    constexpr bool is_object() const noexcept
    {
        return m_type == value_t::object;
    }

    /*!
    @brief return whether value is an array

    This function returns true iff the JSON value is an array.

    @return `true` if type is array, `false` otherwise.

    @complexity Constant.

    @exceptionsafety No-throw guarantee: this member function never throws
    exceptions.

    @liveexample{The following code exemplifies `is_array()` for all JSON
    types.,is_array}

    @since version 1.0.0
    */
    constexpr bool is_array() const noexcept
    {
        return m_type == value_t::array;
    }

    /*!
    @brief return whether value is a string

    This function returns true iff the JSON value is a string.

    @return `true` if type is string, `false` otherwise.

    @complexity Constant.

    @exceptionsafety No-throw guarantee: this member function never throws
    exceptions.

    @liveexample{The following code exemplifies `is_string()` for all JSON
    types.,is_string}

    @since version 1.0.0
    */
    constexpr bool is_string() const noexcept
    {
        return m_type == value_t::string;
    }

    /*!
    @brief return whether value is discarded

    This function returns true iff the JSON value was discarded during parsing
    with a callback function (see @ref parser_callback_t).

    @note This function will always be `false` for JSON values after parsing.
    That is, discarded values can only occur during parsing, but will be
    removed when inside a structured value or replaced by null in other cases.

    @return `true` if type is discarded, `false` otherwise.

    @complexity Constant.

    @exceptionsafety No-throw guarantee: this member function never throws
    exceptions.

    @liveexample{The following code exemplifies `is_discarded()` for all JSON
    types.,is_discarded}

    @since version 1.0.0
    */
    constexpr bool is_discarded() const noexcept
    {
        return m_type == value_t::discarded;
    }

    /*!
    @brief return the type of the JSON value (implicit)

    Implicitly return the type of the JSON value as a value from the @ref
    value_t enumeration.

    @return the type of the JSON value

    @complexity Constant.

    @exceptionsafety No-throw guarantee: this member function never throws
    exceptions.

    @liveexample{The following code exemplifies the @ref value_t operator for
    all JSON types.,operator__value_t}

    @since version 1.0.0
    */
    constexpr operator value_t() const noexcept
    {
        return m_type;
    }

    /// @}

  private:
    //////////////////
    // value access //
    //////////////////

    /// get a boolean (explicit)
    boolean_t get_impl(boolean_t* /*unused*/) const
    {
        if (is_boolean())
        {
            return m_value.boolean;
        }

        JSON_THROW(std::domain_error("type must be boolean, but is " + type_name()));
    }

    /// get a pointer to the value (object)
    object_t* get_impl_ptr(object_t* /*unused*/) noexcept
    {
        return is_object() ? m_value.object : nullptr;
    }

    /// get a pointer to the value (object)
    constexpr const object_t* get_impl_ptr(const object_t* /*unused*/) const noexcept
    {
        return is_object() ? m_value.object : nullptr;
    }

    /// get a pointer to the value (array)
    array_t* get_impl_ptr(array_t* /*unused*/) noexcept
    {
        return is_array() ? m_value.array : nullptr;
    }

    /// get a pointer to the value (array)
    constexpr const array_t* get_impl_ptr(const array_t* /*unused*/) const noexcept
    {
        return is_array() ? m_value.array : nullptr;
    }

    /// get a pointer to the value (string)
    string_t* get_impl_ptr(string_t* /*unused*/) noexcept
    {
        return is_string() ? m_value.string : nullptr;
    }

    /// get a pointer to the value (string)
    constexpr const string_t* get_impl_ptr(const string_t* /*unused*/) const noexcept
    {
        return is_string() ? m_value.string : nullptr;
    }

    /// get a pointer to the value (boolean)
    boolean_t* get_impl_ptr(boolean_t* /*unused*/) noexcept
    {
        return is_boolean() ? &m_value.boolean : nullptr;
    }

    /// get a pointer to the value (boolean)
    constexpr const boolean_t* get_impl_ptr(const boolean_t* /*unused*/) const noexcept
    {
        return is_boolean() ? &m_value.boolean : nullptr;
    }

    /// get a pointer to the value (integer number)
    number_integer_t* get_impl_ptr(number_integer_t* /*unused*/) noexcept
    {
        return is_number_integer() ? &m_value.number_integer : nullptr;
    }

    /// get a pointer to the value (integer number)
    constexpr const number_integer_t* get_impl_ptr(const number_integer_t* /*unused*/) const noexcept
    {
        return is_number_integer() ? &m_value.number_integer : nullptr;
    }

    /// get a pointer to the value (unsigned number)
    number_unsigned_t* get_impl_ptr(number_unsigned_t* /*unused*/) noexcept
    {
        return is_number_unsigned() ? &m_value.number_unsigned : nullptr;
    }

    /// get a pointer to the value (unsigned number)
    constexpr const number_unsigned_t* get_impl_ptr(const number_unsigned_t* /*unused*/) const noexcept
    {
        return is_number_unsigned() ? &m_value.number_unsigned : nullptr;
    }

    /// get a pointer to the value (floating-point number)
    number_float_t* get_impl_ptr(number_float_t* /*unused*/) noexcept
    {
        return is_number_float() ? &m_value.number_float : nullptr;
    }

    /// get a pointer to the value (floating-point number)
    constexpr const number_float_t* get_impl_ptr(const number_float_t* /*unused*/) const noexcept
    {
        return is_number_float() ? &m_value.number_float : nullptr;
    }

    /*!
    @brief helper function to implement get_ref()

    This funcion helps to implement get_ref() without code duplication for
    const and non-const overloads

    @tparam ThisType will be deduced as `basic_json` or `const basic_json`

    @throw std::domain_error if ReferenceType does not match underlying value
    type of the current JSON
    */
    template<typename ReferenceType, typename ThisType>
    static ReferenceType get_ref_impl(ThisType& obj)
    {
        // helper type
        using PointerType = typename std::add_pointer<ReferenceType>::type;

        // delegate the call to get_ptr<>()
        auto ptr = obj.template get_ptr<PointerType>();

        if (ptr != nullptr)
        {
            return *ptr;
        }

        JSON_THROW(std::domain_error("incompatible ReferenceType for get_ref, actual type is " +
                                     obj.type_name()));
    }

  public:
    /// @name value access
    /// Direct access to the stored value of a JSON value.
    /// @{

    /*!
    @brief get special-case overload

    This overloads avoids a lot of template boilerplate, it can be seen as the
    identity method

    @tparam BasicJsonType == @ref basic_json

    @return a copy of *this

    @complexity Constant.

    @since version 2.1.0
    */
    template <
        typename BasicJsonType,
        detail::enable_if_t<std::is_same<typename std::remove_const<BasicJsonType>::type,
                                         basic_json_t>::value,
                            int> = 0 >
    basic_json get() const
    {
        return *this;
    }

    /*!
    @brief get a value (explicit)

    Explicit type conversion between the JSON value and a compatible value
    which is [CopyConstructible](http://en.cppreference.com/w/cpp/concept/CopyConstructible)
    and [DefaultConstructible](http://en.cppreference.com/w/cpp/concept/DefaultConstructible).
    The value is converted by calling the @ref json_serializer<ValueType>
    `from_json()` method.

    The function is equivalent to executing
    @code {.cpp}
    ValueType ret;
    JSONSerializer<ValueType>::from_json(*this, ret);
    return ret;
    @endcode

    This overloads is chosen if:
    - @a ValueType is not @ref basic_json,
    - @ref json_serializer<ValueType> has a `from_json()` method of the form
      `void from_json(const @ref basic_json&, ValueType&)`, and
    - @ref json_serializer<ValueType> does not have a `from_json()` method of
      the form `ValueType from_json(const @ref basic_json&)`

    @tparam ValueTypeCV the provided value type
    @tparam ValueType the returned value type

    @return copy of the JSON value, converted to @a ValueType

    @throw what @ref json_serializer<ValueType> `from_json()` method throws

    @liveexample{The example below shows several conversions from JSON values
    to other types. There a few things to note: (1) Floating-point numbers can
    be converted to integers\, (2) A JSON array can be converted to a standard
    `std::vector<short>`\, (3) A JSON object can be converted to C++
    associative containers such as `std::unordered_map<std::string\,
    json>`.,get__ValueType_const}

    @since version 2.1.0
    */
    template <
        typename ValueTypeCV,
        typename ValueType = detail::uncvref_t<ValueTypeCV>,
        detail::enable_if_t <
            not std::is_same<basic_json_t, ValueType>::value and
            detail::has_from_json<basic_json_t, ValueType>::value and
            not detail::has_non_default_from_json<basic_json_t, ValueType>::value,
            int > = 0 >
    ValueType get() const noexcept(noexcept(
                                       JSONSerializer<ValueType>::from_json(std::declval<const basic_json_t&>(), std::declval<ValueType&>())))
    {
        // we cannot static_assert on ValueTypeCV being non-const, because
        // there is support for get<const basic_json_t>(), which is why we
        // still need the uncvref
        static_assert(not std::is_reference<ValueTypeCV>::value,
                      "get() cannot be used with reference types, you might want to use get_ref()");
        static_assert(std::is_default_constructible<ValueType>::value,
                      "types must be DefaultConstructible when used with get()");

        ValueType ret;
        JSONSerializer<ValueType>::from_json(*this, ret);
        return ret;
    }

    /*!
    @brief get a value (explicit); special case

    Explicit type conversion between the JSON value and a compatible value
    which is **not** [CopyConstructible](http://en.cppreference.com/w/cpp/concept/CopyConstructible)
    and **not** [DefaultConstructible](http://en.cppreference.com/w/cpp/concept/DefaultConstructible).
    The value is converted by calling the @ref json_serializer<ValueType>
    `from_json()` method.

    The function is equivalent to executing
    @code {.cpp}
    return JSONSerializer<ValueTypeCV>::from_json(*this);
    @endcode

    This overloads is chosen if:
    - @a ValueType is not @ref basic_json and
    - @ref json_serializer<ValueType> has a `from_json()` method of the form
      `ValueType from_json(const @ref basic_json&)`

    @note If @ref json_serializer<ValueType> has both overloads of
    `from_json()`, this one is chosen.

    @tparam ValueTypeCV the provided value type
    @tparam ValueType the returned value type

    @return copy of the JSON value, converted to @a ValueType

    @throw what @ref json_serializer<ValueType> `from_json()` method throws

    @since version 2.1.0
    */
    template <
        typename ValueTypeCV,
        typename ValueType = detail::uncvref_t<ValueTypeCV>,
        detail::enable_if_t<not std::is_same<basic_json_t, ValueType>::value and
                            detail::has_non_default_from_json<basic_json_t,
                                    ValueType>::value, int> = 0 >
    ValueType get() const noexcept(noexcept(
                                       JSONSerializer<ValueTypeCV>::from_json(std::declval<const basic_json_t&>())))
    {
        static_assert(not std::is_reference<ValueTypeCV>::value,
                      "get() cannot be used with reference types, you might want to use get_ref()");
        return JSONSerializer<ValueTypeCV>::from_json(*this);
    }

    /*!
    @brief get a pointer value (explicit)

    Explicit pointer access to the internally stored JSON value. No copies are
    made.

    @warning The pointer becomes invalid if the underlying JSON object
    changes.

    @tparam PointerType pointer type; must be a pointer to @ref array_t, @ref
    object_t, @ref string_t, @ref boolean_t, @ref number_integer_t,
    @ref number_unsigned_t, or @ref number_float_t.

    @return pointer to the internally stored JSON value if the requested
    pointer type @a PointerType fits to the JSON value; `nullptr` otherwise

    @complexity Constant.

    @liveexample{The example below shows how pointers to internal values of a
    JSON value can be requested. Note that no type conversions are made and a
    `nullptr` is returned if the value and the requested pointer type does not
    match.,get__PointerType}

    @sa @ref get_ptr() for explicit pointer-member access

    @since version 1.0.0
    */
    template<typename PointerType, typename std::enable_if<
                 std::is_pointer<PointerType>::value, int>::type = 0>
    PointerType get() noexcept
    {
        // delegate the call to get_ptr
        return get_ptr<PointerType>();
    }

    /*!
    @brief get a pointer value (explicit)
    @copydoc get()
    */
    template<typename PointerType, typename std::enable_if<
                 std::is_pointer<PointerType>::value, int>::type = 0>
    constexpr const PointerType get() const noexcept
    {
        // delegate the call to get_ptr
        return get_ptr<PointerType>();
    }

    /*!
    @brief get a pointer value (implicit)

    Implicit pointer access to the internally stored JSON value. No copies are
    made.

    @warning Writing data to the pointee of the result yields an undefined
    state.

    @tparam PointerType pointer type; must be a pointer to @ref array_t, @ref
    object_t, @ref string_t, @ref boolean_t, @ref number_integer_t,
    @ref number_unsigned_t, or @ref number_float_t. Enforced by a static
    assertion.

    @return pointer to the internally stored JSON value if the requested
    pointer type @a PointerType fits to the JSON value; `nullptr` otherwise

    @complexity Constant.

    @liveexample{The example below shows how pointers to internal values of a
    JSON value can be requested. Note that no type conversions are made and a
    `nullptr` is returned if the value and the requested pointer type does not
    match.,get_ptr}

    @since version 1.0.0
    */
    template<typename PointerType, typename std::enable_if<
                 std::is_pointer<PointerType>::value, int>::type = 0>
    PointerType get_ptr() noexcept
    {
        // get the type of the PointerType (remove pointer and const)
        using pointee_t = typename std::remove_const<typename
                          std::remove_pointer<typename
                          std::remove_const<PointerType>::type>::type>::type;
        // make sure the type matches the allowed types
        static_assert(
            std::is_same<object_t, pointee_t>::value
            or std::is_same<array_t, pointee_t>::value
            or std::is_same<string_t, pointee_t>::value
            or std::is_same<boolean_t, pointee_t>::value
            or std::is_same<number_integer_t, pointee_t>::value
            or std::is_same<number_unsigned_t, pointee_t>::value
            or std::is_same<number_float_t, pointee_t>::value
            , "incompatible pointer type");

        // delegate the call to get_impl_ptr<>()
        return get_impl_ptr(static_cast<PointerType>(nullptr));
    }

    /*!
    @brief get a pointer value (implicit)
    @copydoc get_ptr()
    */
    template<typename PointerType, typename std::enable_if<
                 std::is_pointer<PointerType>::value and
                 std::is_const<typename std::remove_pointer<PointerType>::type>::value, int>::type = 0>
    constexpr const PointerType get_ptr() const noexcept
    {
        // get the type of the PointerType (remove pointer and const)
        using pointee_t = typename std::remove_const<typename
                          std::remove_pointer<typename
                          std::remove_const<PointerType>::type>::type>::type;
        // make sure the type matches the allowed types
        static_assert(
            std::is_same<object_t, pointee_t>::value
            or std::is_same<array_t, pointee_t>::value
            or std::is_same<string_t, pointee_t>::value
            or std::is_same<boolean_t, pointee_t>::value
            or std::is_same<number_integer_t, pointee_t>::value
            or std::is_same<number_unsigned_t, pointee_t>::value
            or std::is_same<number_float_t, pointee_t>::value
            , "incompatible pointer type");

        // delegate the call to get_impl_ptr<>() const
        return get_impl_ptr(static_cast<const PointerType>(nullptr));
    }

    /*!
    @brief get a reference value (implicit)

    Implicit reference access to the internally stored JSON value. No copies
    are made.

    @warning Writing data to the referee of the result yields an undefined
    state.

    @tparam ReferenceType reference type; must be a reference to @ref array_t,
    @ref object_t, @ref string_t, @ref boolean_t, @ref number_integer_t, or
    @ref number_float_t. Enforced by static assertion.

    @return reference to the internally stored JSON value if the requested
    reference type @a ReferenceType fits to the JSON value; throws
    std::domain_error otherwise

    @throw std::domain_error in case passed type @a ReferenceType is
    incompatible with the stored JSON value

    @complexity Constant.

    @liveexample{The example shows several calls to `get_ref()`.,get_ref}

    @since version 1.1.0
    */
    template<typename ReferenceType, typename std::enable_if<
                 std::is_reference<ReferenceType>::value, int>::type = 0>
    ReferenceType get_ref()
    {
        // delegate call to get_ref_impl
        return get_ref_impl<ReferenceType>(*this);
    }

    /*!
    @brief get a reference value (implicit)
    @copydoc get_ref()
    */
    template<typename ReferenceType, typename std::enable_if<
                 std::is_reference<ReferenceType>::value and
                 std::is_const<typename std::remove_reference<ReferenceType>::type>::value, int>::type = 0>
    ReferenceType get_ref() const
    {
        // delegate call to get_ref_impl
        return get_ref_impl<ReferenceType>(*this);
    }

    /*!
    @brief get a value (implicit)

    Implicit type conversion between the JSON value and a compatible value.
    The call is realized by calling @ref get() const.

    @tparam ValueType non-pointer type compatible to the JSON value, for
    instance `int` for JSON integer numbers, `bool` for JSON booleans, or
    `std::vector` types for JSON arrays. The character type of @ref string_t
    as well as an initializer list of this type is excluded to avoid
    ambiguities as these types implicitly convert to `std::string`.

    @return copy of the JSON value, converted to type @a ValueType

    @throw std::domain_error in case passed type @a ValueType is incompatible
    to JSON, thrown by @ref get() const

    @complexity Linear in the size of the JSON value.

    @liveexample{The example below shows several conversions from JSON values
    to other types. There a few things to note: (1) Floating-point numbers can
    be converted to integers\, (2) A JSON array can be converted to a standard
    `std::vector<short>`\, (3) A JSON object can be converted to C++
    associative containers such as `std::unordered_map<std::string\,
    json>`.,operator__ValueType}

    @since version 1.0.0
    */
    template < typename ValueType, typename std::enable_if <
                   not std::is_pointer<ValueType>::value and
                   not std::is_same<ValueType, typename string_t::value_type>::value
#ifndef _MSC_VER  // fix for issue #167 operator<< ambiguity under VS2015
                   and not std::is_same<ValueType, std::initializer_list<typename string_t::value_type>>::value
#endif
                   , int >::type = 0 >
    operator ValueType() const
    {
        // delegate the call to get<>() const
        return get<ValueType>();
    }

    /// @}


    ////////////////////
    // element access //
    ////////////////////

    /// @name element access
    /// Access to the JSON value.
    /// @{

    /*!
    @brief access specified array element with bounds checking

    Returns a reference to the element at specified location @a idx, with
    bounds checking.

    @param[in] idx  index of the element to access

    @return reference to the element at index @a idx

    @throw std::domain_error if the JSON value is not an array; example:
    `"cannot use at() with string"`
    @throw std::out_of_range if the index @a idx is out of range of the array;
    that is, `idx >= size()`; example: `"array index 7 is out of range"`

    @complexity Constant.

    @liveexample{The example below shows how array elements can be read and
    written using `at()`.,at__size_type}

    @since version 1.0.0
    */
    reference at(size_type idx)
    {
        // at only works for arrays
        if (is_array())
        {
            JSON_TRY
            {
                return m_value.array->at(idx);
            }
            JSON_CATCH (std::out_of_range&)
            {
                // create better exception explanation
                JSON_THROW(std::out_of_range("array index " + std::to_string(idx) + " is out of range"));
            }
        }
        else
        {
            JSON_THROW(std::domain_error("cannot use at() with " + type_name()));
        }
    }

    /*!
    @brief access specified array element with bounds checking

    Returns a const reference to the element at specified location @a idx,
    with bounds checking.

    @param[in] idx  index of the element to access

    @return const reference to the element at index @a idx

    @throw std::domain_error if the JSON value is not an array; example:
    `"cannot use at() with string"`
    @throw std::out_of_range if the index @a idx is out of range of the array;
    that is, `idx >= size()`; example: `"array index 7 is out of range"`

    @complexity Constant.

    @liveexample{The example below shows how array elements can be read using
    `at()`.,at__size_type_const}

    @since version 1.0.0
    */
    const_reference at(size_type idx) const
    {
        // at only works for arrays
        if (is_array())
        {
            JSON_TRY
            {
                return m_value.array->at(idx);
            }
            JSON_CATCH (std::out_of_range&)
            {
                // create better exception explanation
                JSON_THROW(std::out_of_range("array index " + std::to_string(idx) + " is out of range"));
            }
        }
        else
        {
            JSON_THROW(std::domain_error("cannot use at() with " + type_name()));
        }
    }

    /*!
    @brief access specified object element with bounds checking

    Returns a reference to the element at with specified key @a key, with
    bounds checking.

    @param[in] key  key of the element to access

    @return reference to the element at key @a key

    @throw std::domain_error if the JSON value is not an object; example:
    `"cannot use at() with boolean"`
    @throw std::out_of_range if the key @a key is is not stored in the object;
    that is, `find(key) == end()`; example: `"key "the fast" not found"`

    @complexity Logarithmic in the size of the container.

    @liveexample{The example below shows how object elements can be read and
    written using `at()`.,at__object_t_key_type}

    @sa @ref operator[](const typename object_t::key_type&) for unchecked
    access by reference
    @sa @ref value() for access by value with a default value

    @since version 1.0.0
    */
    reference at(const typename object_t::key_type& key)
    {
        // at only works for objects
        if (is_object())
        {
            JSON_TRY
            {
                return m_value.object->at(key);
            }
            JSON_CATCH (std::out_of_range&)
            {
                // create better exception explanation
                JSON_THROW(std::out_of_range("key '" + key + "' not found"));
            }
        }
        else
        {
            JSON_THROW(std::domain_error("cannot use at() with " + type_name()));
        }
    }

    /*!
    @brief access specified object element with bounds checking

    Returns a const reference to the element at with specified key @a key,
    with bounds checking.

    @param[in] key  key of the element to access

    @return const reference to the element at key @a key

    @throw std::domain_error if the JSON value is not an object; example:
    `"cannot use at() with boolean"`
    @throw std::out_of_range if the key @a key is is not stored in the object;
    that is, `find(key) == end()`; example: `"key "the fast" not found"`

    @complexity Logarithmic in the size of the container.

    @liveexample{The example below shows how object elements can be read using
    `at()`.,at__object_t_key_type_const}

    @sa @ref operator[](const typename object_t::key_type&) for unchecked
    access by reference
    @sa @ref value() for access by value with a default value

    @since version 1.0.0
    */
    const_reference at(const typename object_t::key_type& key) const
    {
        // at only works for objects
        if (is_object())
        {
            JSON_TRY
            {
                return m_value.object->at(key);
            }
            JSON_CATCH (std::out_of_range&)
            {
                // create better exception explanation
                JSON_THROW(std::out_of_range("key '" + key + "' not found"));
            }
        }
        else
        {
            JSON_THROW(std::domain_error("cannot use at() with " + type_name()));
        }
    }

    /*!
    @brief access specified array element

    Returns a reference to the element at specified location @a idx.

    @note If @a idx is beyond the range of the array (i.e., `idx >= size()`),
    then the array is silently filled up with `null` values to make `idx` a
    valid reference to the last stored element.

    @param[in] idx  index of the element to access

    @return reference to the element at index @a idx

    @throw std::domain_error if JSON is not an array or null; example:
    `"cannot use operator[] with string"`

    @complexity Constant if @a idx is in the range of the array. Otherwise
    linear in `idx - size()`.

    @liveexample{The example below shows how array elements can be read and
    written using `[]` operator. Note the addition of `null`
    values.,operatorarray__size_type}

    @since version 1.0.0
    */
    reference operator[](size_type idx)
    {
        // implicitly convert null value to an empty array
        if (is_null())
        {
            m_type = value_t::array;
            m_value.array = create<array_t>();
            assert_invariant();
        }

        // operator[] only works for arrays
        if (is_array())
        {
            // fill up array with null values if given idx is outside range
            if (idx >= m_value.array->size())
            {
                m_value.array->insert(m_value.array->end(),
                                      idx - m_value.array->size() + 1,
                                      basic_json());
            }

            return m_value.array->operator[](idx);
        }

        JSON_THROW(std::domain_error("cannot use operator[] with " + type_name()));
    }

    /*!
    @brief access specified array element

    Returns a const reference to the element at specified location @a idx.

    @param[in] idx  index of the element to access

    @return const reference to the element at index @a idx

    @throw std::domain_error if JSON is not an array; example: `"cannot use
    operator[] with null"`

    @complexity Constant.

    @liveexample{The example below shows how array elements can be read using
    the `[]` operator.,operatorarray__size_type_const}

    @since version 1.0.0
    */
    const_reference operator[](size_type idx) const
    {
        // const operator[] only works for arrays
        if (is_array())
        {
            return m_value.array->operator[](idx);
        }

        JSON_THROW(std::domain_error("cannot use operator[] with " + type_name()));
    }

    /*!
    @brief access specified object element

    Returns a reference to the element at with specified key @a key.

    @note If @a key is not found in the object, then it is silently added to
    the object and filled with a `null` value to make `key` a valid reference.
    In case the value was `null` before, it is converted to an object.

    @param[in] key  key of the element to access

    @return reference to the element at key @a key

    @throw std::domain_error if JSON is not an object or null; example:
    `"cannot use operator[] with string"`

    @complexity Logarithmic in the size of the container.

    @liveexample{The example below shows how object elements can be read and
    written using the `[]` operator.,operatorarray__key_type}

    @sa @ref at(const typename object_t::key_type&) for access by reference
    with range checking
    @sa @ref value() for access by value with a default value

    @since version 1.0.0
    */
    reference operator[](const typename object_t::key_type& key)
    {
        // implicitly convert null value to an empty object
        if (is_null())
        {
            m_type = value_t::object;
            m_value.object = create<object_t>();
            assert_invariant();
        }

        // operator[] only works for objects
        if (is_object())
        {
            return m_value.object->operator[](key);
        }

        JSON_THROW(std::domain_error("cannot use operator[] with " + type_name()));
    }

    /*!
    @brief read-only access specified object element

    Returns a const reference to the element at with specified key @a key. No
    bounds checking is performed.

    @warning If the element with key @a key does not exist, the behavior is
    undefined.

    @param[in] key  key of the element to access

    @return const reference to the element at key @a key

    @pre The element with key @a key must exist. **This precondition is
         enforced with an assertion.**

    @throw std::domain_error if JSON is not an object; example: `"cannot use
    operator[] with null"`

    @complexity Logarithmic in the size of the container.

    @liveexample{The example below shows how object elements can be read using
    the `[]` operator.,operatorarray__key_type_const}

    @sa @ref at(const typename object_t::key_type&) for access by reference
    with range checking
    @sa @ref value() for access by value with a default value

    @since version 1.0.0
    */
    const_reference operator[](const typename object_t::key_type& key) const
    {
        // const operator[] only works for objects
        if (is_object())
        {
            assert(m_value.object->find(key) != m_value.object->end());
            return m_value.object->find(key)->second;
        }

        JSON_THROW(std::domain_error("cannot use operator[] with " + type_name()));
    }

    /*!
    @brief access specified object element

    Returns a reference to the element at with specified key @a key.

    @note If @a key is not found in the object, then it is silently added to
    the object and filled with a `null` value to make `key` a valid reference.
    In case the value was `null` before, it is converted to an object.

    @param[in] key  key of the element to access

    @return reference to the element at key @a key

    @throw std::domain_error if JSON is not an object or null; example:
    `"cannot use operator[] with string"`

    @complexity Logarithmic in the size of the container.

    @liveexample{The example below shows how object elements can be read and
    written using the `[]` operator.,operatorarray__key_type}

    @sa @ref at(const typename object_t::key_type&) for access by reference
    with range checking
    @sa @ref value() for access by value with a default value

    @since version 1.0.0
    */
    template<typename T, std::size_t n>
    reference operator[](T * (&key)[n])
    {
        return operator[](static_cast<const T>(key));
    }

    /*!
    @brief read-only access specified object element

    Returns a const reference to the element at with specified key @a key. No
    bounds checking is performed.

    @warning If the element with key @a key does not exist, the behavior is
    undefined.

    @note This function is required for compatibility reasons with Clang.

    @param[in] key  key of the element to access

    @return const reference to the element at key @a key

    @throw std::domain_error if JSON is not an object; example: `"cannot use
    operator[] with null"`

    @complexity Logarithmic in the size of the container.

    @liveexample{The example below shows how object elements can be read using
    the `[]` operator.,operatorarray__key_type_const}

    @sa @ref at(const typename object_t::key_type&) for access by reference
    with range checking
    @sa @ref value() for access by value with a default value

    @since version 1.0.0
    */
    template<typename T, std::size_t n>
    const_reference operator[](T * (&key)[n]) const
    {
        return operator[](static_cast<const T>(key));
    }

    /*!
    @brief access specified object element

    Returns a reference to the element at with specified key @a key.

    @note If @a key is not found in the object, then it is silently added to
    the object and filled with a `null` value to make `key` a valid reference.
    In case the value was `null` before, it is converted to an object.

    @param[in] key  key of the element to access

    @return reference to the element at key @a key

    @throw std::domain_error if JSON is not an object or null; example:
    `"cannot use operator[] with string"`

    @complexity Logarithmic in the size of the container.

    @liveexample{The example below shows how object elements can be read and
    written using the `[]` operator.,operatorarray__key_type}

    @sa @ref at(const typename object_t::key_type&) for access by reference
    with range checking
    @sa @ref value() for access by value with a default value

    @since version 1.1.0
    */
    template<typename T>
    reference operator[](T* key)
    {
        // implicitly convert null to object
        if (is_null())
        {
            m_type = value_t::object;
            m_value = value_t::object;
            assert_invariant();
        }

        // at only works for objects
        if (is_object())
        {
            return m_value.object->operator[](key);
        }

        JSON_THROW(std::domain_error("cannot use operator[] with " + type_name()));
    }

    /*!
    @brief read-only access specified object element

    Returns a const reference to the element at with specified key @a key. No
    bounds checking is performed.

    @warning If the element with key @a key does not exist, the behavior is
    undefined.

    @param[in] key  key of the element to access

    @return const reference to the element at key @a key

    @pre The element with key @a key must exist. **This precondition is
         enforced with an assertion.**

    @throw std::domain_error if JSON is not an object; example: `"cannot use
    operator[] with null"`

    @complexity Logarithmic in the size of the container.

    @liveexample{The example below shows how object elements can be read using
    the `[]` operator.,operatorarray__key_type_const}

    @sa @ref at(const typename object_t::key_type&) for access by reference
    with range checking
    @sa @ref value() for access by value with a default value

    @since version 1.1.0
    */
    template<typename T>
    const_reference operator[](T* key) const
    {
        // at only works for objects
        if (is_object())
        {
            assert(m_value.object->find(key) != m_value.object->end());
            return m_value.object->find(key)->second;
        }

        JSON_THROW(std::domain_error("cannot use operator[] with " + type_name()));
    }

    /*!
    @brief access specified object element with default value

    Returns either a copy of an object's element at the specified key @a key
    or a given default value if no element with key @a key exists.

    The function is basically equivalent to executing
    @code {.cpp}
    try {
        return at(key);
    } catch(std::out_of_range) {
        return default_value;
    }
    @endcode

    @note Unlike @ref at(const typename object_t::key_type&), this function
    does not throw if the given key @a key was not found.

    @note Unlike @ref operator[](const typename object_t::key_type& key), this
    function does not implicitly add an element to the position defined by @a
    key. This function is furthermore also applicable to const objects.

    @param[in] key  key of the element to access
    @param[in] default_value  the value to return if @a key is not found

    @tparam ValueType type compatible to JSON values, for instance `int` for
    JSON integer numbers, `bool` for JSON booleans, or `std::vector` types for
    JSON arrays. Note the type of the expected value at @a key and the default
    value @a default_value must be compatible.

    @return copy of the element at key @a key or @a default_value if @a key
    is not found

    @throw std::domain_error if JSON is not an object; example: `"cannot use
    value() with null"`

    @complexity Logarithmic in the size of the container.

    @liveexample{The example below shows how object elements can be queried
    with a default value.,basic_json__value}

    @sa @ref at(const typename object_t::key_type&) for access by reference
    with range checking
    @sa @ref operator[](const typename object_t::key_type&) for unchecked
    access by reference

    @since version 1.0.0
    */
    template<class ValueType, typename std::enable_if<
                 std::is_convertible<basic_json_t, ValueType>::value, int>::type = 0>
    ValueType value(const typename object_t::key_type& key, ValueType default_value) const
    {
        // at only works for objects
        if (is_object())
        {
            // if key is found, return value and given default value otherwise
            const auto it = find(key);
            if (it != end())
            {
                return *it;
            }

            return default_value;
        }
        else
        {
            JSON_THROW(std::domain_error("cannot use value() with " + type_name()));
        }
    }

    /*!
    @brief overload for a default value of type const char*
    @copydoc basic_json::value(const typename object_t::key_type&, ValueType) const
    */
    string_t value(const typename object_t::key_type& key, const char* default_value) const
    {
        return value(key, string_t(default_value));
    }

    /*!
    @brief access specified object element via JSON Pointer with default value

    Returns either a copy of an object's element at the specified key @a key
    or a given default value if no element with key @a key exists.

    The function is basically equivalent to executing
    @code {.cpp}
    try {
        return at(ptr);
    } catch(std::out_of_range) {
        return default_value;
    }
    @endcode

    @note Unlike @ref at(const json_pointer&), this function does not throw
    if the given key @a key was not found.

    @param[in] ptr  a JSON pointer to the element to access
    @param[in] default_value  the value to return if @a ptr found no value

    @tparam ValueType type compatible to JSON values, for instance `int` for
    JSON integer numbers, `bool` for JSON booleans, or `std::vector` types for
    JSON arrays. Note the type of the expected value at @a key and the default
    value @a default_value must be compatible.

    @return copy of the element at key @a key or @a default_value if @a key
    is not found

    @throw std::domain_error if JSON is not an object; example: `"cannot use
    value() with null"`

    @complexity Logarithmic in the size of the container.

    @liveexample{The example below shows how object elements can be queried
    with a default value.,basic_json__value_ptr}

    @sa @ref operator[](const json_pointer&) for unchecked access by reference

    @since version 2.0.2
    */
    template<class ValueType, typename std::enable_if<
                 std::is_convertible<basic_json_t, ValueType>::value, int>::type = 0>
    ValueType value(const json_pointer& ptr, ValueType default_value) const
    {
        // at only works for objects
        if (is_object())
        {
            // if pointer resolves a value, return it or use default value
            JSON_TRY
            {
                return ptr.get_checked(this);
            }
            JSON_CATCH (std::out_of_range&)
            {
                return default_value;
            }
        }

        JSON_THROW(std::domain_error("cannot use value() with " + type_name()));
    }

    /*!
    @brief overload for a default value of type const char*
    @copydoc basic_json::value(const json_pointer&, ValueType) const
    */
    string_t value(const json_pointer& ptr, const char* default_value) const
    {
        return value(ptr, string_t(default_value));
    }

    /*!
    @brief access the first element

    Returns a reference to the first element in the container. For a JSON
    container `c`, the expression `c.front()` is equivalent to `*c.begin()`.

    @return In case of a structured type (array or object), a reference to the
    first element is returned. In case of number, string, or boolean values, a
    reference to the value is returned.

    @complexity Constant.

    @pre The JSON value must not be `null` (would throw `std::out_of_range`)
    or an empty array or object (undefined behavior, **guarded by
    assertions**).
    @post The JSON value remains unchanged.

    @throw std::out_of_range when called on `null` value

    @liveexample{The following code shows an example for `front()`.,front}

    @sa @ref back() -- access the last element

    @since version 1.0.0
    */
    reference front()
    {
        return *begin();
    }

    /*!
    @copydoc basic_json::front()
    */
    const_reference front() const
    {
        return *cbegin();
    }

    /*!
    @brief access the last element

    Returns a reference to the last element in the container. For a JSON
    container `c`, the expression `c.back()` is equivalent to
    @code {.cpp}
    auto tmp = c.end();
    --tmp;
    return *tmp;
    @endcode

    @return In case of a structured type (array or object), a reference to the
    last element is returned. In case of number, string, or boolean values, a
    reference to the value is returned.

    @complexity Constant.

    @pre The JSON value must not be `null` (would throw `std::out_of_range`)
    or an empty array or object (undefined behavior, **guarded by
    assertions**).
    @post The JSON value remains unchanged.

    @throw std::out_of_range when called on `null` value.

    @liveexample{The following code shows an example for `back()`.,back}

    @sa @ref front() -- access the first element

    @since version 1.0.0
    */
    reference back()
    {
        auto tmp = end();
        --tmp;
        return *tmp;
    }

    /*!
    @copydoc basic_json::back()
    */
    const_reference back() const
    {
        auto tmp = cend();
        --tmp;
        return *tmp;
    }

    /*!
    @brief remove element given an iterator

    Removes the element specified by iterator @a pos. The iterator @a pos must
    be valid and dereferenceable. Thus the `end()` iterator (which is valid,
    but is not dereferenceable) cannot be used as a value for @a pos.

    If called on a primitive type other than `null`, the resulting JSON value
    will be `null`.

    @param[in] pos iterator to the element to remove
    @return Iterator following the last removed element. If the iterator @a
    pos refers to the last element, the `end()` iterator is returned.

    @tparam IteratorType an @ref iterator or @ref const_iterator

    @post Invalidates iterators and references at or after the point of the
    erase, including the `end()` iterator.

    @throw std::domain_error if called on a `null` value; example: `"cannot
    use erase() with null"`
    @throw std::domain_error if called on an iterator which does not belong to
    the current JSON value; example: `"iterator does not fit current value"`
    @throw std::out_of_range if called on a primitive type with invalid
    iterator (i.e., any iterator which is not `begin()`); example: `"iterator
    out of range"`

    @complexity The complexity depends on the type:
    - objects: amortized constant
    - arrays: linear in distance between @a pos and the end of the container
    - strings: linear in the length of the string
    - other types: constant

    @liveexample{The example shows the result of `erase()` for different JSON
    types.,erase__IteratorType}

    @sa @ref erase(IteratorType, IteratorType) -- removes the elements in
    the given range
    @sa @ref erase(const typename object_t::key_type&) -- removes the element
    from an object at the given key
    @sa @ref erase(const size_type) -- removes the element from an array at
    the given index

    @since version 1.0.0
    */
    template<class IteratorType, typename std::enable_if<
                 std::is_same<IteratorType, typename basic_json_t::iterator>::value or
                 std::is_same<IteratorType, typename basic_json_t::const_iterator>::value, int>::type
             = 0>
    IteratorType erase(IteratorType pos)
    {
        // make sure iterator fits the current value
        if (this != pos.m_object)
        {
            JSON_THROW(std::domain_error("iterator does not fit current value"));
        }

        IteratorType result = end();

        switch (m_type)
        {
            case value_t::boolean:
            case value_t::number_float:
            case value_t::number_integer:
            case value_t::number_unsigned:
            case value_t::string:
            {
                if (not pos.m_it.primitive_iterator.is_begin())
                {
                    JSON_THROW(std::out_of_range("iterator out of range"));
                }

                if (is_string())
                {
                    AllocatorType<string_t> alloc;
                    alloc.destroy(m_value.string);
                    alloc.deallocate(m_value.string, 1);
                    m_value.string = nullptr;
                }

                m_type = value_t::null;
                assert_invariant();
                break;
            }

            case value_t::object:
            {
                result.m_it.object_iterator = m_value.object->erase(pos.m_it.object_iterator);
                break;
            }

            case value_t::array:
            {
                result.m_it.array_iterator = m_value.array->erase(pos.m_it.array_iterator);
                break;
            }

            default:
            {
                JSON_THROW(std::domain_error("cannot use erase() with " + type_name()));
            }
        }

        return result;
    }

    /*!
    @brief remove elements given an iterator range

    Removes the element specified by the range `[first; last)`. The iterator
    @a first does not need to be dereferenceable if `first == last`: erasing
    an empty range is a no-op.

    If called on a primitive type other than `null`, the resulting JSON value
    will be `null`.

    @param[in] first iterator to the beginning of the range to remove
    @param[in] last iterator past the end of the range to remove
    @return Iterator following the last removed element. If the iterator @a
    second refers to the last element, the `end()` iterator is returned.

    @tparam IteratorType an @ref iterator or @ref const_iterator

    @post Invalidates iterators and references at or after the point of the
    erase, including the `end()` iterator.

    @throw std::domain_error if called on a `null` value; example: `"cannot
    use erase() with null"`
    @throw std::domain_error if called on iterators which does not belong to
    the current JSON value; example: `"iterators do not fit current value"`
    @throw std::out_of_range if called on a primitive type with invalid
    iterators (i.e., if `first != begin()` and `last != end()`); example:
    `"iterators out of range"`

    @complexity The complexity depends on the type:
    - objects: `log(size()) + std::distance(first, last)`
    - arrays: linear in the distance between @a first and @a last, plus linear
      in the distance between @a last and end of the container
    - strings: linear in the length of the string
    - other types: constant

    @liveexample{The example shows the result of `erase()` for different JSON
    types.,erase__IteratorType_IteratorType}

    @sa @ref erase(IteratorType) -- removes the element at a given position
    @sa @ref erase(const typename object_t::key_type&) -- removes the element
    from an object at the given key
    @sa @ref erase(const size_type) -- removes the element from an array at
    the given index

    @since version 1.0.0
    */
    template<class IteratorType, typename std::enable_if<
                 std::is_same<IteratorType, typename basic_json_t::iterator>::value or
                 std::is_same<IteratorType, typename basic_json_t::const_iterator>::value, int>::type
             = 0>
    IteratorType erase(IteratorType first, IteratorType last)
    {
        // make sure iterator fits the current value
        if (this != first.m_object or this != last.m_object)
        {
            JSON_THROW(std::domain_error("iterators do not fit current value"));
        }

        IteratorType result = end();

        switch (m_type)
        {
            case value_t::boolean:
            case value_t::number_float:
            case value_t::number_integer:
            case value_t::number_unsigned:
            case value_t::string:
            {
                if (not first.m_it.primitive_iterator.is_begin() or not last.m_it.primitive_iterator.is_end())
                {
                    JSON_THROW(std::out_of_range("iterators out of range"));
                }

                if (is_string())
                {
                    AllocatorType<string_t> alloc;
                    alloc.destroy(m_value.string);
                    alloc.deallocate(m_value.string, 1);
                    m_value.string = nullptr;
                }

                m_type = value_t::null;
                assert_invariant();
                break;
            }

            case value_t::object:
            {
                result.m_it.object_iterator = m_value.object->erase(first.m_it.object_iterator,
                                              last.m_it.object_iterator);
                break;
            }

            case value_t::array:
            {
                result.m_it.array_iterator = m_value.array->erase(first.m_it.array_iterator,
                                             last.m_it.array_iterator);
                break;
            }

            default:
            {
                JSON_THROW(std::domain_error("cannot use erase() with " + type_name()));
            }
        }

        return result;
    }

    /*!
    @brief remove element from a JSON object given a key

    Removes elements from a JSON object with the key value @a key.

    @param[in] key value of the elements to remove

    @return Number of elements removed. If @a ObjectType is the default
    `std::map` type, the return value will always be `0` (@a key was not
    found) or `1` (@a key was found).

    @post References and iterators to the erased elements are invalidated.
    Other references and iterators are not affected.

    @throw std::domain_error when called on a type other than JSON object;
    example: `"cannot use erase() with null"`

    @complexity `log(size()) + count(key)`

    @liveexample{The example shows the effect of `erase()`.,erase__key_type}

    @sa @ref erase(IteratorType) -- removes the element at a given position
    @sa @ref erase(IteratorType, IteratorType) -- removes the elements in
    the given range
    @sa @ref erase(const size_type) -- removes the element from an array at
    the given index

    @since version 1.0.0
    */
    size_type erase(const typename object_t::key_type& key)
    {
        // this erase only works for objects
        if (is_object())
        {
            return m_value.object->erase(key);
        }

        JSON_THROW(std::domain_error("cannot use erase() with " + type_name()));
    }

    /*!
    @brief remove element from a JSON array given an index

    Removes element from a JSON array at the index @a idx.

    @param[in] idx index of the element to remove

    @throw std::domain_error when called on a type other than JSON array;
    example: `"cannot use erase() with null"`
    @throw std::out_of_range when `idx >= size()`; example: `"array index 17
    is out of range"`

    @complexity Linear in distance between @a idx and the end of the container.

    @liveexample{The example shows the effect of `erase()`.,erase__size_type}

    @sa @ref erase(IteratorType) -- removes the element at a given position
    @sa @ref erase(IteratorType, IteratorType) -- removes the elements in
    the given range
    @sa @ref erase(const typename object_t::key_type&) -- removes the element
    from an object at the given key

    @since version 1.0.0
    */
    void erase(const size_type idx)
    {
        // this erase only works for arrays
        if (is_array())
        {
            if (idx >= size())
            {
                JSON_THROW(std::out_of_range("array index " + std::to_string(idx) + " is out of range"));
            }

            m_value.array->erase(m_value.array->begin() + static_cast<difference_type>(idx));
        }
        else
        {
            JSON_THROW(std::domain_error("cannot use erase() with " + type_name()));
        }
    }

    /// @}


    ////////////
    // lookup //
    ////////////

    /// @name lookup
    /// @{

    /*!
    @brief find an element in a JSON object

    Finds an element in a JSON object with key equivalent to @a key. If the
    element is not found or the JSON value is not an object, end() is
    returned.

    @note This method always returns @ref end() when executed on a JSON type
          that is not an object.

    @param[in] key key value of the element to search for

    @return Iterator to an element with key equivalent to @a key. If no such
    element is found or the JSON value is not an object, past-the-end (see
    @ref end()) iterator is returned.

    @complexity Logarithmic in the size of the JSON object.

    @liveexample{The example shows how `find()` is used.,find__key_type}

    @since version 1.0.0
    */
    iterator find(typename object_t::key_type key)
    {
        auto result = end();

        if (is_object())
        {
            result.m_it.object_iterator = m_value.object->find(key);
        }

        return result;
    }

    /*!
    @brief find an element in a JSON object
    @copydoc find(typename object_t::key_type)
    */
    const_iterator find(typename object_t::key_type key) const
    {
        auto result = cend();

        if (is_object())
        {
            result.m_it.object_iterator = m_value.object->find(key);
        }

        return result;
    }

    /*!
    @brief returns the number of occurrences of a key in a JSON object

    Returns the number of elements with key @a key. If ObjectType is the
    default `std::map` type, the return value will always be `0` (@a key was
    not found) or `1` (@a key was found).

    @note This method always returns `0` when executed on a JSON type that is
          not an object.

    @param[in] key key value of the element to count

    @return Number of elements with key @a key. If the JSON value is not an
    object, the return value will be `0`.

    @complexity Logarithmic in the size of the JSON object.

    @liveexample{The example shows how `count()` is used.,count}

    @since version 1.0.0
    */
    size_type count(typename object_t::key_type key) const
    {
        // return 0 for all nonobject types
        return is_object() ? m_value.object->count(key) : 0;
    }

    /// @}


    ///////////////
    // iterators //
    ///////////////

    /// @name iterators
    /// @{

    /*!
    @brief returns an iterator to the first element

    Returns an iterator to the first element.

    @image html range-begin-end.svg "Illustration from cppreference.com"

    @return iterator to the first element

    @complexity Constant.

    @requirement This function helps `basic_json` satisfying the
    [Container](http://en.cppreference.com/w/cpp/concept/Container)
    requirements:
    - The complexity is constant.

    @liveexample{The following code shows an example for `begin()`.,begin}

    @sa @ref cbegin() -- returns a const iterator to the beginning
    @sa @ref end() -- returns an iterator to the end
    @sa @ref cend() -- returns a const iterator to the end

    @since version 1.0.0
    */
    iterator begin() noexcept
    {
        iterator result(this);
        result.set_begin();
        return result;
    }

    /*!
    @copydoc basic_json::cbegin()
    */
    const_iterator begin() const noexcept
    {
        return cbegin();
    }

    /*!
    @brief returns a const iterator to the first element

    Returns a const iterator to the first element.

    @image html range-begin-end.svg "Illustration from cppreference.com"

    @return const iterator to the first element

    @complexity Constant.

    @requirement This function helps `basic_json` satisfying the
    [Container](http://en.cppreference.com/w/cpp/concept/Container)
    requirements:
    - The complexity is constant.
    - Has the semantics of `const_cast<const basic_json&>(*this).begin()`.

    @liveexample{The following code shows an example for `cbegin()`.,cbegin}

    @sa @ref begin() -- returns an iterator to the beginning
    @sa @ref end() -- returns an iterator to the end
    @sa @ref cend() -- returns a const iterator to the end

    @since version 1.0.0
    */
    const_iterator cbegin() const noexcept
    {
        const_iterator result(this);
        result.set_begin();
        return result;
    }

    /*!
    @brief returns an iterator to one past the last element

    Returns an iterator to one past the last element.

    @image html range-begin-end.svg "Illustration from cppreference.com"

    @return iterator one past the last element

    @complexity Constant.

    @requirement This function helps `basic_json` satisfying the
    [Container](http://en.cppreference.com/w/cpp/concept/Container)
    requirements:
    - The complexity is constant.

    @liveexample{The following code shows an example for `end()`.,end}

    @sa @ref cend() -- returns a const iterator to the end
    @sa @ref begin() -- returns an iterator to the beginning
    @sa @ref cbegin() -- returns a const iterator to the beginning

    @since version 1.0.0
    */
    iterator end() noexcept
    {
        iterator result(this);
        result.set_end();
        return result;
    }

    /*!
    @copydoc basic_json::cend()
    */
    const_iterator end() const noexcept
    {
        return cend();
    }

    /*!
    @brief returns a const iterator to one past the last element

    Returns a const iterator to one past the last element.

    @image html range-begin-end.svg "Illustration from cppreference.com"

    @return const iterator one past the last element

    @complexity Constant.

    @requirement This function helps `basic_json` satisfying the
    [Container](http://en.cppreference.com/w/cpp/concept/Container)
    requirements:
    - The complexity is constant.
    - Has the semantics of `const_cast<const basic_json&>(*this).end()`.

    @liveexample{The following code shows an example for `cend()`.,cend}

    @sa @ref end() -- returns an iterator to the end
    @sa @ref begin() -- returns an iterator to the beginning
    @sa @ref cbegin() -- returns a const iterator to the beginning

    @since version 1.0.0
    */
    const_iterator cend() const noexcept
    {
        const_iterator result(this);
        result.set_end();
        return result;
    }

    /*!
    @brief returns an iterator to the reverse-beginning

    Returns an iterator to the reverse-beginning; that is, the last element.

    @image html range-rbegin-rend.svg "Illustration from cppreference.com"

    @complexity Constant.

    @requirement This function helps `basic_json` satisfying the
    [ReversibleContainer](http://en.cppreference.com/w/cpp/concept/ReversibleContainer)
    requirements:
    - The complexity is constant.
    - Has the semantics of `reverse_iterator(end())`.

    @liveexample{The following code shows an example for `rbegin()`.,rbegin}

    @sa @ref crbegin() -- returns a const reverse iterator to the beginning
    @sa @ref rend() -- returns a reverse iterator to the end
    @sa @ref crend() -- returns a const reverse iterator to the end

    @since version 1.0.0
    */
    reverse_iterator rbegin() noexcept
    {
        return reverse_iterator(end());
    }

    /*!
    @copydoc basic_json::crbegin()
    */
    const_reverse_iterator rbegin() const noexcept
    {
        return crbegin();
    }

    /*!
    @brief returns an iterator to the reverse-end

    Returns an iterator to the reverse-end; that is, one before the first
    element.

    @image html range-rbegin-rend.svg "Illustration from cppreference.com"

    @complexity Constant.

    @requirement This function helps `basic_json` satisfying the
    [ReversibleContainer](http://en.cppreference.com/w/cpp/concept/ReversibleContainer)
    requirements:
    - The complexity is constant.
    - Has the semantics of `reverse_iterator(begin())`.

    @liveexample{The following code shows an example for `rend()`.,rend}

    @sa @ref crend() -- returns a const reverse iterator to the end
    @sa @ref rbegin() -- returns a reverse iterator to the beginning
    @sa @ref crbegin() -- returns a const reverse iterator to the beginning

    @since version 1.0.0
    */
    reverse_iterator rend() noexcept
    {
        return reverse_iterator(begin());
    }

    /*!
    @copydoc basic_json::crend()
    */
    const_reverse_iterator rend() const noexcept
    {
        return crend();
    }

    /*!
    @brief returns a const reverse iterator to the last element

    Returns a const iterator to the reverse-beginning; that is, the last
    element.

    @image html range-rbegin-rend.svg "Illustration from cppreference.com"

    @complexity Constant.

    @requirement This function helps `basic_json` satisfying the
    [ReversibleContainer](http://en.cppreference.com/w/cpp/concept/ReversibleContainer)
    requirements:
    - The complexity is constant.
    - Has the semantics of `const_cast<const basic_json&>(*this).rbegin()`.

    @liveexample{The following code shows an example for `crbegin()`.,crbegin}

    @sa @ref rbegin() -- returns a reverse iterator to the beginning
    @sa @ref rend() -- returns a reverse iterator to the end
    @sa @ref crend() -- returns a const reverse iterator to the end

    @since version 1.0.0
    */
    const_reverse_iterator crbegin() const noexcept
    {
        return const_reverse_iterator(cend());
    }

    /*!
    @brief returns a const reverse iterator to one before the first

    Returns a const reverse iterator to the reverse-end; that is, one before
    the first element.

    @image html range-rbegin-rend.svg "Illustration from cppreference.com"

    @complexity Constant.

    @requirement This function helps `basic_json` satisfying the
    [ReversibleContainer](http://en.cppreference.com/w/cpp/concept/ReversibleContainer)
    requirements:
    - The complexity is constant.
    - Has the semantics of `const_cast<const basic_json&>(*this).rend()`.

    @liveexample{The following code shows an example for `crend()`.,crend}

    @sa @ref rend() -- returns a reverse iterator to the end
    @sa @ref rbegin() -- returns a reverse iterator to the beginning
    @sa @ref crbegin() -- returns a const reverse iterator to the beginning

    @since version 1.0.0
    */
    const_reverse_iterator crend() const noexcept
    {
        return const_reverse_iterator(cbegin());
    }

  private:
    // forward declaration
    template<typename IteratorType> class iteration_proxy;

  public:
    /*!
    @brief wrapper to access iterator member functions in range-based for

    This function allows to access @ref iterator::key() and @ref
    iterator::value() during range-based for loops. In these loops, a
    reference to the JSON values is returned, so there is no access to the
    underlying iterator.

    @note The name of this function is not yet final and may change in the
    future.
    */
    static iteration_proxy<iterator> iterator_wrapper(reference cont)
    {
        return iteration_proxy<iterator>(cont);
    }

    /*!
    @copydoc iterator_wrapper(reference)
    */
    static iteration_proxy<const_iterator> iterator_wrapper(const_reference cont)
    {
        return iteration_proxy<const_iterator>(cont);
    }

    /// @}


    //////////////
    // capacity //
    //////////////

    /// @name capacity
    /// @{

    /*!
    @brief checks whether the container is empty

    Checks if a JSON value has no elements.

    @return The return value depends on the different types and is
            defined as follows:
            Value type  | return value
            ----------- | -------------
            null        | `true`
            boolean     | `false`
            string      | `false`
            number      | `false`
            object      | result of function `object_t::empty()`
            array       | result of function `array_t::empty()`

    @note This function does not return whether a string stored as JSON value
    is empty - it returns whether the JSON container itself is empty which is
    false in the case of a string.

    @complexity Constant, as long as @ref array_t and @ref object_t satisfy
    the Container concept; that is, their `empty()` functions have constant
    complexity.

    @requirement This function helps `basic_json` satisfying the
    [Container](http://en.cppreference.com/w/cpp/concept/Container)
    requirements:
    - The complexity is constant.
    - Has the semantics of `begin() == end()`.

    @liveexample{The following code uses `empty()` to check if a JSON
    object contains any elements.,empty}

    @sa @ref size() -- returns the number of elements

    @since version 1.0.0
    */
    bool empty() const noexcept
    {
        switch (m_type)
        {
            case value_t::null:
            {
                // null values are empty
                return true;
            }

            case value_t::array:
            {
                // delegate call to array_t::empty()
                return m_value.array->empty();
            }

            case value_t::object:
            {
                // delegate call to object_t::empty()
                return m_value.object->empty();
            }

            default:
            {
                // all other types are nonempty
                return false;
            }
        }
    }

    /*!
    @brief returns the number of elements

    Returns the number of elements in a JSON value.

    @return The return value depends on the different types and is
            defined as follows:
            Value type  | return value
            ----------- | -------------
            null        | `0`
            boolean     | `1`
            string      | `1`
            number      | `1`
            object      | result of function object_t::size()
            array       | result of function array_t::size()

    @note This function does not return the length of a string stored as JSON
    value - it returns the number of elements in the JSON value which is 1 in
    the case of a string.

    @complexity Constant, as long as @ref array_t and @ref object_t satisfy
    the Container concept; that is, their size() functions have constant
    complexity.

    @requirement This function helps `basic_json` satisfying the
    [Container](http://en.cppreference.com/w/cpp/concept/Container)
    requirements:
    - The complexity is constant.
    - Has the semantics of `std::distance(begin(), end())`.

    @liveexample{The following code calls `size()` on the different value
    types.,size}

    @sa @ref empty() -- checks whether the container is empty
    @sa @ref max_size() -- returns the maximal number of elements

    @since version 1.0.0
    */
    size_type size() const noexcept
    {
        switch (m_type)
        {
            case value_t::null:
            {
                // null values are empty
                return 0;
            }

            case value_t::array:
            {
                // delegate call to array_t::size()
                return m_value.array->size();
            }

            case value_t::object:
            {
                // delegate call to object_t::size()
                return m_value.object->size();
            }

            default:
            {
                // all other types have size 1
                return 1;
            }
        }
    }

    /*!
    @brief returns the maximum possible number of elements

    Returns the maximum number of elements a JSON value is able to hold due to
    system or library implementation limitations, i.e. `std::distance(begin(),
    end())` for the JSON value.

    @return The return value depends on the different types and is
            defined as follows:
            Value type  | return value
            ----------- | -------------
            null        | `0` (same as `size()`)
            boolean     | `1` (same as `size()`)
            string      | `1` (same as `size()`)
            number      | `1` (same as `size()`)
            object      | result of function `object_t::max_size()`
            array       | result of function `array_t::max_size()`

    @complexity Constant, as long as @ref array_t and @ref object_t satisfy
    the Container concept; that is, their `max_size()` functions have constant
    complexity.

    @requirement This function helps `basic_json` satisfying the
    [Container](http://en.cppreference.com/w/cpp/concept/Container)
    requirements:
    - The complexity is constant.
    - Has the semantics of returning `b.size()` where `b` is the largest
      possible JSON value.

    @liveexample{The following code calls `max_size()` on the different value
    types. Note the output is implementation specific.,max_size}

    @sa @ref size() -- returns the number of elements

    @since version 1.0.0
    */
    size_type max_size() const noexcept
    {
        switch (m_type)
        {
            case value_t::array:
            {
                // delegate call to array_t::max_size()
                return m_value.array->max_size();
            }

            case value_t::object:
            {
                // delegate call to object_t::max_size()
                return m_value.object->max_size();
            }

            default:
            {
                // all other types have max_size() == size()
                return size();
            }
        }
    }

    /// @}


    ///////////////
    // modifiers //
    ///////////////

    /// @name modifiers
    /// @{

    /*!
    @brief clears the contents

    Clears the content of a JSON value and resets it to the default value as
    if @ref basic_json(value_t) would have been called:

    Value type  | initial value
    ----------- | -------------
    null        | `null`
    boolean     | `false`
    string      | `""`
    number      | `0`
    object      | `{}`
    array       | `[]`

    @complexity Linear in the size of the JSON value.

    @liveexample{The example below shows the effect of `clear()` to different
    JSON types.,clear}

    @since version 1.0.0
    */
    void clear() noexcept
    {
        switch (m_type)
        {
            case value_t::number_integer:
            {
                m_value.number_integer = 0;
                break;
            }

            case value_t::number_unsigned:
            {
                m_value.number_unsigned = 0;
                break;
            }

            case value_t::number_float:
            {
                m_value.number_float = 0.0;
                break;
            }

            case value_t::boolean:
            {
                m_value.boolean = false;
                break;
            }

            case value_t::string:
            {
                m_value.string->clear();
                break;
            }

            case value_t::array:
            {
                m_value.array->clear();
                break;
            }

            case value_t::object:
            {
                m_value.object->clear();
                break;
            }

            default:
            {
                break;
            }
        }
    }

    /*!
    @brief add an object to an array

    Appends the given element @a val to the end of the JSON value. If the
    function is called on a JSON null value, an empty array is created before
    appending @a val.

    @param[in] val the value to add to the JSON array

    @throw std::domain_error when called on a type other than JSON array or
    null; example: `"cannot use push_back() with number"`

    @complexity Amortized constant.

    @liveexample{The example shows how `push_back()` and `+=` can be used to
    add elements to a JSON array. Note how the `null` value was silently
    converted to a JSON array.,push_back}

    @since version 1.0.0
    */
    void push_back(basic_json&& val)
    {
        // push_back only works for null objects or arrays
        if (not(is_null() or is_array()))
        {
            JSON_THROW(std::domain_error("cannot use push_back() with " + type_name()));
        }

        // transform null object into an array
        if (is_null())
        {
            m_type = value_t::array;
            m_value = value_t::array;
            assert_invariant();
        }

        // add element to array (move semantics)
        m_value.array->push_back(std::move(val));
        // invalidate object
        val.m_type = value_t::null;
    }

    /*!
    @brief add an object to an array
    @copydoc push_back(basic_json&&)
    */
    reference operator+=(basic_json&& val)
    {
        push_back(std::move(val));
        return *this;
    }

    /*!
    @brief add an object to an array
    @copydoc push_back(basic_json&&)
    */
    void push_back(const basic_json& val)
    {
        // push_back only works for null objects or arrays
        if (not(is_null() or is_array()))
        {
            JSON_THROW(std::domain_error("cannot use push_back() with " + type_name()));
        }

        // transform null object into an array
        if (is_null())
        {
            m_type = value_t::array;
            m_value = value_t::array;
            assert_invariant();
        }

        // add element to array
        m_value.array->push_back(val);
    }

    /*!
    @brief add an object to an array
    @copydoc push_back(basic_json&&)
    */
    reference operator+=(const basic_json& val)
    {
        push_back(val);
        return *this;
    }

    /*!
    @brief add an object to an object

    Inserts the given element @a val to the JSON object. If the function is
    called on a JSON null value, an empty object is created before inserting
    @a val.

    @param[in] val the value to add to the JSON object

    @throw std::domain_error when called on a type other than JSON object or
    null; example: `"cannot use push_back() with number"`

    @complexity Logarithmic in the size of the container, O(log(`size()`)).

    @liveexample{The example shows how `push_back()` and `+=` can be used to
    add elements to a JSON object. Note how the `null` value was silently
    converted to a JSON object.,push_back__object_t__value}

    @since version 1.0.0
    */
    void push_back(const typename object_t::value_type& val)
    {
        // push_back only works for null objects or objects
        if (not(is_null() or is_object()))
        {
            JSON_THROW(std::domain_error("cannot use push_back() with " + type_name()));
        }

        // transform null object into an object
        if (is_null())
        {
            m_type = value_t::object;
            m_value = value_t::object;
            assert_invariant();
        }

        // add element to array
        m_value.object->insert(val);
    }

    /*!
    @brief add an object to an object
    @copydoc push_back(const typename object_t::value_type&)
    */
    reference operator+=(const typename object_t::value_type& val)
    {
        push_back(val);
        return *this;
    }

    /*!
    @brief add an object to an object

    This function allows to use `push_back` with an initializer list. In case

    1. the current value is an object,
    2. the initializer list @a init contains only two elements, and
    3. the first element of @a init is a string,

    @a init is converted into an object element and added using
    @ref push_back(const typename object_t::value_type&). Otherwise, @a init
    is converted to a JSON value and added using @ref push_back(basic_json&&).

    @param init  an initializer list

    @complexity Linear in the size of the initializer list @a init.

    @note This function is required to resolve an ambiguous overload error,
          because pairs like `{"key", "value"}` can be both interpreted as
          `object_t::value_type` or `std::initializer_list<basic_json>`, see
          https://github.com/nlohmann/json/issues/235 for more information.

    @liveexample{The example shows how initializer lists are treated as
    objects when possible.,push_back__initializer_list}
    */
    void push_back(std::initializer_list<basic_json> init)
    {
        if (is_object() and init.size() == 2 and init.begin()->is_string())
        {
            const string_t key = *init.begin();
            push_back(typename object_t::value_type(key, *(init.begin() + 1)));
        }
        else
        {
            push_back(basic_json(init));
        }
    }

    /*!
    @brief add an object to an object
    @copydoc push_back(std::initializer_list<basic_json>)
    */
    reference operator+=(std::initializer_list<basic_json> init)
    {
        push_back(init);
        return *this;
    }

    /*!
    @brief add an object to an array

    Creates a JSON value from the passed parameters @a args to the end of the
    JSON value. If the function is called on a JSON null value, an empty array
    is created before appending the value created from @a args.

    @param[in] args arguments to forward to a constructor of @ref basic_json
    @tparam Args compatible types to create a @ref basic_json object

    @throw std::domain_error when called on a type other than JSON array or
    null; example: `"cannot use emplace_back() with number"`

    @complexity Amortized constant.

    @liveexample{The example shows how `push_back()` can be used to add
    elements to a JSON array. Note how the `null` value was silently converted
    to a JSON array.,emplace_back}

    @since version 2.0.8
    */
    template<class... Args>
    void emplace_back(Args&& ... args)
    {
        // emplace_back only works for null objects or arrays
        if (not(is_null() or is_array()))
        {
            JSON_THROW(std::domain_error("cannot use emplace_back() with " + type_name()));
        }

        // transform null object into an array
        if (is_null())
        {
            m_type = value_t::array;
            m_value = value_t::array;
            assert_invariant();
        }

        // add element to array (perfect forwarding)
        m_value.array->emplace_back(std::forward<Args>(args)...);
    }

    /*!
    @brief add an object to an object if key does not exist

    Inserts a new element into a JSON object constructed in-place with the
    given @a args if there is no element with the key in the container. If the
    function is called on a JSON null value, an empty object is created before
    appending the value created from @a args.

    @param[in] args arguments to forward to a constructor of @ref basic_json
    @tparam Args compatible types to create a @ref basic_json object

    @return a pair consisting of an iterator to the inserted element, or the
            already-existing element if no insertion happened, and a bool
            denoting whether the insertion took place.

    @throw std::domain_error when called on a type other than JSON object or
    null; example: `"cannot use emplace() with number"`

    @complexity Logarithmic in the size of the container, O(log(`size()`)).

    @liveexample{The example shows how `emplace()` can be used to add elements
    to a JSON object. Note how the `null` value was silently converted to a
    JSON object. Further note how no value is added if there was already one
    value stored with the same key.,emplace}

    @since version 2.0.8
    */
    template<class... Args>
    std::pair<iterator, bool> emplace(Args&& ... args)
    {
        // emplace only works for null objects or arrays
        if (not(is_null() or is_object()))
        {
            JSON_THROW(std::domain_error("cannot use emplace() with " + type_name()));
        }

        // transform null object into an object
        if (is_null())
        {
            m_type = value_t::object;
            m_value = value_t::object;
            assert_invariant();
        }

        // add element to array (perfect forwarding)
        auto res = m_value.object->emplace(std::forward<Args>(args)...);
        // create result iterator and set iterator to the result of emplace
        auto it = begin();
        it.m_it.object_iterator = res.first;

        // return pair of iterator and boolean
        return {it, res.second};
    }

    /*!
    @brief inserts element

    Inserts element @a val before iterator @a pos.

    @param[in] pos iterator before which the content will be inserted; may be
    the end() iterator
    @param[in] val element to insert
    @return iterator pointing to the inserted @a val.

    @throw std::domain_error if called on JSON values other than arrays;
    example: `"cannot use insert() with string"`
    @throw std::domain_error if @a pos is not an iterator of *this; example:
    `"iterator does not fit current value"`

    @complexity Constant plus linear in the distance between @a pos and end of
    the container.

    @liveexample{The example shows how `insert()` is used.,insert}

    @since version 1.0.0
    */
    iterator insert(const_iterator pos, const basic_json& val)
    {
        // insert only works for arrays
        if (is_array())
        {
            // check if iterator pos fits to this JSON value
            if (pos.m_object != this)
            {
                JSON_THROW(std::domain_error("iterator does not fit current value"));
            }

            // insert to array and return iterator
            iterator result(this);
            result.m_it.array_iterator = m_value.array->insert(pos.m_it.array_iterator, val);
            return result;
        }

        JSON_THROW(std::domain_error("cannot use insert() with " + type_name()));
    }

    /*!
    @brief inserts element
    @copydoc insert(const_iterator, const basic_json&)
    */
    iterator insert(const_iterator pos, basic_json&& val)
    {
        return insert(pos, val);
    }

    /*!
    @brief inserts elements

    Inserts @a cnt copies of @a val before iterator @a pos.

    @param[in] pos iterator before which the content will be inserted; may be
    the end() iterator
    @param[in] cnt number of copies of @a val to insert
    @param[in] val element to insert
    @return iterator pointing to the first element inserted, or @a pos if
    `cnt==0`

    @throw std::domain_error if called on JSON values other than arrays;
    example: `"cannot use insert() with string"`
    @throw std::domain_error if @a pos is not an iterator of *this; example:
    `"iterator does not fit current value"`

    @complexity Linear in @a cnt plus linear in the distance between @a pos
    and end of the container.

    @liveexample{The example shows how `insert()` is used.,insert__count}

    @since version 1.0.0
    */
    iterator insert(const_iterator pos, size_type cnt, const basic_json& val)
    {
        // insert only works for arrays
        if (is_array())
        {
            // check if iterator pos fits to this JSON value
            if (pos.m_object != this)
            {
                JSON_THROW(std::domain_error("iterator does not fit current value"));
            }

            // insert to array and return iterator
            iterator result(this);
            result.m_it.array_iterator = m_value.array->insert(pos.m_it.array_iterator, cnt, val);
            return result;
        }

        JSON_THROW(std::domain_error("cannot use insert() with " + type_name()));
    }

    /*!
    @brief inserts elements

    Inserts elements from range `[first, last)` before iterator @a pos.

    @param[in] pos iterator before which the content will be inserted; may be
    the end() iterator
    @param[in] first begin of the range of elements to insert
    @param[in] last end of the range of elements to insert

    @throw std::domain_error if called on JSON values other than arrays;
    example: `"cannot use insert() with string"`
    @throw std::domain_error if @a pos is not an iterator of *this; example:
    `"iterator does not fit current value"`
    @throw std::domain_error if @a first and @a last do not belong to the same
    JSON value; example: `"iterators do not fit"`
    @throw std::domain_error if @a first or @a last are iterators into
    container for which insert is called; example: `"passed iterators may not
    belong to container"`

    @return iterator pointing to the first element inserted, or @a pos if
    `first==last`

    @complexity Linear in `std::distance(first, last)` plus linear in the
    distance between @a pos and end of the container.

    @liveexample{The example shows how `insert()` is used.,insert__range}

    @since version 1.0.0
    */
    iterator insert(const_iterator pos, const_iterator first, const_iterator last)
    {
        // insert only works for arrays
        if (not is_array())
        {
            JSON_THROW(std::domain_error("cannot use insert() with " + type_name()));
        }

        // check if iterator pos fits to this JSON value
        if (pos.m_object != this)
        {
            JSON_THROW(std::domain_error("iterator does not fit current value"));
        }

        // check if range iterators belong to the same JSON object
        if (first.m_object != last.m_object)
        {
            JSON_THROW(std::domain_error("iterators do not fit"));
        }

        if (first.m_object == this or last.m_object == this)
        {
            JSON_THROW(std::domain_error("passed iterators may not belong to container"));
        }

        // insert to array and return iterator
        iterator result(this);
        result.m_it.array_iterator = m_value.array->insert(
                                         pos.m_it.array_iterator,
                                         first.m_it.array_iterator,
                                         last.m_it.array_iterator);
        return result;
    }

    /*!
    @brief inserts elements

    Inserts elements from initializer list @a ilist before iterator @a pos.

    @param[in] pos iterator before which the content will be inserted; may be
    the end() iterator
    @param[in] ilist initializer list to insert the values from

    @throw std::domain_error if called on JSON values other than arrays;
    example: `"cannot use insert() with string"`
    @throw std::domain_error if @a pos is not an iterator of *this; example:
    `"iterator does not fit current value"`

    @return iterator pointing to the first element inserted, or @a pos if
    `ilist` is empty

    @complexity Linear in `ilist.size()` plus linear in the distance between
    @a pos and end of the container.

    @liveexample{The example shows how `insert()` is used.,insert__ilist}

    @since version 1.0.0
    */
    iterator insert(const_iterator pos, std::initializer_list<basic_json> ilist)
    {
        // insert only works for arrays
        if (not is_array())
        {
            JSON_THROW(std::domain_error("cannot use insert() with " + type_name()));
        }

        // check if iterator pos fits to this JSON value
        if (pos.m_object != this)
        {
            JSON_THROW(std::domain_error("iterator does not fit current value"));
        }

        // insert to array and return iterator
        iterator result(this);
        result.m_it.array_iterator = m_value.array->insert(pos.m_it.array_iterator, ilist);
        return result;
    }

    /*!
    @brief exchanges the values

    Exchanges the contents of the JSON value with those of @a other. Does not
    invoke any move, copy, or swap operations on individual elements. All
    iterators and references remain valid. The past-the-end iterator is
    invalidated.

    @param[in,out] other JSON value to exchange the contents with

    @complexity Constant.

    @liveexample{The example below shows how JSON values can be swapped with
    `swap()`.,swap__reference}

    @since version 1.0.0
    */
    void swap(reference other) noexcept (
        std::is_nothrow_move_constructible<value_t>::value and
        std::is_nothrow_move_assignable<value_t>::value and
        std::is_nothrow_move_constructible<json_value>::value and
        std::is_nothrow_move_assignable<json_value>::value
    )
    {
        std::swap(m_type, other.m_type);
        std::swap(m_value, other.m_value);
        assert_invariant();
    }

    /*!
    @brief exchanges the values

    Exchanges the contents of a JSON array with those of @a other. Does not
    invoke any move, copy, or swap operations on individual elements. All
    iterators and references remain valid. The past-the-end iterator is
    invalidated.

    @param[in,out] other array to exchange the contents with

    @throw std::domain_error when JSON value is not an array; example:
    `"cannot use swap() with string"`

    @complexity Constant.

    @liveexample{The example below shows how arrays can be swapped with
    `swap()`.,swap__array_t}

    @since version 1.0.0
    */
    void swap(array_t& other)
    {
        // swap only works for arrays
        if (is_array())
        {
            std::swap(*(m_value.array), other);
        }
        else
        {
            JSON_THROW(std::domain_error("cannot use swap() with " + type_name()));
        }
    }

    /*!
    @brief exchanges the values

    Exchanges the contents of a JSON object with those of @a other. Does not
    invoke any move, copy, or swap operations on individual elements. All
    iterators and references remain valid. The past-the-end iterator is
    invalidated.

    @param[in,out] other object to exchange the contents with

    @throw std::domain_error when JSON value is not an object; example:
    `"cannot use swap() with string"`

    @complexity Constant.

    @liveexample{The example below shows how objects can be swapped with
    `swap()`.,swap__object_t}

    @since version 1.0.0
    */
    void swap(object_t& other)
    {
        // swap only works for objects
        if (is_object())
        {
            std::swap(*(m_value.object), other);
        }
        else
        {
            JSON_THROW(std::domain_error("cannot use swap() with " + type_name()));
        }
    }

    /*!
    @brief exchanges the values

    Exchanges the contents of a JSON string with those of @a other. Does not
    invoke any move, copy, or swap operations on individual elements. All
    iterators and references remain valid. The past-the-end iterator is
    invalidated.

    @param[in,out] other string to exchange the contents with

    @throw std::domain_error when JSON value is not a string; example: `"cannot
    use swap() with boolean"`

    @complexity Constant.

    @liveexample{The example below shows how strings can be swapped with
    `swap()`.,swap__string_t}

    @since version 1.0.0
    */
    void swap(string_t& other)
    {
        // swap only works for strings
        if (is_string())
        {
            std::swap(*(m_value.string), other);
        }
        else
        {
            JSON_THROW(std::domain_error("cannot use swap() with " + type_name()));
        }
    }

    /// @}

  public:
    //////////////////////////////////////////
    // lexicographical comparison operators //
    //////////////////////////////////////////

    /// @name lexicographical comparison operators
    /// @{

    /*!
    @brief comparison: equal

    Compares two JSON values for equality according to the following rules:
    - Two JSON values are equal if (1) they are from the same type and (2)
      their stored values are the same.
    - Integer and floating-point numbers are automatically converted before
      comparison. Floating-point numbers are compared indirectly: two
      floating-point numbers `f1` and `f2` are considered equal if neither
      `f1 > f2` nor `f2 > f1` holds.
    - Two JSON null values are equal.

    @param[in] lhs  first JSON value to consider
    @param[in] rhs  second JSON value to consider
    @return whether the values @a lhs and @a rhs are equal

    @complexity Linear.

    @liveexample{The example demonstrates comparing several JSON
    types.,operator__equal}

    @since version 1.0.0
    */
    friend bool operator==(const_reference lhs, const_reference rhs) noexcept
    {
        const auto lhs_type = lhs.type();
        const auto rhs_type = rhs.type();

        if (lhs_type == rhs_type)
        {
            switch (lhs_type)
            {
                case value_t::array:
                {
                    return *lhs.m_value.array == *rhs.m_value.array;
                }
                case value_t::object:
                {
                    return *lhs.m_value.object == *rhs.m_value.object;
                }
                case value_t::null:
                {
                    return true;
                }
                case value_t::string:
                {
                    return *lhs.m_value.string == *rhs.m_value.string;
                }
                case value_t::boolean:
                {
                    return lhs.m_value.boolean == rhs.m_value.boolean;
                }
                case value_t::number_integer:
                {
                    return lhs.m_value.number_integer == rhs.m_value.number_integer;
                }
                case value_t::number_unsigned:
                {
                    return lhs.m_value.number_unsigned == rhs.m_value.number_unsigned;
                }
                case value_t::number_float:
                {
                    return lhs.m_value.number_float == rhs.m_value.number_float;
                }
                default:
                {
                    return false;
                }
            }
        }
        else if (lhs_type == value_t::number_integer and rhs_type == value_t::number_float)
        {
            return static_cast<number_float_t>(lhs.m_value.number_integer) == rhs.m_value.number_float;
        }
        else if (lhs_type == value_t::number_float and rhs_type == value_t::number_integer)
        {
            return lhs.m_value.number_float == static_cast<number_float_t>(rhs.m_value.number_integer);
        }
        else if (lhs_type == value_t::number_unsigned and rhs_type == value_t::number_float)
        {
            return static_cast<number_float_t>(lhs.m_value.number_unsigned) == rhs.m_value.number_float;
        }
        else if (lhs_type == value_t::number_float and rhs_type == value_t::number_unsigned)
        {
            return lhs.m_value.number_float == static_cast<number_float_t>(rhs.m_value.number_unsigned);
        }
        else if (lhs_type == value_t::number_unsigned and rhs_type == value_t::number_integer)
        {
            return static_cast<number_integer_t>(lhs.m_value.number_unsigned) == rhs.m_value.number_integer;
        }
        else if (lhs_type == value_t::number_integer and rhs_type == value_t::number_unsigned)
        {
            return lhs.m_value.number_integer == static_cast<number_integer_t>(rhs.m_value.number_unsigned);
        }

        return false;
    }

    /*!
    @brief comparison: equal
    @copydoc operator==(const_reference, const_reference)
    */
    template<typename ScalarType, typename std::enable_if<
                 std::is_scalar<ScalarType>::value, int>::type = 0>
    friend bool operator==(const_reference lhs, const ScalarType rhs) noexcept
    {
        return (lhs == basic_json(rhs));
    }

    /*!
    @brief comparison: equal
    @copydoc operator==(const_reference, const_reference)
    */
    template<typename ScalarType, typename std::enable_if<
                 std::is_scalar<ScalarType>::value, int>::type = 0>
    friend bool operator==(const ScalarType lhs, const_reference rhs) noexcept
    {
        return (basic_json(lhs) == rhs);
    }

    /*!
    @brief comparison: not equal

    Compares two JSON values for inequality by calculating `not (lhs == rhs)`.

    @param[in] lhs  first JSON value to consider
    @param[in] rhs  second JSON value to consider
    @return whether the values @a lhs and @a rhs are not equal

    @complexity Linear.

    @liveexample{The example demonstrates comparing several JSON
    types.,operator__notequal}

    @since version 1.0.0
    */
    friend bool operator!=(const_reference lhs, const_reference rhs) noexcept
    {
        return not (lhs == rhs);
    }

    /*!
    @brief comparison: not equal
    @copydoc operator!=(const_reference, const_reference)
    */
    template<typename ScalarType, typename std::enable_if<
                 std::is_scalar<ScalarType>::value, int>::type = 0>
    friend bool operator!=(const_reference lhs, const ScalarType rhs) noexcept
    {
        return (lhs != basic_json(rhs));
    }

    /*!
    @brief comparison: not equal
    @copydoc operator!=(const_reference, const_reference)
    */
    template<typename ScalarType, typename std::enable_if<
                 std::is_scalar<ScalarType>::value, int>::type = 0>
    friend bool operator!=(const ScalarType lhs, const_reference rhs) noexcept
    {
        return (basic_json(lhs) != rhs);
    }

    /*!
    @brief comparison: less than

    Compares whether one JSON value @a lhs is less than another JSON value @a
    rhs according to the following rules:
    - If @a lhs and @a rhs have the same type, the values are compared using
      the default `<` operator.
    - Integer and floating-point numbers are automatically converted before
      comparison
    - In case @a lhs and @a rhs have different types, the values are ignored
      and the order of the types is considered, see
      @ref operator<(const value_t, const value_t).

    @param[in] lhs  first JSON value to consider
    @param[in] rhs  second JSON value to consider
    @return whether @a lhs is less than @a rhs

    @complexity Linear.

    @liveexample{The example demonstrates comparing several JSON
    types.,operator__less}

    @since version 1.0.0
    */
    friend bool operator<(const_reference lhs, const_reference rhs) noexcept
    {
        const auto lhs_type = lhs.type();
        const auto rhs_type = rhs.type();

        if (lhs_type == rhs_type)
        {
            switch (lhs_type)
            {
                case value_t::array:
                {
                    return *lhs.m_value.array < *rhs.m_value.array;
                }
                case value_t::object:
                {
                    return *lhs.m_value.object < *rhs.m_value.object;
                }
                case value_t::null:
                {
                    return false;
                }
                case value_t::string:
                {
                    return *lhs.m_value.string < *rhs.m_value.string;
                }
                case value_t::boolean:
                {
                    return lhs.m_value.boolean < rhs.m_value.boolean;
                }
                case value_t::number_integer:
                {
                    return lhs.m_value.number_integer < rhs.m_value.number_integer;
                }
                case value_t::number_unsigned:
                {
                    return lhs.m_value.number_unsigned < rhs.m_value.number_unsigned;
                }
                case value_t::number_float:
                {
                    return lhs.m_value.number_float < rhs.m_value.number_float;
                }
                default:
                {
                    return false;
                }
            }
        }
        else if (lhs_type == value_t::number_integer and rhs_type == value_t::number_float)
        {
            return static_cast<number_float_t>(lhs.m_value.number_integer) < rhs.m_value.number_float;
        }
        else if (lhs_type == value_t::number_float and rhs_type == value_t::number_integer)
        {
            return lhs.m_value.number_float < static_cast<number_float_t>(rhs.m_value.number_integer);
        }
        else if (lhs_type == value_t::number_unsigned and rhs_type == value_t::number_float)
        {
            return static_cast<number_float_t>(lhs.m_value.number_unsigned) < rhs.m_value.number_float;
        }
        else if (lhs_type == value_t::number_float and rhs_type == value_t::number_unsigned)
        {
            return lhs.m_value.number_float < static_cast<number_float_t>(rhs.m_value.number_unsigned);
        }
        else if (lhs_type == value_t::number_integer and rhs_type == value_t::number_unsigned)
        {
            return lhs.m_value.number_integer < static_cast<number_integer_t>(rhs.m_value.number_unsigned);
        }
        else if (lhs_type == value_t::number_unsigned and rhs_type == value_t::number_integer)
        {
            return static_cast<number_integer_t>(lhs.m_value.number_unsigned) < rhs.m_value.number_integer;
        }

        // We only reach this line if we cannot compare values. In that case,
        // we compare types. Note we have to call the operator explicitly,
        // because MSVC has problems otherwise.
        return operator<(lhs_type, rhs_type);
    }

    /*!
    @brief comparison: less than or equal

    Compares whether one JSON value @a lhs is less than or equal to another
    JSON value by calculating `not (rhs < lhs)`.

    @param[in] lhs  first JSON value to consider
    @param[in] rhs  second JSON value to consider
    @return whether @a lhs is less than or equal to @a rhs

    @complexity Linear.

    @liveexample{The example demonstrates comparing several JSON
    types.,operator__greater}

    @since version 1.0.0
    */
    friend bool operator<=(const_reference lhs, const_reference rhs) noexcept
    {
        return not (rhs < lhs);
    }

    /*!
    @brief comparison: greater than

    Compares whether one JSON value @a lhs is greater than another
    JSON value by calculating `not (lhs <= rhs)`.

    @param[in] lhs  first JSON value to consider
    @param[in] rhs  second JSON value to consider
    @return whether @a lhs is greater than to @a rhs

    @complexity Linear.

    @liveexample{The example demonstrates comparing several JSON
    types.,operator__lessequal}

    @since version 1.0.0
    */
    friend bool operator>(const_reference lhs, const_reference rhs) noexcept
    {
        return not (lhs <= rhs);
    }

    /*!
    @brief comparison: greater than or equal

    Compares whether one JSON value @a lhs is greater than or equal to another
    JSON value by calculating `not (lhs < rhs)`.

    @param[in] lhs  first JSON value to consider
    @param[in] rhs  second JSON value to consider
    @return whether @a lhs is greater than or equal to @a rhs

    @complexity Linear.

    @liveexample{The example demonstrates comparing several JSON
    types.,operator__greaterequal}

    @since version 1.0.0
    */
    friend bool operator>=(const_reference lhs, const_reference rhs) noexcept
    {
        return not (lhs < rhs);
    }

    /// @}


    ///////////////////
    // serialization //
    ///////////////////

    /// @name serialization
    /// @{

    /*!
    @brief serialize to stream

    Serialize the given JSON value @a j to the output stream @a o. The JSON
    value will be serialized using the @ref dump member function. The
    indentation of the output can be controlled with the member variable
    `width` of the output stream @a o. For instance, using the manipulator
    `std::setw(4)` on @a o sets the indentation level to `4` and the
    serialization result is the same as calling `dump(4)`.

    @param[in,out] o  stream to serialize to
    @param[in] j  JSON value to serialize

    @return the stream @a o

    @complexity Linear.

    @liveexample{The example below shows the serialization with different
    parameters to `width` to adjust the indentation level.,operator_serialize}

    @since version 1.0.0
    */
    friend std::ostream& operator<<(std::ostream& o, const basic_json& j)
    {
        // read width member and use it as indentation parameter if nonzero
        const bool pretty_print = (o.width() > 0);
        const auto indentation = (pretty_print ? o.width() : 0);

        // reset width to 0 for subsequent calls to this stream
        o.width(0);

        // do the actual serialization
        j.dump(o, pretty_print, static_cast<unsigned int>(indentation));

        return o;
    }

    /*!
    @brief serialize to stream
    @copydoc operator<<(std::ostream&, const basic_json&)
    */
    friend std::ostream& operator>>(const basic_json& j, std::ostream& o)
    {
        return o << j;
    }

    /// @}


    /////////////////////
    // deserialization //
    /////////////////////

    /// @name deserialization
    /// @{

    /*!
    @brief deserialize from an array

    This function reads from an array of 1-byte values.

    @pre Each element of the container has a size of 1 byte. Violating this
    precondition yields undefined behavior. **This precondition is enforced
    with a static assertion.**

    @param[in] array  array to read from
    @param[in] cb  a parser callback function of type @ref parser_callback_t
    which is used to control the deserialization by filtering unwanted values
    (optional)

    @return result of the deserialization

    @complexity Linear in the length of the input. The parser is a predictive
    LL(1) parser. The complexity can be higher if the parser callback function
    @a cb has a super-linear complexity.

    @note A UTF-8 byte order mark is silently ignored.

    @liveexample{The example below demonstrates the `parse()` function reading
    from an array.,parse__array__parser_callback_t}

    @since version 2.0.3
    */
    template<class T, std::size_t N>
    static basic_json parse(T (&array)[N],
                            const parser_callback_t cb = nullptr)
    {
        // delegate the call to the iterator-range parse overload
        return parse(std::begin(array), std::end(array), cb);
    }

    /*!
    @brief deserialize from string literal

    @tparam CharT character/literal type with size of 1 byte
    @param[in] s  string literal to read a serialized JSON value from
    @param[in] cb a parser callback function of type @ref parser_callback_t
    which is used to control the deserialization by filtering unwanted values
    (optional)

    @return result of the deserialization

    @complexity Linear in the length of the input. The parser is a predictive
    LL(1) parser. The complexity can be higher if the parser callback function
    @a cb has a super-linear complexity.

    @note A UTF-8 byte order mark is silently ignored.
    @note String containers like `std::string` or @ref string_t can be parsed
          with @ref parse(const ContiguousContainer&, const parser_callback_t)

    @liveexample{The example below demonstrates the `parse()` function with
    and without callback function.,parse__string__parser_callback_t}

    @sa @ref parse(std::istream&, const parser_callback_t) for a version that
    reads from an input stream

    @since version 1.0.0 (originally for @ref string_t)
    */
    template<typename CharT, typename std::enable_if<
                 std::is_pointer<CharT>::value and
                 std::is_integral<typename std::remove_pointer<CharT>::type>::value and
                 sizeof(typename std::remove_pointer<CharT>::type) == 1, int>::type = 0>
    static basic_json parse(const CharT s,
                            const parser_callback_t cb = nullptr)
    {
        return parser(reinterpret_cast<const char*>(s), cb).parse();
    }

    /*!
    @brief deserialize from stream

    @param[in,out] i  stream to read a serialized JSON value from
    @param[in] cb a parser callback function of type @ref parser_callback_t
    which is used to control the deserialization by filtering unwanted values
    (optional)

    @return result of the deserialization

    @complexity Linear in the length of the input. The parser is a predictive
    LL(1) parser. The complexity can be higher if the parser callback function
    @a cb has a super-linear complexity.

    @note A UTF-8 byte order mark is silently ignored.

    @liveexample{The example below demonstrates the `parse()` function with
    and without callback function.,parse__istream__parser_callback_t}

    @sa @ref parse(const CharT, const parser_callback_t) for a version
    that reads from a string

    @since version 1.0.0
    */
    static basic_json parse(std::istream& i,
                            const parser_callback_t cb = nullptr)
    {
        return parser(i, cb).parse();
    }

    /*!
    @copydoc parse(std::istream&, const parser_callback_t)
    */
    static basic_json parse(std::istream&& i,
                            const parser_callback_t cb = nullptr)
    {
        return parser(i, cb).parse();
    }

    /*!
    @brief deserialize from an iterator range with contiguous storage

    This function reads from an iterator range of a container with contiguous
    storage of 1-byte values. Compatible container types include
    `std::vector`, `std::string`, `std::array`, `std::valarray`, and
    `std::initializer_list`. Furthermore, C-style arrays can be used with
    `std::begin()`/`std::end()`. User-defined containers can be used as long
    as they implement random-access iterators and a contiguous storage.

    @pre The iterator range is contiguous. Violating this precondition yields
    undefined behavior. **This precondition is enforced with an assertion.**
    @pre Each element in the range has a size of 1 byte. Violating this
    precondition yields undefined behavior. **This precondition is enforced
    with a static assertion.**

    @warning There is no way to enforce all preconditions at compile-time. If
             the function is called with noncompliant iterators and with
             assertions switched off, the behavior is undefined and will most
             likely yield segmentation violation.

    @tparam IteratorType iterator of container with contiguous storage
    @param[in] first  begin of the range to parse (included)
    @param[in] last  end of the range to parse (excluded)
    @param[in] cb  a parser callback function of type @ref parser_callback_t
    which is used to control the deserialization by filtering unwanted values
    (optional)

    @return result of the deserialization

    @complexity Linear in the length of the input. The parser is a predictive
    LL(1) parser. The complexity can be higher if the parser callback function
    @a cb has a super-linear complexity.

    @note A UTF-8 byte order mark is silently ignored.

    @liveexample{The example below demonstrates the `parse()` function reading
    from an iterator range.,parse__iteratortype__parser_callback_t}

    @since version 2.0.3
    */
    template<class IteratorType, typename std::enable_if<
                 std::is_base_of<
                     std::random_access_iterator_tag,
                     typename std::iterator_traits<IteratorType>::iterator_category>::value, int>::type = 0>
    static basic_json parse(IteratorType first, IteratorType last,
                            const parser_callback_t cb = nullptr)
    {
        // assertion to check that the iterator range is indeed contiguous,
        // see http://stackoverflow.com/a/35008842/266378 for more discussion
        assert(std::accumulate(first, last, std::pair<bool, int>(true, 0),
                               [&first](std::pair<bool, int> res, decltype(*first) val)
        {
            res.first &= (val == *(std::next(std::addressof(*first), res.second++)));
            return res;
        }).first);

        // assertion to check that each element is 1 byte long
        static_assert(sizeof(typename std::iterator_traits<IteratorType>::value_type) == 1,