ROOT::Experimental::IsCollectionProxy< T, typename > Struct Template Reference

template<typename T, typename = void>
struct ROOT::Experimental::IsCollectionProxy< T, typename >

The point here is that we can only tell at run time if a class has an associated collection proxy.

For compile time, in the first iteration of this PR we had an extra template argument that acted as a "tag" to differentiate the RField specialization for classes with an associated collection proxy (inherits RProxiedCollectionField) from the RField primary template definition (RClassField-derived), as in:

auto field = std::make_unique<RField<MyClass>>("klass");
// vs
auto otherField = std::make_unique<RField<MyClass, ROOT::Experimental::TagIsCollectionProxy>>("klass");

That is convenient only for non-nested types, i.e. it doesn't work with, e.g. RField<std::vector<MyClass>, ROOT::Experimental::TagIsCollectionProxy>, as the tag is not forwarded to the instantiation of the inner RField (that for the value type of the vector). The following two possible solutions were considered:

• A wrapper type (much like ntuple/v7/inc/ROOT/RNTupleUtil.hxx:49), that helps to differentiate both cases. There we would have:

  auto field = std::make_unique<RField<RProxiedCollection<MyClass>>>("klass"); // Using collection proxy

• A helper IsCollectionProxy<T> type, that can be used in a similar way to those in the <type_traits> header. We found this more convenient and is the implemented thing below. Here, classes can be marked as a collection proxy with either of the following two forms (whichever is more convenient for the user):

  template <>
  struct IsCollectionProxy<MyClass> : std::true_type {};

  or by adding a member type to the class as follows:

  class MyClass {
  public:
     using IsCollectionProxy = std::true_type;
  };

Of course, there is another possible solution which is to have a single RClassField that implements both the regular-class and the collection-proxy behaviors, and always chooses appropriately at run time. We found that less clean and probably has more overhead, as most probably it involves an additional branch + call in each of the member functions.

Definition at line 247 of file RFieldProxiedCollection.hxx.

#include <ROOT/RField/RFieldProxiedCollection.hxx>

Inheritance diagram for ROOT::Experimental::IsCollectionProxy< T, typename >:

Collaboration diagram for ROOT::Experimental::IsCollectionProxy< T, typename >:

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The documentation for this struct was generated from the following file:

• tree/ntuple/v7/inc/ROOT/RField/RFieldProxiedCollection.hxx