Re: copy

From: OKUMURA, Akira <>
Date: Thu, 22 Jun 2006 19:36:49 +0900

Hello Axel,

Thank you so much.

> These copy constructors /
> op= are declared but not implemented to _prevent_ the compiler from
> generating a default one

Where are they declared ? In TSpectrum.h/.cxx ? I could not locate the declaration.
So I wondered why they appeared in the html document.


Institute for Cosmic Ray Research, University of Tokyo 5-1-5 Kashiwanoha Kashiwa Chiba 277-8582 Japan Phone/Fax : +81 4-7136-3153
Skype : okumura.akira

On 2006/06/22, at 17:48, Axel Naumann wrote:

> Hi,
> OKUMURA, Akira wrote:
>> Hello ROOTers,
>> I have several questions about copy constructors and assignment
>> operators.
>> 1) Automatic copy constructor and assignment operator
>> ROOT classes have copy constructor and assignment operator which are
>> documented in html.
>> However some of them are not implemented in their sources nor
>> headers.
>> For example,
>> Does this mean that copy constructors and assignment operators
>> will be
>> created by compilers automatically ?
>> If so, it is very confusing if the automatic ones are listed in
>> the html
>> document.
> That's a good point - the documentation part. These copy
> constructors /
> op= are declared but not implemented to _prevent_ the compiler from
> generating a default one, because the author of the class has decided
> that the execution of its proper, hand-written version is too costly,
> and these objects should not be _accidentally_ copied, only with an
> explicit call to Clone(), and the automatically generated version is
> insufficient. It's also about shallow copies (which are the ones
> provided by the compiler) - they usually have side effects that the
> author of the class foresaw, and wanted to prevent. Uh, I see you're
> mentioning this yourself.
> So it's perfectly legal to declare them but not implement them (so the
> user accidentally using them gets a linker error). But this should be
> documented, and currently a) THtml doesn't try to document which
> methods
> don't have an implementation, and b) we don't have a doc syntax that
> fits to methods without method blocks. The latter will change in about
> two weeks.
>> 2) Deep copy and shallow copy
>> As to TSpectrum, its assignment operator seems to copy objects as
>> 'shallow copy' in spite of its pointer members.
>> Meanwhile e.g. TH1D has own (not automatic) assignment operator which
>> uses ::Copy() inside it.
>> What is the difference between these two coding styles ?
> From what you describe this is inconsistent, and it should be fixed in
> TSpectrum - but I didn't look at the code. I'm sure the authors
> and / or
> Rene will comment.
>> 3) assignment operator, copy constructor, Copy() and Clone()
>> How to determine appropriate copier from the above without reading
>> the
>> ROOT source ?
> copy c'tor: shallow copy, use if there's no allocation which would be
> shared by two objects. ROOT tries to forbid the usage of this copy
> c'tor
> if a shallow copy is not enough, by declaring but not implementing it.
> assignment op: same thing when it comes to shallowness.
> Theoretically a
> bit less efficient (due to the return value, a temporary object, which
> should get optimizerd away by any reasonable modern compiler).
> Copy(): like Clone, except where stated. A little less deep copy,
> usually ignoring members that are not always needed.
> Clone(): a full, complete, deep copy of an object.
> Cheers, Axel.
Received on Thu Jun 22 2006 - 12:37:06 MEST

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