[cxx-abi-dev] The magic of uncalled copy constructors; documenting the bug

[John McCall]

On Feb 9, 2014, at 6:17 PM, Richard Smith <richardsmith at google.com> wrote:
> On 9 February 2014 05:37, Hubert Tong <hstong at ca.ibm.com> wrote:
> Core Issue 1590 mentions that the trivial copy or move constructor should be non-deleted and accessible.
> Replacing the pair of copy constructors below with private, deleted copy and move constructors seems to make GCC avoid the "magic" copy, but not Clang.
> Which compiler has the preferred behaviour?
> 
> I think GCC does, whereas Clang obeys the current description of the ABI.
> 
> Under the current wording of core issue 1590, we're not allowed to perform a 'trivial' copy here. I think we should update the ABI document (in 3.1.1/1) to make this work (perhaps something equivalent to replacing "non-trivial" with "non-trivial, non-public, or deleted" each time it occurs).

gcc’s rule appears to just be “non-trivial or deleted”, which I think is reasonable for the ABI to adopt.  Would you like to write up a proposal for that?

“Accessible” does not mean “public”.  That’s a standards bug: we need to be able to decide the ABI signature of a function at its point of declaration, not have it be dependent on whether there exist (or might exist) calls to it which would not be allowed to use a privately-defaulted trivial copy constructor.

Note that the pre-C++11 idiom for suppressing the copy-constructor would also make it non-trivial, so that’s okay.

John.
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