On Feb 9, 2014, at 6:17 PM, Richard Smith <[email protected]> wrote:
> On 9 February 2014 05:37, Hubert Tong <[email protected]> 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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