Re: Guaranteed copy elision

[Jonathan Wakely <jwakely.gcc@gmail.com>]

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On Tue, 15 Nov 2022, 05:48 Yubin Ruan via Gcc-help, <gcc-help@gcc.gnu.org>
wrote:

> Hi,
>
> As mentioned in cppreference:
> https://en.cppreference.com/w/cpp/language/copy_elision
>
> it is guaranteed in c++17 that copy elision must be applied for some cases
> like
>
>     SomeBigObject SomeBigObject::Factory(...) {
>         SomeBigObject local;
>          ...
>          return local;
>     }
>

No, this is not guaranteed. As the cppreference page explains, it's only
guaranteed for copying temporaries. This is not a temporary.

Elision here is allowed, but not required. It's commonly implemented though.




> (examples taken from https://abseil.io/tips/11 )
>
> but not for cases like
>
>     SomeBigObject SomeBigObject::Factory(...) {
>         SomeBigObject local1;
>         SomeBigObject local2;
>          ...
>
>          if (cond_1) {
>              return local1;
>          } else {
>              return local2;
>          }
>     }
>
> For a c++ user, it is somewhat difficult to be 100% sure that copy elision
> / NVO is applied to the functions' return value above.
>
> To be sure that a object would not be copied, we usually write something
> like
>
>          SomeBigObject obj;
>          func(&obj);
>
> while in most of the cases a one-liner like
>
>          SomeBigObject obj = func();
>
> would suffice.
>
> Is there any language facility to help us guarantee that at compile time
> (such as some kind of static_assert() ) so that we can be confident writing
> those one-liner ?
>

Make it cheap to move, and then it will be moved instead of copied. The
move isn't always elided, but if it's cheap then the code will still be
efficient.


> I know that marking the copy constructor deleted would do the good, but
> copy is needed in some cases.
>
> Thanks
> Yubin
>