Re: Guaranteed copy elision

[Yubin Ruan <ablacktshirt@gmail.com>]

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On Sat, Nov 19, 2022 at 6:12 PM Jonathan Wakely <jwakely.gcc@gmail.com>
wrote:

>
>
> 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.
>

Thank you all.

It seems that currently the only way to guarantee  efficiency is to use
move instead.

--
Yubin