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From: Martin B. on 15 Jun 2010 22:56
Mathias Gaunard wrote:
> On 15 juin, 10:05, Peng Yu <pengyu...(a)gmail.com> wrote:
>
>> If there is return value optimization (RVO), I think that I can return
>> tr1::unordered_map as the compile can optimize away unnecessary coping
>> in certain cases. But I think that there cases where RVO doesn't help.
>> For example, if tr1::unordered_map is returned from a function that is
>> only available in the object format (through linkage), the copying can
>> not be avoided, right?
>
> RVO, if implemented, is consistent. NRVO should also be consistent if
> all exit paths from the function return the same named object.
> You should expect any compiler of good quality to implement NRVO, and
> any decent compiler to implement RVO.
>
> Therefore you should return by value if that's what semantically makes
> sense. There is no useless copy (or even move) going on when returning
> a local variable by value (assuming the return type is the same of the
> type of the return expression).
>

Except of course if someone wrote:
container_t fn() { ... }
...
container_t obj;
obj = fn();
In this case (N)RVO cannot work and you always have to copy(move).

br,
Martin

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From: Martin B. on 18 Jun 2010 00:30
Mathias Gaunard wrote:
> On 16 juin, 14:56, "Martin B." <0xCDCDC...(a)gmx.at> wrote:
>
>> Except of course if someone wrote:
>> container_t fn() { ... }
>> ...
>> container_t obj;
>> obj = fn();
>> In this case (N)RVO cannot work and you always have to copy(move).
>
> NRVO or RVO remove useless copies, not assignments.

Well, for me in day-to-day language, an assignment *is* a copy.

> That's a different thing altogether and that simply cannot be done
> because:
> 1) this is not allowed by the language
> 2) this doesn't work when fn may throw.
>

True. That's why I pointed out that return-by-value may be more
expensive that other alternatives when doing an assignment instead of
initialization. (I do think it does not really matter with C++0x move
semantics, because this is also applied when assigning?)

br,
Martin

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