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Subject: Re: [boost] [operators] What is the correct overload set for a binary operator?
From: Marc Glisse (marc.glisse_at_[hidden])
Date: 2013-04-28 16:50:12

On Sun, 28 Apr 2013, Andrew Ho wrote:

> Yeah, I did some testing with VS2012 (/O2 optimizations) using your original
> set and with Marc's set. Both work, but Marc's set does result in fewer
> temporaries all-together. To be specific, all internal moves seem to be
> optimized away, only move required is moving into the result variable. I
> think this provides all of the benefits of your previous rvalue-ref code
> without the unsafe behavior.

Er, no, there are many unnecessary moves in this version that would go
away if we returned a reference.

> One interesting behavior I noticed while testing:
> 1)
> MyClass operator+( const MyClass& lhs, const MyClass& rhs )
> {
> MyClass nrv(lhs);
> nrv += rhs;
> return nrv;
> }
> 2)
> MyClass operator+( const MyClass& lhs, const MyClass& rhs )
> {
> return std::move(MyClass(lhs) += rhs);
> }
> 1) and 2) do not compile equivalently. 2) for some reason does not allow the
> compiler to perform return value optimizations (at least the compilers I
> tested with), potentially resulting in extra unnecessary temporaries.

Copy elision is extremely limited. The committee basically wrote 3
examples and said it was allowed for all 3 and nothing else. One trick to
help reason about it: std::move involves a reference, and the elision is
for values only (the return value of += is also a reference).

Marc Glisse

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