Re: [boost] [operators] What is the correct overload set for a binary operator?

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