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 12:14:36

On Sun, 28 Apr 2013, Marc Glisse wrote:

> On Sun, 28 Apr 2013, Daniel Frey wrote:
>
>> after some more experiments, I convinced myself that I should explore the 
>> option of returning an rvalue (not an rvalue reference) in all cases when 
>> implementing operator+ based on operator+= (as an example). But I wonder 
>> what the most efficient overload set would look like. With this post I'll 
>> concentrate on same-type operations (T+T).
>> 
>> Given a class T with operator+= like this:
>>
>>   T& T::operator+=( const T& );
>>   T& T::operator+=( T&& ); // if useful the class provides it and it should 
>> be used if applicable
>> 
>> I think the following is currently the best overload set for operators:
>>
>>    T operator+( T lhs, const T& rhs )
>>    {
>>        lhs += rhs;
>>        return lhs; // no std::move needed
>>    }
>
> Splitting this into const& and && overloads of lhs would save a move for:
> T r = std::move(a) + b;
> (passing an xvalue and not a prvalue)
>
> In practice, it would also save a move for T r=a+b, although in theory it 
> shouldn't (it should cost one extra move instead),

Forgot to mention that this extra move would be for a+b+c, not a+b, sorry.

> but compilers are bad at 
> eliding the copy between an argument and the return (it would be a 
> cross-function optimization in the front-end).
>
> Sometimes you would want to overload on lvalue vs xvalue vs prvalue for 
> optimal results, but the language will not allow it.

-- 
Marc Glisse