Re: [boost] [move] auto-generated operator=

Subject: Re: [boost] [move] auto-generated operator=
From: David Abrahams (dave_at_[hidden])
Date: 2009-09-08 06:22:12

on Tue Sep 08 2009, Ion Gaztañaga <igaztanaga-AT-gmail.com> wrote:

> David Abrahams escribió:
>>> Well, maybe benefits don't outweigh problems... I added that overload so that
>>> assignment moves rvalues instead of copying them, 
>
> In the first version of the library we had two overloads ("const T &" and "rv<T> &")
> for copy/move constructor and assignments. But if T& operator=(const T&) is defined
> then:
>
> T produce();
>
> T t;
> //...
> //Copy assignment called instead of move-assignment!
> t = produce();
>
> This is suboptimal for assignments, because a move is replaced with a copy. In theory
> this also happens for copy-constructors but fortunately RVO is a much better approach
> and in practice this problem only happens with assignments:

...and only with assignments that don't already take their argument by
value.  std::vector assignment would be an example of something that can't.

> So right know I have two options: the old approach that produces suboptimal
> assignments from non-const rvalues and the newer one that is optimal but hurts the
> compiler generated copy assignment.
>
> We need a third alternative: a new overload set that maintains "const T &" and
> properly catches non-const rvalues.

Hmm, tough one.

-- 
Dave Abrahams
BoostPro Computing
http://www.boostpro.com