Re: [boost] [variant] Please vote for behavior

Subject: Re: [boost] [variant] Please vote for behavior
From: Dave Abrahams (dave_at_[hidden])
Date: 2013-02-07 15:25:16

on Thu Feb 07 2013, Sebastian Redl <sebastian.redl-AT-getdesigned.at> wrote:

> On 06.02.2013, at 22:10, Dave Abrahams wrote:
>
>> 
>> on Sat Feb 02 2013, Sebastian Redl <sebastian.redl-AT-getdesigned.at> wrote:
>> 
>>> Anything that gives the target object the state the source object had
>>> and is cheaper than a copy is a successful move, really. That is what
>>> a move ultimately is: an optimization of copying.
>> 
>> I never liked that characterization.  If move were an optimization of
>> copy it would have all the semantics of copy...
>
> I disagree. That would be the case if move were a universally
> applicable optimization, but it isn't. It's a special case
> optimization. In intent, it optimizes copying for the case where the
> resulting state of the source object is irrelevant. In practice, the
> source object must retain some valid state.
>
> This is really similar to how the compiler can only apply some
> optimizations if it can prove certain properties of the code,
> e.g. that two pointers don't alias (various redundant load or store
> eliminations) or that a piece of code is side effect free (loop
> fusion, common subexpression elimination).

Yes, but those are semantics-preserving transformations.  I suppose you
can call move an optimization if you presume the irrelevancy of the
source object has been proven, but usually that proof is part of the
optimization process.

-- 
Dave Abrahams