Re: [boost] [move][container] Review Request (new versions ofBoost.Move and Boost.Container in sandbox and vault)

Subject: Re: [boost] [move][container] Review Request (new versions ofBoost.Move and Boost.Container in sandbox and vault)
From: Christopher Jefferson (chris_at_[hidden])
Date: 2009-09-07 18:14:37

On 7 Sep 2009, at 23:03, David Abrahams wrote:

>
> on Mon Sep 07 2009, "Peter Dimov" <pdimov-AT-pdimov.com> wrote:
>
>>> Rationale:
>>> The move assignment operator should be fast enough that an extra if-
>>> test may adversely impact performance (above the noise level).
>>
>> The problem with making it UB is that y = move(x) now needs to be  
>> guarded with if( &x
>> != &y ) everywhere in client code. Sometimes the user will know  
>> that x is not y, but
>> is this really the case most of the time?
>
> Most of the time, yeah.  Definitely, in most of the generic  
> algorithms.
> But I'm not sure whether it's true enough of the time.  Consider our  
> new
> generic std::swap:
>
>  template <class T>
>  void swap(T& x, T& y)
>  {
>      T z = std::move(x);
>      x = std::move(y); // <== uh oh?
>      y = std::move(z);
>  }
>
> Do we think swap(x,y) is also an error when x == y?

Last time I checked (which was a couple of years ago) STLport did a  
self-swap when std::reverse was given an odd-length list, and wouldn't  
accept a patch to change the behaviour.

Chris