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: David Abrahams (dave_at_[hidden])
Date: 2009-09-07 20:37:54

on Mon Sep 07 2009, Howard Hinnant <howard.hinnant-AT-gmail.com> wrote:

> std::swap will only do a self-move-assignment using a moved-from
> object.  

You mean the target will be a moved-from object.  OK, so?

> Of course
> vector will never use this definition.  But if it  did, then:
>
> template <class _Tp, class _Allocator>
> inline
> vector<_Tp, _Allocator>&
> vector<_Tp, _Allocator>::operator=(vector&& __x)
> {
>    clear();
>    swap(__x);
>    return *this;
> }
>
> would work just fine.  clear() is a no-op, and then you swap an empty capacity vector
> with itself.
>
> My point is that I don't want to be told I need to put a if(this !=
> &x) in my move assignment operator.  It is a useless performance
> penalty.  Everyone should not have to pay (in terms of lost
> performance) for someone's bad code just to make sure self-move-
> assignment does a no-op instead of crashes.

And the reason this argument doesn't also apply to copy assignment
is...?

> If you want to cast an lvalue to an rvalue, that's fine.  Just make
> sure your code logic is such that such a cast is safe.  I believe
> std::swap's use of move assignment is safe because by the time it
> move-assigns, if it is self referencing, all of your resources have
> already been moved to a local temp.

Then what is the coding guideline, exactly?  "Make sure it's safe"
doesn't qualify.

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