Re: [boost] auto_vector - vector with auto_ptr semantics

From: Hamish Mackenzie (boost_at_[hidden])
Date: 2002-02-05 09:08:14

On Tue, 2002-02-05 at 01:07, Howard Hinnant wrote:
> On Monday, February 4, 2002, at 07:37  PM, Jon Kalb wrote:
> 
> > Is move what we really want? Or is it swap? I've thought for awhile that
> > swap was a missing "operator." It seems to be such a fundamental
> > operation.
> 
> I agree 100% that swap is fundamental, and I would love to see a swap 
> operator.  I suspect we're talking about the "next" language, but you 
> never know.  However, I think move is even more fundamental than swap.  
> Consider:  if scalars support move semantics (as they do swap), then 
> "moving" a scalar would be 3 times as expensive as you need:
> 
> template <class T>
> void
> swap(T& a, T& b)
> {
> 	T tmp(a);
> 	a = b;
> 	b = tmp;
> }
> 
> Three assignments instead of one!
> 
> However if moving a scalar could be accomplished by a single assignment, 
> then swap could be built on top of move:
> 
> template <class T>
> void
> swap(T& a, T& b)
> {
> 	T tmp(move(a));
> 	a = move(b);
> 	b = move(tmp);
> }
> 
> (courtesy Andrew Koenig)
> 
> And if move is /defined/ to be a non-throwing operation, then unlike the 
> current std::swap, this version built on move could be immediately 
> applicable to every class that implemented move semantics *and* be 
> non-throwing.

How about

template< class T >
void move( T & destination, T & source )
{
  std::swap( destination, source );
}

void move( int & destination, int & source )
{
  destination = source;
}

...

With similar overloads for classes made up of only scaler types.  This
leaves it to the designer of the class to decide if assignment is a
valid optimization.

> I agree that we could do move with swap.  I just think it would be 
> higher quality to do swap with move.

I am not convinced. Swap in my opinion is more basic as it does not
create or destory any objects.

Hamish Mackenzie