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