From: Howard Hinnant (hinnant_at_[hidden])
Date: 2002-02-06 19:42:29
On Wednesday, February 6, 2002, at 03:56 PM, Sean Parent wrote:
> It was correctly pointed out, that swap can be implemented using
> constructing moves (in the same way it is done with copying, through a
> temp). But in the case of a non-POD class, I don't think this is a win
> unless you define the source from the move to have been destructed as a
> result of the move (call this relocate).
> The move vs. relocate semantics would be:
> move - moving a source A into uninitialized memory B. The result leaves
> A in
> an indeterminate but destructible state. "Copy" construction of an
> is an example.
> relocate - moving a source A into uninitialized memory B. The result
> A in a destructed state. Requires the source A is also managed with
> placement new and delete.
I like these definitions, except I would also like to see moving into
initialized memory defined (e.g. auto_ptr assignment). They make it
easier to discuss the variations.
I'm not understanding your comments with respect to swap implemented
with move though:
template <class T>
swap(T& a, T& b)
T tmp(move(a)); // move construct
a = move(b); // move assign
b = move(tmp); // move assign
None of these was intended to be a relocation, though the last move from
tmp certainly could be since the next (implied) statement is the
destruction of tmp.
> The relocate construct would be useful for container optimization -
> but I
> think the complexities of implementing it for anything that can't be
> as a POD (just do a memcopy for the relocate) is difficult. It may be
> having a trait though for objects that can use memcopy to relocate.
I don't think relocation will be useful for container optimization, at
least not vector. I went into this in some detail under the auto_vector
subject. I remain unconvinced (but open minded) about its usefulness in
> The first item, I think is simplest to implement using a non-throwing
> constructor (one that constructs an object upon which the only valid
> operation is destruction) and a swap.
I suspect we've got a time delay thing going on. I just got your post,
but it is dated hours earlier. Have you had a chance to look at
Hamish's proposed code, and my response to it? Your comments on this
would be most valued.
Boost list run by bdawes at acm.org, gregod at cs.rpi.edu, cpdaniel at pacbell.net, john at johnmaddock.co.uk