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