Add weak_use_count to weak_ptr as a debugging aid

[Peter Dimov]

​http://lists.boost.org/Archives/boost/2006/03/102620.php

This is a feature request to add weak_use_count() to weak_ptr.

I am not interested in this feature for anything else than a debugging aid. Recently, a weakness was highlighted in one of my projects, I had stale weak_ptr objects that would remain in core even if all copies of weak and shared pointer objects should have been eliminated by now.

To confirm the bug fix and audit the rest of the code base, a debugger was used to set a conditional break point on internal members of weak_ptr. I would rather have the ability to write assertions when needed hence weak_use_count().

[Carsten Neumann <carsten_neumann@…>]

weak_use_count() implementation, docs, some testing coverage