add support for weak_ptr

[pwinston@…]

Today shared_ptr works very well, but it makes it difficult for the C++ application to retain control over the lifetime of its objects. Variables in the interpreter which are hard to clear can prevent an object from being deleting.

weak_ptr support would be the ideal solution. A application could hand out only weak_ptrs to Python. The object could then be deleted as normal.

Attempting to use a weak_ptr in Python if the object has been released should throw. Python code which knows its objects might be deleted out from under it can catch this exception.

See thread w/ Dave Abrahams titled "application exit and shared_ptr" in C++-sig.

[Dave Abrahams]

I'd like to have a (currently-failing) test case that demonstrates what you want to work. Without that, I can't be sure you're getting what you really want.

[anonymous]

See ​https://svn.boost.org/trac/boost/ticket/3673 for example