[boost] making bind and mem_fn results default constructible?

From: Fredrik Blomqvist (fredrik_blomqvist_at_[hidden])
Date: 2004-09-16 11:53:32

Hi

Consider this example of mem_fn usage:

struct X { int bar; };
std::vector<X> vec;
....
boost::make_transform_iterator(vec.begin(), boost::mem_fn(&X::bar));

The above is, strictly speaking, _not_ a Random Access Traversal Iterator,
or even a Forward Traversal Iterator, and would fail in otherwise concept
conforming code (or in a potential concept-check).

The problems lies in the function objects returned by boost::mem_fn and
boost::bind not being default constructible, thus making the resulting
iterator not default constructible which is required by the Forward
Traversal concept.
http://boost.org/libs/iterator/doc/new-iter-concepts.html#forward-traversal-iterators-lib-forward-traversal-iterators
(The problem is even more pronounced if using the "old" Iterator Concepts.)

I found an old (2002) mail by David Abrahams where he also noted the
problem: http://lists.boost.org/MailArchives/boost/msg30420.php
But it didn't seem to have resulted in any change of the codebase.

I think use-cases similar to the one above are good motivation and hope this
could be considered an acceptable improvement.

Regards
// Fredrik Blomqvist

P.S. I realize it is very close to the branch for release and don't expect
this to be added before that.