title: Is there a standard C++ function object for taking apart a std::pair?
body: <p>Does anyone know if there's a de-facto standard (i.e., TR1 or Boost) C++ function object for accessing the elements of a std::pair?  Twice in the past 24 hours I've wished I had something like the <code>keys</code> function for Perl hashes.  For example, it would be nice to run std::transform on a std::map object and dump all the keys (or values) to another container.  I could certainly write such a function object but I'd prefer to reuse something that's had a lot of eyeballs on it.</p>