It is often the case that one is dealing with key value pairs when using the STL containers. Sometimes, I have found the need to extract either the key or the value from these pairs as part of a call to a standard C++ algorithm. Unfortunatelly, unless one is using the SGI implementation of STL, function objects such as select1st and select2nd are not available. So, I started to look for a boost equivalent and finally found it in the bind library. Here are the mechanisms, with the std prefix omitted for brevity:
MyMap mymap; // A map of some key type to some value type MyVector myvector; // A vector of elements either of key type or value
(...)
// Implementation using boost::bind transform(mymap.begin(),mymap.end(),back_inserter (myvector),boost::bind(&MyMap::value_type::second,_1));
And here's the most straighforward of them, but overlooked, implementation using BOOST_FOREACH, no std-omitting was necessary at all: BOOST_FOREACH(const MyMap::value_type &v, mymap) myvector.push_back(v.first); cheers, Marcin