Providing an iterator for the first element of a container of pairs

#include <map>
#include <algorithm>
#include <iostream>
#include <string>
#include <iterator>
#include <boost/iterator/transform_iterator.hpp>
#include <boost/bind.hpp>
#include <boost/function.hpp>

int main() {
    typedef std::map<std::string, int>::value_type value_type;
    std::map<std::string, int> a;
    a["one"] = 1;
    a["two"] = 2;

    // returns the second element
    boost::function<int(value_type&)> f = boost::bind(&value_type::second, _1);
    std::copy(boost::make_transform_iterator(a.begin(), f),
              boost::make_transform_iterator(a.end(), f),
              std::ostream_iterator<int>(std::cout, " "));

}