Question

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 keys 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.

Answer 1

boost::bind is what you look for.

boost::bind(&std::pair::second, _1); // returns the value of a pair 

Example:

typedef std::map<std::string, int> map_type;  std::vector<int> values; // will contain all values map_type map; std::transform(map.begin(),                 map.end(),                 std::back_inserter(values),                 boost::bind(&map_type::value_type::second, _1));