I'd like to propose adding collection_keys and collection_values to iterators library. ======== Motivation ======== I find myself very often having to iterate over all keys or values of a map (or multimap).  The best I can do is to use BOOST_FOREACH to iterate over the collection value_type pairs and then work with .first or .second.  That makes for awkward and unclear code.   BOOST_FOREACH( const foo_bar_map_t::value_type &foo_bar_pair, my_map )   {      baz(foo_bar_pair.second);   } Instead, I'd like to do something like:   BOOST_FOREACH( const bar_t &bar, collection_values(my_map) )   {      baz(bar);   }   BOOST_FOREACH( const foo_t &foo, collection_keys(my_map) )   {      baz(foo);   } ======== Proposal ======== Add this to iterators library as collection_iterators.hpp Thanks, - Igor #ifndef .... #define .... #include <boost/iterator/transform_iterator.hpp> namespace boost {   namespace detail   {     template<typename CollectionT>     struct key_function : public std::unary_function<const typename CollectionT::value_type&,                                                     const typename CollectionT::key_type&>     {       typename key_function::result_type operator()(typename key_function::argument_type p) const       {         return p.first;       }     };     template<typename CollectionT>     struct value_function : public std::unary_function<const typename CollectionT::value_type&,                                                       const typename CollectionT::mapped_type&>     {       typename value_function::result_type operator()(typename value_function::argument_type p) const       {         return p.second;       }     };     template<typename CollectionT, typename AdapterF>     class collection_adapter     {       const CollectionT &c;     public:       typedef boost::transform_iterator<AdapterF, typename CollectionT::const_iterator> const_iterator;       typedef const_iterator                                                            iterator;       collection_adapter(const CollectionT &c) : c(c) {}       const_iterator begin() const { return boost::make_transform_iterator(c.begin(), AdapterF()); }       const_iterator end()   const { return boost::make_transform_iterator(c.end(),   AdapterF()); }     };   } // Adapter for map-like collection -> iterable keys collection template<typename CollectionT> const detail::collection_adapter<CollectionT, detail::key_function<CollectionT> > collection_keys(const CollectionT &c) {   return detail::collection_adapter<CollectionT, detail::key_function<CollectionT> >(c); } // Adapter for map-like collection -> iterable values collection template<typename CollectionT> const detail::collection_adapter<CollectionT, detail::value_function<CollectionT> > collection_values(const CollectionT &c) {   return detail::collection_adapter<CollectionT, detail::value_function<CollectionT> >(c); } }   // namespace boost #endif  // ....