[Boost-users] [MultiIndex] How to extract a key-extractor?

Hello, The following code is expected to get the key-extractor, but it doesn't compile. How to do this correctly? Thanks. #include #include #include struct A { int f() const { return 0; } }; typedef boost::multi_index::multi_index_container< int, boost::multi_index::indexed_by< boost::multi_index::ordered_unique< boost::multi_index::const_mem_fun > >

container_type;

typedef container_type::key_from_value_type extractor_type; // doesn't compile typedef container_type::nth_index<0>::key_from_value_type extractor_type; // doesn't compile