Following some ideas discussed about a year ago, I've added a new boost::multi_index::key construct allowing for a very terse specification of predefined key extractors in C++17 compliant environments. For instance, given     struct phonebook_entry     {       std::string family_name;       std::string given_name;       std::string phone_number()const;     };     int num_calls(const phonebook_entry&); a classical definition like     typedef multi_index_container<       phonebook_entry,       indexed_by<         ordered_unique< const_mem_fun<phonebook_entry,std::string,&phonebook_entry::phone_number>         >,         ordered_unique<           global_fun<const phonebook_entry&,int,num_calls>         >,         ordered_non_unique<           composite_key<             phonebook_entry, member<phonebook_entry,std::string,&phonebook_entry::family_name>, member<phonebook_entry,std::string,&phonebook_entry::given_name>           >         >       >     > phonebook; can now be written as     typedef multi_index_container<       phonebook_entry,       indexed_by<         ordered_unique<           key<&phonebook_entry::phone_number>         >,         ordered_unique<           key<num_calls>         >,         ordered_non_unique< key<&phonebook_entry::family_name,&phonebook_entry::given_name>         >       >     > phonebook; If you have a C++17 compiler, you can play with this new feature by simply copying https://github.com/boostorg/multi_index/blob/develop/include/boost/multi_ind... into your local Boost installation. More info at https://www.boost.org/doc/libs/develop/libs/multi_index/doc/tutorial/key_ext... https://www.boost.org/doc/libs/develop/libs/multi_index/doc/reference/key_ex... Comments and suggestions, specially before Boost 1.69 ships and this becomes more or less frozen, are most welcome. Best regards, Joaquín M López Muñoz