...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
boost::proto::_state::impl
// In header: <boost/proto/transform/arg.hpp> template<typename Expr, typename State, typename Data> struct impl : proto::transform_impl< Expr, State, Data > { // types typedef State result_type; // public member functions State operator()(typename impl::expr_param, typename impl::state_param, typename impl::data_param) const; };