Boost
  C++ Libraries
  
  ...one of the most highly
  regarded and expertly designed C++ library projects in the
  world.
 — Herb Sutter and Andrei
  Alexandrescu, C++
  Coding Standards
boost::pfr::hash — std::hash like functor that returns boost::pfr::hash_value (x)
// In header: <boost/pfr/functors.hpp> template<typename T> struct hash { // public member functions std::size_t operator()(const T &) const; };