...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
boost::numeric::functional::promote_base
template<typename To, typename From, typename EnableIf> struct promote_base : public std::unary_function< From, To > { // public member functions To operator()(From &) const; };