...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
boost::type_erasure::is_placeholder
// In header: <boost/type_erasure/is_placeholder.hpp> template<typename T> struct is_placeholder : public boost::is_base_and_derived< placeholder, T > { };