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

Struct template tag_of

boost::proto::tag_of — A metafunction that returns the tag type of a Proto expression.

// In header: <boost/proto/traits.hpp>

template<typename Expr> 
struct tag_of {
  // types
  typedef typename Expr::proto_tag type;
};