...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::arity_of — A metafunction that returns the arity of a Proto expression.
// In header: <boost/proto/traits.hpp> template<typename Expr> struct arity_of : Expr::proto_arity { };