...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::unexpr — Lets you inherit the interface of an expression while hiding from Proto the fact that the type is a Proto expression.
template<typename Expr> struct unexpr { // construct/copy/destruct unexpr(Expr const &); };