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 is_expr

boost::yap::is_expr

// In header: <boost/yap/algorithm_fwd.hpp>

template<typename Expr> 
struct is_expr {
};

A metafunction that evaluates to std::true_type if Expr is an Expression, and std::false_type otherwise.