boost::proto::is_expr — A Boolean metafunction that indicates whether a given type T is a Proto expression type.
T
template<typename T> struct is_expr : mpl::bool_<true-or-false> { };
true-or-false
If T is an instantiation of proto::expr<> or is an extension (via proto::extends<> or BOOST_PROTO_EXTENDS()) of such an instantiation, proto::is_expr<T>::value is true. Otherwise, proto::is_expr<T>::value is false.
proto::expr<>
proto::is_expr<T>::value
true
false