Struct is_proto_exprboost::proto::is_proto_expr — Empty type to be used as a dummy template parameter of POD expression wrappers. It allows argument-dependent lookup to find Proto's operator overloads. Synopsisstruct is_proto_expr { }; Description
template<typename T, typename Dummy = proto::is_proto_expr> struct my_terminal { BOOST_PROTO_BASIC_EXTENDS( typename proto::terminal<T>::type , my_terminal<T> , proto::default_domain ) }; // ... my_terminal<int> _1, _2; _1 + _2; // OK, uses proto::operator+
Without the second |