Struct is_proto

Struct is_proto_expr

boost::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.

Synopsis

// In header: <boost/proto/extends.hpp>


struct is_proto_expr {
};

Description

proto::is_proto_expr allows argument-dependent lookup to find Proto's operator overloads. For example:

 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 Dummy template parameter, Proto's operator overloads would not be considered by name lookup.