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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.
// In header: <boost/proto/extends.hpp> struct is_proto_expr { };
proto::is_proto_expr
allows argument-dependent lookup to find Proto's operator overloads. For example:
proto::is_proto_expr> BOOST_PROTO_BASIC_EXTENDS( proto::terminal<proto::default_domain ) }; // ...
Without the second Dummy
template parameter, Proto's operator overloads
would not be considered by name lookup.