STRUCT _(3)

NAME
boost::proto::_ - A wildcard grammar element that matches any expression, and a transform that returns the current expression unchanged.

SYNOPSIS

// In header: <boost/proto/matches.hpp>
struct _ :  proto::transform<_> {
  // types
  typedef _ proto_grammar;
  // member classes/structs/unions
  template<typename Expr, typename State, typename Data>
  struct impl :  proto::transform_impl<Expr, State, Data> {
    // types
    typedef Expr result_type;
    // public member functions
    Expr operator()(typename impl::expr_param, typename impl::state_param,
                    typename impl::data_param) const;
  };
};

DESCRIPTION

The wildcard type, proto::_, is a grammar element such that proto::matches<E, proto::_>::value is true for any expression type E.

The wildcard can also be used as a stand-in for a template argument when matching terminals. For instance, the following is a grammar that will match any std::complex<> terminal:

    BOOST_MPL_ASSERT((
      proto::matches<
        proto::terminal<std::complex<double> >::type,
        proto::terminal<std::complex< proto::_ > >
      >
    ));

When used as a transform, proto::_ returns the current expression unchanged. For instance, in the following, proto::_ is used with the proto::fold<> transform to fold the children of a node:

    struct CountChildren :
      proto::or_<
        // Terminals have no children
        proto::when<proto::terminal<proto::_>, mpl::int_<0>()>,
        // Use proto::fold<> to count the children of non-terminals
        proto::otherwise<
          proto::fold<
            proto::_, // <-- fold the current expression
            mpl::int_<0>(),
            mpl::plus<proto::_state, mpl::int_<1> >()
          >
        >
      >
    {};