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Struct _

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


// 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;


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:

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