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boost/xpressive/proto/transform/arg.hpp

///////////////////////////////////////////////////////////////////////////////
/// \file arg.hpp
/// Proto transforms for extracting arguments from expressions.
//
//  Copyright 2007 Eric Niebler. Distributed under the Boost
//  Software License, Version 1.0. (See accompanying file
//  LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)

#ifndef BOOST_PROTO_TRANSFORM_ARG_HPP_EAN_12_16_2006
#define BOOST_PROTO_TRANSFORM_ARG_HPP_EAN_12_16_2006

#include <boost/xpressive/proto/detail/prefix.hpp>
#include <boost/xpressive/proto/proto_fwd.hpp>
#include <boost/xpressive/proto/traits.hpp>
#include <boost/xpressive/proto/detail/suffix.hpp>

namespace boost { namespace proto { namespace transform
{
    // A transform that simply extracts the arg from an expression
    template<typename Grammar, typename N>
    struct arg
      : Grammar
    {
        arg() {}

        template<typename Expr, typename State, typename Visitor>
        struct apply
          : proto::result_of::arg<typename Grammar::template apply<Expr, State, Visitor>::type, N>
        {};

        template<typename Expr, typename State, typename Visitor>
        static typename apply<Expr, State, Visitor>::type //reference
        call(Expr const &expr, State const &state, Visitor &visitor)
        {
            // NOTE Grammar::call could return a temporary!
            // Don't return a dangling reference
            return proto::arg<N>(Grammar::call(expr, state, visitor));
        }
    };

    // A transform that simply extracts the arg from an expression
    template<typename Grammar, long N>
    struct arg_c
      : Grammar
    {
        arg_c() {}

        template<typename Expr, typename State, typename Visitor>
        struct apply
          : proto::result_of::arg_c<typename Grammar::template apply<Expr, State, Visitor>::type, N>
        {};

        template<typename Expr, typename State, typename Visitor>
        static typename apply<Expr, State, Visitor>::type //const &
        call(Expr const &expr, State const &state, Visitor &visitor)
        {
            return proto::arg_c<N>(Grammar::call(expr, state, visitor));
        }
    };

    // A transform that simply extracts the left arg from an expression
    template<typename Grammar>
    struct left
      : Grammar
    {
        left() {}

        template<typename Expr, typename State, typename Visitor>
        struct apply
          : proto::result_of::left<typename Grammar::template apply<Expr, State, Visitor>::type>
        {};

        template<typename Expr, typename State, typename Visitor>
        static typename apply<Expr, State, Visitor>::type //const &
        call(Expr const &expr, State const &state, Visitor &visitor)
        {
            return proto::left(Grammar::call(expr, state, visitor));
        }
    };

    // A transform that simply extracts the right arg from an expression
    template<typename Grammar>
    struct right
      : Grammar
    {
        right() {}

        template<typename Expr, typename State, typename Visitor>
        struct apply
          : proto::result_of::right<typename Grammar::template apply<Expr, State, Visitor>::type>
        {};

        template<typename Expr, typename State, typename Visitor>
        static typename apply<Expr, State, Visitor>::type //const &
        call(Expr const &expr, State const &state, Visitor &visitor)
        {
            return proto::right(Grammar::call(expr, state, visitor));
        }
    };

    // Just return the passed in Expr
    template<typename Grammar>
    struct identity
      : Grammar
    {
        identity() {}
        BOOST_PROTO_IDENTITY_TRANSFORM();
    };

    // Just return the state
    template<typename Grammar>
    struct state
      : Grammar
    {
        state() {}

        template<typename, typename State, typename>
        struct apply
        {
            typedef State type;
        };

        template<typename Expr, typename State, typename Visitor>
        static State const &
        call(Expr const &, State const &state_, Visitor &)
        {
            return state_;
        }
    };

    // Just return the visitor
    template<typename Grammar>
    struct visitor
      : Grammar
    {
        visitor() {}

        template<typename, typename, typename Visitor>
        struct apply
        {
            typedef Visitor type;
        };

        template<typename Expr, typename State, typename Visitor>
        static Visitor &
        call(Expr const &, State const &, Visitor &visitor_)
        {
            return visitor_;
        }
    };

}}}

namespace boost { namespace proto
{
    template<typename Grammar, typename N>
    struct is_transform<transform::arg<Grammar, N> >
      : mpl::true_
    {};

    template<typename Grammar, long N>
    struct is_transform<transform::arg_c<Grammar, N> >
      : mpl::true_
    {};

    template<typename Grammar>
    struct is_transform<transform::left<Grammar> >
      : mpl::true_
    {};

    template<typename Grammar>
    struct is_transform<transform::right<Grammar> >
      : mpl::true_
    {};

    template<typename Grammar>
    struct is_transform<transform::identity<Grammar> >
      : mpl::true_
    {};

    template<typename Grammar>
    struct is_transform<transform::state<Grammar> >
      : mpl::true_
    {};

    template<typename Grammar>
    struct is_transform<transform::visitor<Grammar> >
      : mpl::true_
    {};
}}

#endif