as_variant.hpp

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/*=============================================================================
    Copyright (c) 2001-2007 Joel de Guzman

    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_PP_IS_ITERATING
#if !defined(BOOST_SPIRIT_AS_VARIANT_NOV_16_2007_0420PM)
#define BOOST_SPIRIT_AS_VARIANT_NOV_16_2007_0420PM

#include <boost/preprocessor/iterate.hpp>
#include <boost/preprocessor/repetition/enum_params.hpp>
#include <boost/preprocessor/repetition/enum_binary_params.hpp>
#include <boost/preprocessor/repetition/enum_params_with_a_default.hpp>
#include <boost/preprocessor/repetition/repeat_from_to.hpp>
#include <boost/variant/variant_fwd.hpp>
#include <boost/mpl/fold.hpp>
#include <boost/mpl/vector.hpp>
#include <boost/mpl/push_back.hpp>
#include <boost/mpl/contains.hpp>
#include <boost/type_traits/is_same.hpp>

namespace boost { namespace spirit { namespace detail
{
    template <int size>
    struct as_variant;

    template <>
    struct as_variant<0>
    {
        template <typename Iterator>
        struct apply
        {
            typedef variant<> type;
        };
    };

#define BOOST_FUSION_NEXT_ITERATOR(z, n, data)                                  \
    typedef typename fusion::result_of::next<BOOST_PP_CAT(I, n)>::type          \
        BOOST_PP_CAT(I, BOOST_PP_INC(n));

#define BOOST_FUSION_NEXT_CALL_ITERATOR(z, n, data)                             \
    typename gen::BOOST_PP_CAT(I, BOOST_PP_INC(n))                              \
        BOOST_PP_CAT(i, BOOST_PP_INC(n)) = fusion::next(BOOST_PP_CAT(i, n));

#define BOOST_FUSION_VALUE_OF_ITERATOR(z, n, data)                              \
    typedef typename fusion::result_of::value_of<BOOST_PP_CAT(I, n)>::type      \
        BOOST_PP_CAT(T, n);

#define BOOST_PP_FILENAME_1 <boost/spirit/home/support/as_variant.hpp>
#define BOOST_PP_ITERATION_LIMITS (1, BOOST_VARIANT_LIMIT_TYPES)
#include BOOST_PP_ITERATE()

#undef BOOST_FUSION_NEXT_ITERATOR
#undef BOOST_FUSION_NEXT_CALL_ITERATOR
#undef BOOST_FUSION_VALUE_OF_ITERATOR

    template <typename Sequence>
    struct generate_variant
    {
        // build a variant generator being able to generate a variant holding 
        // all of the types as given in the typelist 
        typedef typename
            detail::as_variant<fusion::result_of::size<Sequence>::value>
        gen;
        
        // use this generator to create the actual variant
        typedef typename gen::template apply<
                typename fusion::result_of::begin<Sequence>::type
            >::type 
        type;
    };
    
}}}

namespace boost { namespace spirit
{
    template <typename Sequence>
    struct as_variant
    {
        // make sure each of the types occurs only once in the type list
        typedef typename 
            mpl::fold<
                Sequence, mpl::vector<>,
                mpl::if_<
                    mpl::contains<mpl::_1, mpl::_2>, 
                    mpl::_1, mpl::push_back<mpl::_1, mpl::_2> 
                > 
            >::type 
        new_sequence;

        // if there is only one type in the list of types we strip off the 
        // variant all together
        typedef typename 
            mpl::eval_if<
                mpl::equal_to<mpl::size<new_sequence>, mpl::int_<1> >,
                mpl::deref<mpl::front<Sequence> >,
                detail::generate_variant<new_sequence>
            >::type 
        type;
    };
}}

#endif
#else // defined(BOOST_PP_IS_ITERATING)
///////////////////////////////////////////////////////////////////////////////
//
//  Preprocessor vertical repetition code
//
///////////////////////////////////////////////////////////////////////////////

#define N BOOST_PP_ITERATION()

    template <>
    struct as_variant<N>
    {
        template <typename I0>
        struct apply
        {
            BOOST_PP_REPEAT(N, BOOST_FUSION_NEXT_ITERATOR, _)
            BOOST_PP_REPEAT(N, BOOST_FUSION_VALUE_OF_ITERATOR, _)
            typedef variant<BOOST_PP_ENUM_PARAMS(N, T)> type;
        };
    };

#undef N
#endif // defined(BOOST_PP_IS_ITERATING)