at_impl.hpp

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//-----------------------------------------------------------------------------
// boost mpl/aux_/at_impl.hpp header file
// See http://www.boost.org for updates, documentation, and revision history.
//-----------------------------------------------------------------------------
//
// Copyright (c) 2000-02
// Aleksey Gurtovoy
//
// Permission to use, copy, modify, distribute and sell this software
// and its documentation for any purpose is hereby granted without fee, 
// provided that the above copyright notice appears in all copies and 
// that both the copyright notice and this permission notice appear in 
// supporting documentation. No representations are made about the 
// suitability of this software for any purpose. It is provided "as is" 
// without express or implied warranty.

#ifndef BOOST_MPL_AUX_AT_IMPL_HPP_INCLUDED
#define BOOST_MPL_AUX_AT_IMPL_HPP_INCLUDED

#include "boost/mpl/begin_end.hpp"
#include "boost/mpl/advance.hpp"
#include "boost/mpl/aux_/deref_wknd.hpp"
#include "boost/mpl/aux_/traits_lambda_spec.hpp"

namespace boost {
namespace mpl {

// default implementation; conrete sequences might override it by 
// specializing either the |at_traits| or the primary |at| template

template< typename Tag >
struct at_traits
{
    template< typename Sequence, typename N > struct algorithm
    {
        typedef typename advance<
              typename begin<Sequence>::type
            , N
            >::type iter_;

        typedef typename BOOST_MPL_AUX_DEREF_WNKD(iter_) type;
    };
};

BOOST_MPL_ALGORITM_TRAITS_LAMBDA_SPEC(2,at_traits)

} // namespace mpl
} // namespace boost

#endif // BOOST_MPL_AUX_AT_IMPL_HPP_INCLUDED