mpl/is_placeholder.hpp

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#ifndef BOOST_MPL_IS_PLACEHOLDER_HPP_INCLUDED
#define BOOST_MPL_IS_PLACEHOLDER_HPP_INCLUDED
 
// Copyright Aleksey Gurtovoy 2001-2004
//
// 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)
//
// See http://www.boost.org/libs/mpl for documentation.
 
// $Id: is_placeholder.hpp 13472 2017-08-22 07:53:44Z richean $
// $Date: 2017-08-22 09:53:44 +0200 (Di, 22 Aug 2017) $
// $Revision: 13472 $
 
#include <boost/mpl/arg_fwd.hpp>
#include <boost/mpl/bool.hpp>
#include <boost/mpl/aux_/yes_no.hpp>
#include <boost/mpl/aux_/type_wrapper.hpp>
#include <boost/mpl/aux_/nttp_decl.hpp>
#include <boost/mpl/aux_/config/ctps.hpp>
#include <boost/mpl/aux_/config/static_constant.hpp>
 
namespace boost { namespace mpl {
 
#if !defined(BOOST_NO_TEMPLATE_PARTIAL_SPECIALIZATION)
 
template< typename T >
struct is_placeholder
    : bool_<false>
{
};
 
template< BOOST_MPL_AUX_NTTP_DECL(int, N) >
struct is_placeholder< arg<N> >
    : bool_<true>
{
};
 
#else
 
namespace aux {
 
aux::no_tag is_placeholder_helper(...);
 
template< BOOST_MPL_AUX_NTTP_DECL(int, N) >
aux::yes_tag is_placeholder_helper(aux::type_wrapper< arg<N> >*);
 
} // namespace aux
 
template< typename T >
struct is_placeholder
{
    static aux::type_wrapper<T>* get();
    BOOST_STATIC_CONSTANT(bool, value = 
          sizeof(aux::is_placeholder_helper(get())) == sizeof(aux::yes_tag)
        );
    
    typedef bool_<value> type;
};
 
#endif // BOOST_NO_TEMPLATE_PARTIAL_SPECIALIZATION
 
}}
 
#endif // BOOST_MPL_IS_PLACEHOLDER_HPP_INCLUDED