flyweight/detail/value_tag.hpp

0001 /* Copyright 2006-2008 Joaquin M Lopez Munoz.
0002  * Distributed under the Boost Software License, Version 1.0.
0003  * (See accompanying file LICENSE_1_0.txt or copy at
0004  * http://www.boost.org/LICENSE_1_0.txt)
0005  *
0006  * See http://www.boost.org/libs/flyweight for library home page.
0007  */
0008
0009 #ifndef BOOST_FLYWEIGHT_DETAIL_VALUE_TAG_HPP
0010 #define BOOST_FLYWEIGHT_DETAIL_VALUE_TAG_HPP
0011
0012 #if defined(_MSC_VER)
0013 #pragma once
0014 #endif
0015
0016 #include <boost/config.hpp> /* keep it first to prevent nasty warns in MSVC */
0017 #include <boost/parameter/parameters.hpp>
0018 #include <boost/type_traits/is_base_and_derived.hpp>
0019
0020 namespace boost{
0021
0022 namespace flyweights{
0023
0024 namespace detail{
0025
0026 /* Three ways to indicate that a given class T is a value policy:
0027  *   1. Make it derived from value_marker.
0028  *   2. Specialize is_value to evaluate to boost::mpl::true_.
0029  *   3. Pass it as value<T> when defining a flyweight type.
0030  *
0031  * For the time being the interface of value policies is not public.
0032  */
0033
0034 struct value_marker{};
0035
0036 template<typename T>
0037 struct is_value:is_base_and_derived<value_marker,T>
0038 {};
0039
0040 template<typename T=parameter::void_>
0041 struct value:parameter::template_keyword<value<>,T>
0042 {};
0043
0044 } /* namespace flyweights::detail */
0045
0046 } /* namespace flyweights */
0047
0048 } /* namespace boost */
0049
0050 #endif