| // |
| // cpp03/can_prefer_not_preferable_free_prefer.cpp |
| // ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
| // |
| // Copyright (c) 2003-2021 Christopher M. Kohlhoff (chris at kohlhoff dot com) |
| // |
| // 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) |
| // |
| |
| #include <boost/asio/prefer.hpp> |
| #include <cassert> |
| |
| template <int> |
| struct prop |
| { |
| static const bool is_preferable = false; |
| }; |
| |
| template <int> |
| struct object |
| { |
| template <int N> |
| friend object<N> prefer(const object&, prop<N>) |
| { |
| return object<N>(); |
| } |
| }; |
| |
| namespace boost { |
| namespace asio { |
| |
| template<int N, int M> |
| struct is_applicable_property<object<N>, prop<M> > |
| { |
| static const bool value = true; |
| }; |
| |
| namespace traits { |
| |
| template<int N, int M> |
| struct prefer_free<object<N>, prop<M> > |
| { |
| static const bool is_valid = true; |
| static const bool is_noexcept = true; |
| typedef object<M> result_type; |
| }; |
| |
| } // namespace traits |
| } // namespace asio |
| } // namespace boost |
| |
| int main() |
| { |
| assert((!boost::asio::can_prefer<object<1>, prop<2> >::value)); |
| assert((!boost::asio::can_prefer<object<1>, prop<2>, prop<3> >::value)); |
| assert((!boost::asio::can_prefer<object<1>, prop<2>, prop<3>, prop<4> >::value)); |
| assert((!boost::asio::can_prefer<const object<1>, prop<2> >::value)); |
| assert((!boost::asio::can_prefer<const object<1>, prop<2>, prop<3> >::value)); |
| assert((!boost::asio::can_prefer<const object<1>, prop<2>, prop<3>, prop<4> >::value)); |
| } |