1 //
2 // cpp03/prefer_free_prefer.cpp
3 // ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
4 //
5 // Copyright (c) 2003-2021 Christopher M. Kohlhoff (chris at kohlhoff dot com)
6 //
7 // Distributed under the Boost Software License, Version 1.0. (See accompanying
8 // file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
9 //
10
11 #include <boost/asio/prefer.hpp>
12 #include <cassert>
13
14 template <int>
15 struct prop
16 {
17 static const bool is_preferable = true;
18 };
19
20 template <int>
21 struct object
22 {
23 template <int N>
prefer(const object &,prop<N>)24 friend object<N> prefer(const object&, prop<N>)
25 {
26 return object<N>();
27 }
28 };
29
30 namespace boost {
31 namespace asio {
32
33 template<int N, int M>
34 struct is_applicable_property<object<N>, prop<M> >
35 {
36 static const bool value = true;
37 };
38
39 namespace traits {
40
41 template<int N, int M>
42 struct prefer_free<object<N>, prop<M> >
43 {
44 static const bool is_valid = true;
45 static const bool is_noexcept = true;
46 typedef object<M> result_type;
47 };
48
49 } // namespace traits
50 } // namespace asio
51 } // namespace boost
52
main()53 int main()
54 {
55 object<1> o1 = {};
56 object<2> o2 = boost::asio::prefer(o1, prop<2>());
57 object<3> o3 = boost::asio::prefer(o1, prop<2>(), prop<3>());
58 object<4> o4 = boost::asio::prefer(o1, prop<2>(), prop<3>(), prop<4>());
59 (void)o2;
60 (void)o3;
61 (void)o4;
62
63 const object<1> o5 = {};
64 object<2> o6 = boost::asio::prefer(o5, prop<2>());
65 object<3> o7 = boost::asio::prefer(o5, prop<2>(), prop<3>());
66 object<4> o8 = boost::asio::prefer(o5, prop<2>(), prop<3>(), prop<4>());
67 (void)o6;
68 (void)o7;
69 (void)o8;
70 }
71