1 //
2 // cpp11/prefer_free_require.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 constexpr bool is_preferable = true;
18 };
19
20 template <int>
21 struct object
22 {
23 template <int N>
require(const object &,prop<N>)24 friend constexpr object<N> require(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 constexpr bool value = true;
37 };
38
39 } // namespace asio
40 } // namespace boost
41
main()42 int main()
43 {
44 object<1> o1 = {};
45 object<2> o2 = boost::asio::prefer(o1, prop<2>());
46 object<3> o3 = boost::asio::prefer(o1, prop<2>(), prop<3>());
47 object<4> o4 = boost::asio::prefer(o1, prop<2>(), prop<3>(), prop<4>());
48 (void)o2;
49 (void)o3;
50 (void)o4;
51
52 const object<1> o5 = {};
53 object<2> o6 = boost::asio::prefer(o5, prop<2>());
54 object<3> o7 = boost::asio::prefer(o5, prop<2>(), prop<3>());
55 object<4> o8 = boost::asio::prefer(o5, prop<2>(), prop<3>(), prop<4>());
56 (void)o6;
57 (void)o7;
58 (void)o8;
59
60 constexpr object<2> o9 = boost::asio::prefer(object<1>(), prop<2>());
61 constexpr object<3> o10 = boost::asio::prefer(object<1>(), prop<2>(), prop<3>());
62 constexpr object<4> o11 = boost::asio::prefer(object<1>(), prop<2>(), prop<3>(), prop<4>());
63 (void)o9;
64 (void)o10;
65 (void)o11;
66 }
67