1 //
2 // cpp14/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 template <typename> static constexpr bool is_applicable_property_v = true;
18 static constexpr bool is_preferable = true;
19 };
20
21 template <int>
22 struct object
23 {
24 template <int N>
require(const object &,prop<N>)25 friend constexpr object<N> require(const object&, prop<N>)
26 {
27 return object<N>();
28 }
29 };
30
main()31 int main()
32 {
33 object<1> o1 = {};
34 object<2> o2 = boost::asio::prefer(o1, prop<2>());
35 object<3> o3 = boost::asio::prefer(o1, prop<2>(), prop<3>());
36 object<4> o4 = boost::asio::prefer(o1, prop<2>(), prop<3>(), prop<4>());
37 (void)o2;
38 (void)o3;
39 (void)o4;
40
41 const object<1> o5 = {};
42 object<2> o6 = boost::asio::prefer(o5, prop<2>());
43 object<3> o7 = boost::asio::prefer(o5, prop<2>(), prop<3>());
44 object<4> o8 = boost::asio::prefer(o5, prop<2>(), prop<3>(), prop<4>());
45 (void)o6;
46 (void)o7;
47 (void)o8;
48
49 constexpr object<2> o9 = boost::asio::prefer(object<1>(), prop<2>());
50 constexpr object<3> o10 = boost::asio::prefer(object<1>(), prop<2>(), prop<3>());
51 constexpr object<4> o11 = boost::asio::prefer(object<1>(), prop<2>(), prop<3>(), prop<4>());
52 (void)o9;
53 (void)o10;
54 (void)o11;
55 }
56