libstdc++
monostate.h
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1 // Definition of std::monostate for <variant> and <utility> -*- C++ -*-
2 
3 // Copyright (C) 2016-2025 Free Software Foundation, Inc.
4 //
5 // This file is part of the GNU ISO C++ Library. This library is free
6 // software; you can redistribute it and/or modify it under the
7 // terms of the GNU General Public License as published by the
8 // Free Software Foundation; either version 3, or (at your option)
9 // any later version.
10 
11 // This library is distributed in the hope that it will be useful,
12 // but WITHOUT ANY WARRANTY; without even the implied warranty of
13 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
14 // GNU General Public License for more details.
15 
16 // Under Section 7 of GPL version 3, you are granted additional
17 // permissions described in the GCC Runtime Library Exception, version
18 // 3.1, as published by the Free Software Foundation.
19 
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24 
25 /** @file bits/monostate.h
26  * This is an internal header file, included by other library headers.
27  * Do not attempt to use it directly. @headername{utility}
28  */
29 
30 #ifndef _GLIBCXX_MONOSTATE_H
31 #define _GLIBCXX_MONOSTATE_H 1
32 
33 #include <bits/version.h>
34 
35 #ifdef __glibcxx_variant // C++ >= 17
36 
37 #include <bits/functional_hash.h>
38 #if __cplusplus >= 202002L
39 # include <compare>
40 #endif
41 
42 namespace std _GLIBCXX_VISIBILITY(default)
43 {
44 _GLIBCXX_BEGIN_NAMESPACE_VERSION
45  struct monostate { };
46 
47  constexpr bool operator==(monostate, monostate) noexcept { return true; }
48 #ifdef __cpp_lib_three_way_comparison
49  constexpr strong_ordering
50  operator<=>(monostate, monostate) noexcept { return strong_ordering::equal; }
51 #else
52  constexpr bool operator!=(monostate, monostate) noexcept { return false; }
53  constexpr bool operator<(monostate, monostate) noexcept { return false; }
54  constexpr bool operator>(monostate, monostate) noexcept { return false; }
55  constexpr bool operator<=(monostate, monostate) noexcept { return true; }
56  constexpr bool operator>=(monostate, monostate) noexcept { return true; }
57 #endif
58 
59  template<>
60  struct hash<monostate>
61  {
62 #if __cplusplus < 202002L
63  using result_type [[__deprecated__]] = size_t;
64  using argument_type [[__deprecated__]] = monostate;
65 #endif
66 
67  size_t
68  operator()(const monostate&) const noexcept
69  {
70  constexpr size_t __magic_monostate_hash = -7777;
71  return __magic_monostate_hash;
72  }
73  };
74 
75 _GLIBCXX_END_NAMESPACE_VERSION
76 } // namespace std
77 #endif // __glibcxx_variant
78 #endif /* _GLIBCXX_MONOSTATE_H */
constexpr bool operator<=(const duration< _Rep1, _Period1 > &__lhs, const duration< _Rep2, _Period2 > &__rhs)
Definition: chrono.h:859
constexpr bool operator>=(const duration< _Rep1, _Period1 > &__lhs, const duration< _Rep2, _Period2 > &__rhs)
Definition: chrono.h:873
constexpr bool operator<(const duration< _Rep1, _Period1 > &__lhs, const duration< _Rep2, _Period2 > &__rhs)
Definition: chrono.h:826
constexpr bool operator>(const duration< _Rep1, _Period1 > &__lhs, const duration< _Rep2, _Period2 > &__rhs)
Definition: chrono.h:866
ISO C++ entities toplevel namespace is std.