libstdc++: monostate.h Source File

libstdc++

monostate.h
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1// Definition of std::monostate for <variant> and <utility> -*- C++ -*-
2
3// Copyright (C) 2016-2026 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
20// You should have received a copy of the GNU General Public License and
21// a copy of the GCC Runtime Library Exception along with this program;
22// see the files COPYING3 and COPYING.RUNTIME respectively. If not, see
23// <http://www.gnu.org/licenses/>.
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
42namespace 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 */
compare
functional_hash.h
version.h
std
ISO C++ entities toplevel namespace is std.
std::hash
Primary class template hash.
Definition functional_hash.h:106