monostate.h
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// Definition of std::monostate for <variant> and <utility> -*- C++ -*-
// Copyright (C) 2016-2026 Free Software Foundation, Inc.
//
// This file is part of the GNU ISO C++ Library. This library is free
// software; you can redistribute it and/or modify it under the
// terms of the GNU General Public License as published by the
// Free Software Foundation; either version 3, or (at your option)
// any later version.
// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU General Public License for more details.
// Under Section 7 of GPL version 3, you are granted additional
// permissions described in the GCC Runtime Library Exception, version
// 3.1, as published by the Free Software Foundation.
// You should have received a copy of the GNU General Public License and
// a copy of the GCC Runtime Library Exception along with this program;
// see the files COPYING3 and COPYING.RUNTIME respectively. If not, see
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/** @file bits/monostate.h
* This is an internal header file, included by other library headers.
* Do not attempt to use it directly. @headername{utility}
*/
#ifndef _GLIBCXX_MONOSTATE_H
#define _GLIBCXX_MONOSTATE_H 1
#include <bits/version.h>
#ifdef __glibcxx_variant // C++ >= 17
#include <bits/functional_hash.h>
#if __cplusplus >= 202002L
# include <compare>
#endif
namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION
struct monostate { };
constexpr bool operator==(monostate, monostate) noexcept { return true; }
#ifdef __cpp_lib_three_way_comparison
constexpr strong_ordering
operator<=>(monostate, monostate) noexcept { return strong_ordering::equal; }
#else
constexpr bool operator!=(monostate, monostate) noexcept { return false; }
constexpr bool operator<(monostate, monostate) noexcept { return false; }
constexpr bool operator>(monostate, monostate) noexcept { return false; }
constexpr bool operator<=(monostate, monostate) noexcept { return true; }
constexpr bool operator>=(monostate, monostate) noexcept { return true; }
#endif
template<>
struct hash<monostate>
{
#if __cplusplus < 202002L
using result_type [[__deprecated__]] = size_t;
using argument_type [[__deprecated__]] = monostate;
#endif
size_t
operator()(const monostate&) const noexcept
{
constexpr size_t __magic_monostate_hash = -7777;
return __magic_monostate_hash;
}
};
_GLIBCXX_END_NAMESPACE_VERSION
} // namespace std
#endif // __glibcxx_variant
#endif /* _GLIBCXX_MONOSTATE_H */
compare
version.h
functional_hash.h
std
ISO C++ entities toplevel namespace is std.
std::hash
Primary class template hash.
Definition functional_hash.h:106