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The C and C++ Include Header Files

cat -n /usr/include/c++/15/bits/monostate.h

     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	
    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	
    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 */