1// -*- C++ -*-
 2//===----------------------------------------------------------------------===//
 3//
 4// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 5// See https://llvm.org/LICENSE.txt for license information.
 6// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 7//
 8//===----------------------------------------------------------------------===//
 9
10#ifndef _LIBCPP___VARIANT_MONOSTATE_H
11#define _LIBCPP___VARIANT_MONOSTATE_H
12
13#include <__compare/ordering.h>
14#include <__config>
15#include <__cstddef/size_t.h>
16#include <__functional/hash.h>
17
18#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
19#  pragma GCC system_header
20#endif
21
22_LIBCPP_BEGIN_NAMESPACE_STD
23
24#if _LIBCPP_STD_VER >= 17
25
26struct monostate {};
27
28_LIBCPP_HIDE_FROM_ABI inline constexpr bool operator==(monostate, monostate) noexcept { return true; }
29
30#  if _LIBCPP_STD_VER >= 20
31
32_LIBCPP_HIDE_FROM_ABI inline constexpr strong_ordering operator<=>(monostate, monostate) noexcept {
33  return strong_ordering::equal;
34}
35
36#  else // _LIBCPP_STD_VER >= 20
37
38_LIBCPP_HIDE_FROM_ABI inline constexpr bool operator!=(monostate, monostate) noexcept { return false; }
39
40_LIBCPP_HIDE_FROM_ABI inline constexpr bool operator<(monostate, monostate) noexcept { return false; }
41
42_LIBCPP_HIDE_FROM_ABI inline constexpr bool operator>(monostate, monostate) noexcept { return false; }
43
44_LIBCPP_HIDE_FROM_ABI inline constexpr bool operator<=(monostate, monostate) noexcept { return true; }
45
46_LIBCPP_HIDE_FROM_ABI inline constexpr bool operator>=(monostate, monostate) noexcept { return true; }
47
48#  endif // _LIBCPP_STD_VER >= 20
49
50template <>
51struct hash<monostate> {
52#  if _LIBCPP_STD_VER <= 17 || defined(_LIBCPP_ENABLE_CXX20_REMOVED_BINDER_TYPEDEFS)
53  using argument_type _LIBCPP_DEPRECATED_IN_CXX17 = monostate;
54  using result_type _LIBCPP_DEPRECATED_IN_CXX17   = size_t;
55#  endif
56
57  inline _LIBCPP_HIDE_FROM_ABI size_t operator()(const monostate&) const noexcept {
58    return 66740831; // return a fundamentally attractive random value.
59  }
60};
61
62#endif // _LIBCPP_STD_VER >= 17
63
64_LIBCPP_END_NAMESPACE_STD
65
66#endif // _LIBCPP___VARIANT_MONOSTATE_H