libcxx/include/__variant/monostate.h

   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
  26 struct _LIBCPP_TEMPLATE_VIS 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
  50 template <>
  51 struct _LIBCPP_TEMPLATE_VIS hash<monostate> {
  52   using argument_type = monostate;
  53   using result_type   = size_t;
  54
  55   inline _LIBCPP_HIDE_FROM_ABI result_type operator()(const argument_type&) const _NOEXCEPT {
  56     return 66740831; // return a fundamentally attractive random value.
  57   }
  58 };
  59
  60 #endif // _LIBCPP_STD_VER >= 17
  61
  62 _LIBCPP_END_NAMESPACE_STD
  63
  64 #endif // _LIBCPP___VARIANT_MONOSTATE_H