v1/__variant/monostate.h

0001 // -*- C++ -*-
0002 //===----------------------------------------------------------------------===//
0003 //
0004 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
0005 // See https://llvm.org/LICENSE.txt for license information.
0006 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
0007 //
0008 //===----------------------------------------------------------------------===//
0009
0010 #ifndef _LIBCPP___VARIANT_MONOSTATE_H
0011 #define _LIBCPP___VARIANT_MONOSTATE_H
0012
0013 #include <__compare/ordering.h>
0014 #include <__config>
0015 #include <__cstddef/size_t.h>
0016 #include <__functional/hash.h>
0017
0018 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
0019 #  pragma GCC system_header
0020 #endif
0021
0022 _LIBCPP_BEGIN_NAMESPACE_STD
0023
0024 #if _LIBCPP_STD_VER >= 17
0025
0026 struct _LIBCPP_TEMPLATE_VIS monostate {};
0027
0028 _LIBCPP_HIDE_FROM_ABI inline constexpr bool operator==(monostate, monostate) noexcept { return true; }
0029
0030 #  if _LIBCPP_STD_VER >= 20
0031
0032 _LIBCPP_HIDE_FROM_ABI inline constexpr strong_ordering operator<=>(monostate, monostate) noexcept {
0033   return strong_ordering::equal;
0034 }
0035
0036 #  else // _LIBCPP_STD_VER >= 20
0037
0038 _LIBCPP_HIDE_FROM_ABI inline constexpr bool operator!=(monostate, monostate) noexcept { return false; }
0039
0040 _LIBCPP_HIDE_FROM_ABI inline constexpr bool operator<(monostate, monostate) noexcept { return false; }
0041
0042 _LIBCPP_HIDE_FROM_ABI inline constexpr bool operator>(monostate, monostate) noexcept { return false; }
0043
0044 _LIBCPP_HIDE_FROM_ABI inline constexpr bool operator<=(monostate, monostate) noexcept { return true; }
0045
0046 _LIBCPP_HIDE_FROM_ABI inline constexpr bool operator>=(monostate, monostate) noexcept { return true; }
0047
0048 #  endif // _LIBCPP_STD_VER >= 20
0049
0050 template <>
0051 struct _LIBCPP_TEMPLATE_VIS hash<monostate> {
0052   using argument_type = monostate;
0053   using result_type   = size_t;
0054
0055   inline _LIBCPP_HIDE_FROM_ABI result_type operator()(const argument_type&) const _NOEXCEPT {
0056     return 66740831; // return a fundamentally attractive random value.
0057   }
0058 };
0059
0060 #endif // _LIBCPP_STD_VER >= 17
0061
0062 _LIBCPP_END_NAMESPACE_STD
0063
0064 #endif // _LIBCPP___VARIANT_MONOSTATE_H