v1/__functional/identity.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___FUNCTIONAL_IDENTITY_H
0011 #define _LIBCPP___FUNCTIONAL_IDENTITY_H
0012
0013 #include <__config>
0014 #include <__fwd/functional.h>
0015 #include <__type_traits/integral_constant.h>
0016 #include <__utility/forward.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 template <class _Tp>
0025 struct __is_identity : false_type {};
0026
0027 struct __identity {
0028   template <class _Tp>
0029   [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR _Tp&& operator()(_Tp&& __t) const _NOEXCEPT {
0030     return std::forward<_Tp>(__t);
0031   }
0032
0033   using is_transparent = void;
0034 };
0035
0036 template <>
0037 struct __is_identity<__identity> : true_type {};
0038 template <>
0039 struct __is_identity<reference_wrapper<__identity> > : true_type {};
0040 template <>
0041 struct __is_identity<reference_wrapper<const __identity> > : true_type {};
0042
0043 #if _LIBCPP_STD_VER >= 20
0044
0045 struct identity {
0046   template <class _Tp>
0047   [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Tp&& operator()(_Tp&& __t) const noexcept {
0048     return std::forward<_Tp>(__t);
0049   }
0050
0051   using is_transparent = void;
0052 };
0053
0054 template <>
0055 struct __is_identity<identity> : true_type {};
0056 template <>
0057 struct __is_identity<reference_wrapper<identity> > : true_type {};
0058 template <>
0059 struct __is_identity<reference_wrapper<const identity> > : true_type {};
0060
0061 #endif // _LIBCPP_STD_VER >= 20
0062
0063 _LIBCPP_END_NAMESPACE_STD
0064
0065 #endif // _LIBCPP___FUNCTIONAL_IDENTITY_H