libcxx/include/__functional/identity.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___FUNCTIONAL_IDENTITY_H
  11 #define _LIBCPP___FUNCTIONAL_IDENTITY_H
  12
  13 #include <__config>
  14 #include <__type_traits/integral_constant.h>
  15 #include <__utility/forward.h>
  16
  17 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
  18 #  pragma GCC system_header
  19 #endif
  20
  21 _LIBCPP_BEGIN_NAMESPACE_STD
  22
  23 template <class _Tp>
  24 struct __is_identity : false_type {};
  25
  26 struct __identity {
  27   template <class _Tp>
  28   _LIBCPP_NODISCARD _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR _Tp&& operator()(_Tp&& __t) const _NOEXCEPT {
  29     return std::forward<_Tp>(__t);
  30   }
  31
  32   using is_transparent = void;
  33 };
  34
  35 template <>
  36 struct __is_identity<__identity> : true_type {};
  37
  38 #if _LIBCPP_STD_VER >= 20
  39
  40 struct identity {
  41     template<class _Tp>
  42     _LIBCPP_NODISCARD_EXT _LIBCPP_HIDE_FROM_ABI constexpr _Tp&& operator()(_Tp&& __t) const noexcept
  43     {
  44         return _VSTD::forward<_Tp>(__t);
  45     }
  46
  47     using is_transparent = void;
  48 };
  49
  50 template <>
  51 struct __is_identity<identity> : true_type {};
  52
  53 #endif // _LIBCPP_STD_VER >= 20
  54
  55 _LIBCPP_END_NAMESPACE_STD
  56
  57 #endif // _LIBCPP___FUNCTIONAL_IDENTITY_H