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 <__fwd/functional.h>
  15 #include <__type_traits/integral_constant.h>
  16 #include <__utility/forward.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 template <class _Tp>
  25 struct __is_identity : false_type {};
  26
  27 struct __identity {
  28   template <class _Tp>
  29   [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR _Tp&& operator()(_Tp&& __t) const _NOEXCEPT {
  30     return std::forward<_Tp>(__t);
  31   }
  32
  33   using is_transparent = void;
  34 };
  35
  36 template <>
  37 struct __is_identity<__identity> : true_type {};
  38 template <>
  39 struct __is_identity<reference_wrapper<__identity> > : true_type {};
  40 template <>
  41 struct __is_identity<reference_wrapper<const __identity> > : true_type {};
  42
  43 #if _LIBCPP_STD_VER >= 20
  44
  45 struct identity {
  46   template <class _Tp>
  47   [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Tp&& operator()(_Tp&& __t) const noexcept {
  48     return std::forward<_Tp>(__t);
  49   }
  50
  51   using is_transparent = void;
  52 };
  53
  54 template <>
  55 struct __is_identity<identity> : true_type {};
  56 template <>
  57 struct __is_identity<reference_wrapper<identity> > : true_type {};
  58 template <>
  59 struct __is_identity<reference_wrapper<const identity> > : true_type {};
  60
  61 #endif // _LIBCPP_STD_VER >= 20
  62
  63 _LIBCPP_END_NAMESPACE_STD
  64
  65 #endif // _LIBCPP___FUNCTIONAL_IDENTITY_H