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
58