libcxx/include/__iterator/projected.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___ITERATOR_PROJECTED_H
  11 #define _LIBCPP___ITERATOR_PROJECTED_H
  12
  13 #include <__config>
  14 #include <__iterator/concepts.h>
  15 #include <__iterator/incrementable_traits.h> // iter_difference_t
  16 #include <__type_traits/remove_cvref.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 #if _LIBCPP_STD_VER >= 20
  25
  26 template <class _It, class _Proj>
  27 struct __projected_impl {
  28   struct __type {
  29     using value_type = remove_cvref_t<indirect_result_t<_Proj&, _It>>;
  30     indirect_result_t<_Proj&, _It> operator*() const; // not defined
  31   };
  32 };
  33
  34 template <weakly_incrementable _It, class _Proj>
  35 struct __projected_impl<_It, _Proj> {
  36   struct __type {
  37     using value_type      = remove_cvref_t<indirect_result_t<_Proj&, _It>>;
  38     using difference_type = iter_difference_t<_It>;
  39     indirect_result_t<_Proj&, _It> operator*() const; // not defined
  40   };
  41 };
  42
  43 // Note that we implement std::projected in a way that satisfies P2538R1 even in standard
  44 // modes before C++26 to avoid breaking the ABI between standard modes (even though ABI
  45 // breaks with std::projected are expected to have essentially no impact).
  46 template <indirectly_readable _It, indirectly_regular_unary_invocable<_It> _Proj>
  47 using projected = typename __projected_impl<_It, _Proj>::__type;
  48
  49 #endif // _LIBCPP_STD_VER >= 20
  50
  51 _LIBCPP_END_NAMESPACE_STD
  52
  53 #endif // _LIBCPP___ITERATOR_PROJECTED_H