libcxx/include/__iterator/prev.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_PREV_H
  11 #define _LIBCPP___ITERATOR_PREV_H
  12
  13 #include <__assert>
  14 #include <__config>
  15 #include <__iterator/advance.h>
  16 #include <__iterator/concepts.h>
  17 #include <__iterator/incrementable_traits.h>
  18 #include <__iterator/iterator_traits.h>
  19 #include <__type_traits/enable_if.h>
  20 #include <__utility/move.h>
  21
  22 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
  23 #  pragma GCC system_header
  24 #endif
  25
  26 _LIBCPP_PUSH_MACROS
  27 #include <__undef_macros>
  28
  29 _LIBCPP_BEGIN_NAMESPACE_STD
  30
  31 template <class _InputIter, __enable_if_t<__has_input_iterator_category<_InputIter>::value, int> = 0>
  32 [[__nodiscard__]] inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 _InputIter
  33 prev(_InputIter __x, typename iterator_traits<_InputIter>::difference_type __n) {
  34   // Calling `advance` with a negative value on a non-bidirectional iterator is a no-op in the current implementation.
  35   // Note that this check duplicates the similar check in `std::advance`.
  36   _LIBCPP_ASSERT_PEDANTIC(__n <= 0 || __has_bidirectional_iterator_category<_InputIter>::value,
  37                           "Attempt to prev(it, n) with a positive n on a non-bidirectional iterator");
  38   std::advance(__x, -__n);
  39   return __x;
  40 }
  41
  42 // LWG 3197
  43 // It is unclear what the implications of "BidirectionalIterator" in the standard are.
  44 // However, calling std::prev(non-bidi-iterator) is obviously an error and we should catch it at compile time.
  45 template <class _InputIter, __enable_if_t<__has_input_iterator_category<_InputIter>::value, int> = 0>
  46 [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 _InputIter prev(_InputIter __it) {
  47   static_assert(__has_bidirectional_iterator_category<_InputIter>::value,
  48                 "Attempt to prev(it) with a non-bidirectional iterator");
  49   return std::prev(std::move(__it), 1);
  50 }
  51
  52 #if _LIBCPP_STD_VER >= 20
  53
  54 // [range.iter.op.prev]
  55
  56 namespace ranges {
  57 struct __prev {
  58   template <bidirectional_iterator _Ip>
  59   [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Ip operator()(_Ip __x) const {
  60     --__x;
  61     return __x;
  62   }
  63
  64   template <bidirectional_iterator _Ip>
  65   [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Ip operator()(_Ip __x, iter_difference_t<_Ip> __n) const {
  66     ranges::advance(__x, -__n);
  67     return __x;
  68   }
  69
  70   template <bidirectional_iterator _Ip>
  71   [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Ip
  72   operator()(_Ip __x, iter_difference_t<_Ip> __n, _Ip __bound_iter) const {
  73     ranges::advance(__x, -__n, __bound_iter);
  74     return __x;
  75   }
  76 };
  77
  78 inline namespace __cpo {
  79 inline constexpr auto prev = __prev{};
  80 } // namespace __cpo
  81 } // namespace ranges
  82
  83 #endif // _LIBCPP_STD_VER >= 20
  84
  85 _LIBCPP_END_NAMESPACE_STD
  86
  87 _LIBCPP_POP_MACROS
  88
  89 #endif // _LIBCPP___ITERATOR_PREV_H