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 
21 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
22 #  pragma GCC system_header
23 #endif
24 
25 _LIBCPP_BEGIN_NAMESPACE_STD
26 
27 template <class _InputIter, __enable_if_t<__has_input_iterator_category<_InputIter>::value, int> = 0>
28 inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 _InputIter
29 prev(_InputIter __x, typename iterator_traits<_InputIter>::difference_type __n = 1) {
30   // Calling `advance` with a negative value on a non-bidirectional iterator is a no-op in the current implementation.
31   // Note that this check duplicates the similar check in `std::advance`.
32   _LIBCPP_ASSERT_PEDANTIC(__n <= 0 || __has_bidirectional_iterator_category<_InputIter>::value,
33                           "Attempt to prev(it, n) with a positive n on a non-bidirectional iterator");
34   std::advance(__x, -__n);
35   return __x;
36 }
37 
38 #if _LIBCPP_STD_VER >= 20
39 
40 // [range.iter.op.prev]
41 
42 namespace ranges {
43 namespace __prev {
44 
45 struct __fn {
46   template <bidirectional_iterator _Ip>
operator__fn47   _LIBCPP_HIDE_FROM_ABI constexpr _Ip operator()(_Ip __x) const {
48     --__x;
49     return __x;
50   }
51 
52   template <bidirectional_iterator _Ip>
operator__fn53   _LIBCPP_HIDE_FROM_ABI constexpr _Ip operator()(_Ip __x, iter_difference_t<_Ip> __n) const {
54     ranges::advance(__x, -__n);
55     return __x;
56   }
57 
58   template <bidirectional_iterator _Ip>
operator__fn59   _LIBCPP_HIDE_FROM_ABI constexpr _Ip operator()(_Ip __x, iter_difference_t<_Ip> __n, _Ip __bound_iter) const {
60     ranges::advance(__x, -__n, __bound_iter);
61     return __x;
62   }
63 };
64 
65 } // namespace __prev
66 
67 inline namespace __cpo {
68 inline constexpr auto prev = __prev::__fn{};
69 } // namespace __cpo
70 } // namespace ranges
71 
72 #endif // _LIBCPP_STD_VER >= 20
73 
74 _LIBCPP_END_NAMESPACE_STD
75 
76 #endif // _LIBCPP___ITERATOR_PREV_H
77