mirror of
https://github.com/llvm/llvm-project.git
synced 2025-05-03 07:36:07 +00:00
a5d919b4b2
The std::prev function appeared to work on non Cpp17BidirectionalIterators, however it
behaved strangely by being the identity function. That was extremely misleading and
potentially dangerous, since several recent iterators that are C++20 bidirectional_iterators
don't satisfy the Cpp17BidirectionalIterator named requirement. For example:
auto zv = std::views::zip(vec1, vec2);
auto it = zv.begin() + 5;
auto it2 = std::prev(it); // "it2" will be the same as "it", instead of the previous one
Here, zip_view::iterator is a c++20 random_access_iterator, but it only satisfies the
Cpp17InputIterator named requirement because its reference type is a proxy type. Hence
`std::prev` would silently accept that iterator but it would do a no-op instead of going
to the previous position.
This patch changes `std::prev(it)` to produce an error at compile-time when instantiated
with a type that is not a Cpp17BidirectionalIterator.
Fixes #109456
90 lines
3.0 KiB
C++
90 lines
3.0 KiB
C++
// -*- C++ -*-
|
|
//===----------------------------------------------------------------------===//
|
|
//
|
|
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
|
|
// See https://llvm.org/LICENSE.txt for license information.
|
|
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
|
|
//
|
|
//===----------------------------------------------------------------------===//
|
|
|
|
#ifndef _LIBCPP___ITERATOR_PREV_H
|
|
#define _LIBCPP___ITERATOR_PREV_H
|
|
|
|
#include <__assert>
|
|
#include <__config>
|
|
#include <__iterator/advance.h>
|
|
#include <__iterator/concepts.h>
|
|
#include <__iterator/incrementable_traits.h>
|
|
#include <__iterator/iterator_traits.h>
|
|
#include <__type_traits/enable_if.h>
|
|
#include <__utility/move.h>
|
|
|
|
#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
|
|
# pragma GCC system_header
|
|
#endif
|
|
|
|
_LIBCPP_PUSH_MACROS
|
|
#include <__undef_macros>
|
|
|
|
_LIBCPP_BEGIN_NAMESPACE_STD
|
|
|
|
template <class _InputIter, __enable_if_t<__has_input_iterator_category<_InputIter>::value, int> = 0>
|
|
[[__nodiscard__]] inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 _InputIter
|
|
prev(_InputIter __x, typename iterator_traits<_InputIter>::difference_type __n) {
|
|
// Calling `advance` with a negative value on a non-bidirectional iterator is a no-op in the current implementation.
|
|
// Note that this check duplicates the similar check in `std::advance`.
|
|
_LIBCPP_ASSERT_PEDANTIC(__n <= 0 || __has_bidirectional_iterator_category<_InputIter>::value,
|
|
"Attempt to prev(it, n) with a positive n on a non-bidirectional iterator");
|
|
std::advance(__x, -__n);
|
|
return __x;
|
|
}
|
|
|
|
// LWG 3197
|
|
// It is unclear what the implications of "BidirectionalIterator" in the standard are.
|
|
// However, calling std::prev(non-bidi-iterator) is obviously an error and we should catch it at compile time.
|
|
template <class _InputIter, __enable_if_t<__has_input_iterator_category<_InputIter>::value, int> = 0>
|
|
[[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 _InputIter prev(_InputIter __it) {
|
|
static_assert(__has_bidirectional_iterator_category<_InputIter>::value,
|
|
"Attempt to prev(it) with a non-bidirectional iterator");
|
|
return std::prev(std::move(__it), 1);
|
|
}
|
|
|
|
#if _LIBCPP_STD_VER >= 20
|
|
|
|
// [range.iter.op.prev]
|
|
|
|
namespace ranges {
|
|
struct __prev {
|
|
template <bidirectional_iterator _Ip>
|
|
[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Ip operator()(_Ip __x) const {
|
|
--__x;
|
|
return __x;
|
|
}
|
|
|
|
template <bidirectional_iterator _Ip>
|
|
[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Ip operator()(_Ip __x, iter_difference_t<_Ip> __n) const {
|
|
ranges::advance(__x, -__n);
|
|
return __x;
|
|
}
|
|
|
|
template <bidirectional_iterator _Ip>
|
|
[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Ip
|
|
operator()(_Ip __x, iter_difference_t<_Ip> __n, _Ip __bound_iter) const {
|
|
ranges::advance(__x, -__n, __bound_iter);
|
|
return __x;
|
|
}
|
|
};
|
|
|
|
inline namespace __cpo {
|
|
inline constexpr auto prev = __prev{};
|
|
} // namespace __cpo
|
|
} // namespace ranges
|
|
|
|
#endif // _LIBCPP_STD_VER >= 20
|
|
|
|
_LIBCPP_END_NAMESPACE_STD
|
|
|
|
_LIBCPP_POP_MACROS
|
|
|
|
#endif // _LIBCPP___ITERATOR_PREV_H
|