[libc++] Remove redundant and somewhat confusing assertions around advance() #133276
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…vance() The std::advance function has a clear precondition that it can only be called with a negative distance when a bidirectional iterator is used. However, prev() and next() don't have such preconditions explicitly, they inherit it from calling advance(). This patch removes assertions in prev() and next() that were duplicates of similar ones in advance(), and removes a copy-pasted comment that was trying to justify the use of _LIBCPP_ASSERT_PEDANTIC but IMO is creating confusion with little benefit.
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@llvm/pr-subscribers-libcxx Author: Louis Dionne (ldionne) ChangesThe std::advance function has a clear precondition that it can only be called with a negative distance when a bidirectional iterator is used. However, prev() and next() don't have such preconditions explicitly, they inherit it from calling advance(). This patch removes assertions in prev() and next() that were duplicates of similar ones in advance(), and removes a copy-pasted comment that was trying to justify the use of _LIBCPP_ASSERT_PEDANTIC but IMO is creating confusion with little benefit. Full diff: https://github.com/llvm/llvm-project/pull/133276.diff 3 Files Affected:
diff --git a/libcxx/include/__iterator/advance.h b/libcxx/include/__iterator/advance.h
index 57b1b845f1afa..f1a8d28f39aa0 100644
--- a/libcxx/include/__iterator/advance.h
+++ b/libcxx/include/__iterator/advance.h
@@ -65,8 +65,7 @@ template < class _InputIter,
_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 void advance(_InputIter& __i, _Distance __orig_n) {
typedef typename iterator_traits<_InputIter>::difference_type _Difference;
_Difference __n = static_cast<_Difference>(std::__convert_to_integral(__orig_n));
- // Calling `advance` with a negative value on a non-bidirectional iterator is a no-op in the current implementation.
- _LIBCPP_ASSERT_PEDANTIC(__n >= 0 || __has_bidirectional_iterator_category<_InputIter>::value,
+ _LIBCPP_ASSERT_PEDANTIC(__has_bidirectional_iterator_category<_InputIter>::value || __n >= 0,
"Attempt to advance(it, n) with negative n on a non-bidirectional iterator");
std::__advance(__i, __n, typename iterator_traits<_InputIter>::iterator_category());
}
@@ -98,9 +97,8 @@ struct __advance {
// Preconditions: If `I` does not model `bidirectional_iterator`, `n` is not negative.
template <input_or_output_iterator _Ip>
_LIBCPP_HIDE_FROM_ABI constexpr void operator()(_Ip& __i, iter_difference_t<_Ip> __n) const {
- // Calling `advance` with a negative value on a non-bidirectional iterator is a no-op in the current implementation.
- _LIBCPP_ASSERT_PEDANTIC(
- __n >= 0 || bidirectional_iterator<_Ip>, "If `n < 0`, then `bidirectional_iterator<I>` must be true.");
+ _LIBCPP_ASSERT_PEDANTIC(bidirectional_iterator<_Ip> || __n >= 0,
+ "ranges::advance: Can only pass a negative `n` with a bidirectional_iterator.");
// If `I` models `random_access_iterator`, equivalent to `i += n`.
if constexpr (random_access_iterator<_Ip>) {
@@ -149,9 +147,9 @@ struct __advance {
template <input_or_output_iterator _Ip, sentinel_for<_Ip> _Sp>
_LIBCPP_HIDE_FROM_ABI constexpr iter_difference_t<_Ip>
operator()(_Ip& __i, iter_difference_t<_Ip> __n, _Sp __bound_sentinel) const {
- // Calling `advance` with a negative value on a non-bidirectional iterator is a no-op in the current implementation.
- _LIBCPP_ASSERT_PEDANTIC((__n >= 0) || (bidirectional_iterator<_Ip> && same_as<_Ip, _Sp>),
- "If `n < 0`, then `bidirectional_iterator<I> && same_as<I, S>` must be true.");
+ _LIBCPP_ASSERT_PEDANTIC(
+ (bidirectional_iterator<_Ip> && same_as<_Ip, _Sp>) || (__n >= 0),
+ "ranges::advance: Can only pass a negative `n` with a bidirectional_iterator coming from a common_range.");
// If `S` and `I` model `sized_sentinel_for<S, I>`:
if constexpr (sized_sentinel_for<_Sp, _Ip>) {
// If |n| >= |bound_sentinel - i|, equivalent to `ranges::advance(i, bound_sentinel)`.
diff --git a/libcxx/include/__iterator/next.h b/libcxx/include/__iterator/next.h
index 1f68a5bec8f39..1143ab31ff7c2 100644
--- a/libcxx/include/__iterator/next.h
+++ b/libcxx/include/__iterator/next.h
@@ -10,7 +10,6 @@
#ifndef _LIBCPP___ITERATOR_NEXT_H
#define _LIBCPP___ITERATOR_NEXT_H
-#include <__assert>
#include <__config>
#include <__iterator/advance.h>
#include <__iterator/concepts.h>
@@ -27,11 +26,6 @@ _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
next(_InputIter __x, typename iterator_traits<_InputIter>::difference_type __n = 1) {
- // 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 next(it, n) with negative n on a non-bidirectional iterator");
-
std::advance(__x, __n);
return __x;
}
diff --git a/libcxx/include/__iterator/prev.h b/libcxx/include/__iterator/prev.h
index bffd5527dc953..97139e067c896 100644
--- a/libcxx/include/__iterator/prev.h
+++ b/libcxx/include/__iterator/prev.h
@@ -10,7 +10,6 @@
#ifndef _LIBCPP___ITERATOR_PREV_H
#define _LIBCPP___ITERATOR_PREV_H
-#include <__assert>
#include <__config>
#include <__iterator/advance.h>
#include <__iterator/concepts.h>
@@ -31,10 +30,6 @@ _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;
}
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mordante
approved these changes
Mar 27, 2025
You can test this locally with the following command:git-clang-format --diff HEAD~1 HEAD --extensions cpp,h -- libcxx/include/__iterator/advance.h libcxx/include/__iterator/next.h libcxx/include/__iterator/prev.h libcxx/test/libcxx/iterators/assert.next.pass.cpp libcxx/test/libcxx/iterators/assert.prev.pass.cppView the diff from clang-format here.diff --git a/libcxx/test/libcxx/iterators/assert.next.pass.cpp b/libcxx/test/libcxx/iterators/assert.next.pass.cpp
index 1e8672308..927070075 100644
--- a/libcxx/test/libcxx/iterators/assert.next.pass.cpp
+++ b/libcxx/test/libcxx/iterators/assert.next.pass.cpp
@@ -25,7 +25,8 @@ int main(int, char**) {
forward_iterator<int *> it(a+1);
(void)std::next(it, 1); // should work fine
(void)std::next(it, 0); // should work fine
- TEST_LIBCPP_ASSERT_FAILURE(std::next(it, -1), "Attempt to advance(it, n) with negative n on a non-bidirectional iterator");
+ TEST_LIBCPP_ASSERT_FAILURE(
+ std::next(it, -1), "Attempt to advance(it, n) with negative n on a non-bidirectional iterator");
return 0;
}
diff --git a/libcxx/test/libcxx/iterators/assert.prev.pass.cpp b/libcxx/test/libcxx/iterators/assert.prev.pass.cpp
index 29b8d6ed5..34b8a7365 100644
--- a/libcxx/test/libcxx/iterators/assert.prev.pass.cpp
+++ b/libcxx/test/libcxx/iterators/assert.prev.pass.cpp
@@ -31,7 +31,8 @@ int main(int, char**) {
forward_iterator<int *> it(a+1);
(void)std::prev(it, -1); // should work fine
(void)std::prev(it, 0); // should work fine
- TEST_LIBCPP_ASSERT_FAILURE(std::prev(it, 1), "Attempt to advance(it, n) with negative n on a non-bidirectional iterator");
+ TEST_LIBCPP_ASSERT_FAILURE(
+ std::prev(it, 1), "Attempt to advance(it, n) with negative n on a non-bidirectional iterator");
return 0;
}
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ldionne
added a commit
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that referenced
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May 6, 2025
…anges::advance As brought up in llvm#133276.
GeorgeARM
pushed a commit
to GeorgeARM/llvm-project
that referenced
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May 7, 2025
…vance() (llvm#133276) The std::advance function has a clear precondition that it can only be called with a negative distance when a bidirectional iterator is used. However, prev() and next() don't have such preconditions explicitly, they inherit it from calling advance(). This patch removes assertions in prev() and next() that were duplicates of similar ones in advance(), and removes a copy-pasted comment that was trying to justify the use of _LIBCPP_ASSERT_PEDANTIC but IMO is creating confusion with little benefit.
llvm-sync bot
pushed a commit
to arm/arm-toolchain
that referenced
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May 7, 2025
…ency with ranges::advance (#138749) As brought up in llvm/llvm-project#133276.
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The std::advance function has a clear precondition that it can only be called with a negative distance when a bidirectional iterator is used. However, prev() and next() don't have such preconditions explicitly, they inherit it from calling advance().
This patch removes assertions in prev() and next() that were duplicates of similar ones in advance(), and removes a copy-pasted comment that was trying to justify the use of _LIBCPP_ASSERT_PEDANTIC but IMO is creating confusion with little benefit.