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New issue from Oscar Slotosch: "Wrong range in [alg.search]"
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| <?xml version='1.0' encoding='utf-8' standalone='no'?> | ||
| <!DOCTYPE issue SYSTEM "lwg-issue.dtd"> | ||
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| <issue num="4179" status="New"> | ||
| <title>Wrong range in [alg.search]</title> | ||
| <section><sref ref="[alg.search]"/></section> | ||
| <submitter>Oscar Slotosch</submitter> | ||
| <date>05 Dec 2024</date> | ||
| <priority>99</priority> | ||
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| <discussion> | ||
| <p> | ||
| Originally reported as editorial request <a href="https://github.com/cplusplus/draft/issues/7474">#7474</a>: | ||
| <p/> | ||
| During the qualification of the C++ STL Validas has pointed me to the following issue: | ||
| <p/> | ||
| The specification of <sref ref="[alg.search]"/> has a wrong range. Currently the range is | ||
| "<tt>[first1, last1 - (last2 - first2))</tt>" (exclusive) but should be | ||
| "<tt>[first1, last1 - (last2 - first2)]</tt>" (inclusive). So please correct the closing ")" to "]". | ||
| Otherwise the last occurrence will not be found. We observed the issue in C++14 and C++17 | ||
| and cppreference.com. | ||
| <p/> | ||
| The implementations do the right thing and work correctly and find even the last occurrence. | ||
| <p/> | ||
| For example in the list `{1, 2, 3, 4, 5}` the pattern `{4, 5}` should be found (obviously). | ||
| In the case the last element is not included it will not be found. | ||
| <p/> | ||
| <a href="https://godbolt.org/z/daMa5nTY9">Demonstration on godbolt</a> shows that the implementation | ||
| is working and finds the pattern. | ||
| </p> | ||
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| <note>2024-12-07; Daniel comments and provides wording</note> | ||
| <p> | ||
| The mentioned wording issue is present in all previous standard versions, including the | ||
| <a href="https://sgistl.github.io/search.html">SGI STL</a>. It needs to be fixed in all search | ||
| variants specified in <sref ref="[alg.search]"/> (except for the form using a `Searcher`). | ||
| </p> | ||
| </discussion> | ||
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| <resolution> | ||
| <p> | ||
| This wording is relative to <paper num="N4993"/>. | ||
| </p> | ||
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| <ol> | ||
| <li><p>Modify <sref ref="[alg.search]"/> as indicated:</p> | ||
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| <blockquote> | ||
| <pre> | ||
| template<class ForwardIterator1, class ForwardIterator2> | ||
| constexpr ForwardIterator1 | ||
| search(ForwardIterator1 first1, ForwardIterator1 last1, | ||
| ForwardIterator2 first2, ForwardIterator2 last2); | ||
| […] | ||
| template<class ExecutionPolicy, class ForwardIterator1, class ForwardIterator2, | ||
| class BinaryPredicate> | ||
| ForwardIterator1 | ||
| search(ExecutionPolicy&& exec, | ||
| ForwardIterator1 first1, ForwardIterator1 last1, | ||
| ForwardIterator2 first2, ForwardIterator2 last2, | ||
| BinaryPredicate pred); | ||
| </pre> | ||
| <blockquote> | ||
| <p> | ||
| -1- <i>Returns</i>: The first iterator `i` in the range <tt>[first1, last1 - (last2 - first2)<ins>]</ins><del>)</del></tt> | ||
| such that for every nonnegative integer `n` less than `last2 - first2` the following | ||
| corresponding conditions hold: `*(i + n) == *(first2 + n), pred(*(i + n)`, `*(first2 + n)) != false`. | ||
| Returns `first1` if `[first2, last2)` is empty, otherwise returns `last1` if no such iterator is found. | ||
| <p/> | ||
| -2- […] | ||
| </p> | ||
| </blockquote> | ||
| <pre> | ||
| template<forward_iterator I1, sentinel_for<I1> S1, forward_iterator I2, | ||
| sentinel_for<I2> S2, class Pred = ranges::equal_to, | ||
| class Proj1 = identity, class Proj2 = identity> | ||
| requires indirectly_comparable<I1, I2, Pred, Proj1, Proj2> | ||
| constexpr subrange<I1> | ||
| ranges::search(I1 first1, S1 last1, I2 first2, S2 last2, Pred pred = {}, | ||
| Proj1 proj1 = {}, Proj2 proj2 = {}); | ||
| template<forward_range R1, forward_range R2, class Pred = ranges::equal_to, | ||
| class Proj1 = identity, class Proj2 = identity> | ||
| requires indirectly_comparable<iterator_t<R1>, iterator_t<R2>, Pred, Proj1, Proj2> | ||
| constexpr borrowed_subrange_t<R1> | ||
| ranges::search(R1&& r1, R2&& r2, Pred pred = {}, | ||
| Proj1 proj1 = {}, Proj2 proj2 = {}); | ||
| </pre> | ||
| <blockquote> | ||
| <p> | ||
| -3- <i>Returns</i>: | ||
| </p> | ||
| <ol style="list-style-type: none"> | ||
| <li><p>(3.1) — `{i, i + (last2 - first2)}`, where `i` is the first iterator in the range | ||
| <tt>[first1, last1 - (last2 - first2)<ins>]</ins><del>)</del></tt> such that for every non-negative integer `n` | ||
| less than `last2 - first2` the condition</p> | ||
| <blockquote><pre> | ||
| bool(invoke(pred, invoke(proj1, *(i + n)), invoke(proj2, *(first2 + n)))) | ||
| </pre></blockquote> | ||
| <p>is `true`.</p> | ||
| </li> | ||
| <li><p>(3.2) — Returns `{last1, last1}` if no such iterator exists.</p></li> | ||
| </ol> | ||
| <p> | ||
| -4- […] | ||
| </p> | ||
| </blockquote> | ||
| <pre> | ||
| template<class ForwardIterator, class Size, class T = iterator_traits<ForwardIterator>::value_type> | ||
| constexpr ForwardIterator | ||
| search_n(ForwardIterator first, ForwardIterator last, | ||
| Size count, const T& value); | ||
| […] | ||
| template<class ExecutionPolicy, class ForwardIterator, class Size, | ||
| class T = iterator_traits<ForwardIterator>::value_type, | ||
| class BinaryPredicate> | ||
| ForwardIterator | ||
| search_n(ExecutionPolicy&& exec, | ||
| ForwardIterator first, ForwardIterator last, | ||
| Size count, const T& value, | ||
| BinaryPredicate pred); | ||
| </pre> | ||
| <blockquote> | ||
| <p> | ||
| -5- […] | ||
| <p/> | ||
| -6- […] | ||
| <p/> | ||
| -7- <i>Returns</i>: The first iterator `i` in the range <tt>[first, last-count<ins>]</ins><del>)</del></tt> | ||
| such that for every non-negative integer `n` less than `count` the condition <tt><i>E</i></tt> is `true`. | ||
| Returns `last` if no such iterator is found. | ||
| <p/> | ||
| -8- […] | ||
| </p> | ||
| </blockquote> | ||
| <pre> | ||
| template<forward_iterator I, sentinel_for<I> S, | ||
| class Pred = ranges::equal_to, class Proj = identity, | ||
| class T = projected_value_t<I, Proj>> | ||
| requires indirectly_comparable<I, const T*, Pred, Proj> | ||
| constexpr subrange<I> | ||
| ranges::search_n(I first, S last, iter_difference_t<I> count, | ||
| const T& value, Pred pred = {}, Proj proj = {}); | ||
| template<forward_range R, class Pred = ranges::equal_to, | ||
| class Proj = identity, class T = projected_value_t<iterator_t<R>, Proj>> | ||
| requires indirectly_comparable<iterator_t<R>, const T*, Pred, Proj> | ||
| constexpr borrowed_subrange_t<R> | ||
| ranges::search_n(R&& r, range_difference_t<R> count, | ||
| const T& value, Pred pred = {}, Proj proj = {}); | ||
| </pre> | ||
| <blockquote> | ||
| <p> | ||
| -9- <i>Returns</i>: `{i, i + count}` where `i` is the first iterator in the range | ||
| <tt>[first, last - count<ins>]</ins><del>)</del></tt> such that for every non-negative integer | ||
| `n` less than `count`, the following condition holds: `invoke(pred, invoke(proj, *(i + n)), value)`. | ||
| Returns `{last, last}` if no such iterator is found. | ||
| <p/> | ||
| -10- […] | ||
| </p> | ||
| </blockquote> | ||
| </blockquote> | ||
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| </li> | ||
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| </ol> | ||
| </resolution> | ||
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| </issue> |