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Showing content from https://cplusplus.github.io/LWG/issue4318 below:

erase_if reevaluate end() to avoid UB

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4318. Have hive::erase_if reevaluate end() to avoid UB

Section: 23.3.9.6 [hive.erasure] Status: New Submitter: Frank Birbacher Opened: 2025-08-16 Last modified: 2025-08-17

Priority: Not Prioritized

View all other issues in [hive.erasure].

View all issues with New status.

Discussion:

Background: https://github.com/cplusplus/draft/pull/8162

For 23.3.9.6 [hive.erasure] p2, the defining code must not cache the end-iterator. In case the last element of the sequence is removed, the past-the-end iterator will be invalidated. This will trigger UB in the loop condition. Instead, re-evaluate end() each time.

Proposed resolution:

This wording is relative to N5014.

[Drafting note: There are other ways to fix this code while keeping the caching behaviour, but I don't see any particular reason to do so for the definition of the effects.]

  1. Modify 23.3.9.6 [hive.erasure] as indicated:

    template<class T, class Allocator, class Predicate>
  typename hive<T, Allocator>::size_type
    erase_if(hive<T, Allocator>& c, Predicate pred);

    -2- Effects: Equivalent to:

    auto original_size = c.size();
for (auto i = c.begin(), last = c.end(); i != lastc.end(); ) {
  if (pred(*i)) {
    i = c.erase(i);
  } else {
    ++i;
  }
}
return original_size - c.size();

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