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4584. std::hive::sort should add Cpp17MoveConstructible to its preconditions on T

Section: 23.3.9.5 [hive.operations] Status: New Submitter: Joaquín M López Muñoz Opened: 2026-05-24 Last modified: 2026-05-24

Priority: Not Prioritized

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Discussion:

std::hive::sort (23.3.9.5 [hive.operations]) requires that T be Cpp17MoveInsertable into hive, Cpp17MoveAssignable, and Cpp17Swappable. It does not require T to be Cpp17MoveConstructible, which is a prerequisite for the internal use of std::sort by the implementation. Note that, in general, Cpp17MoveInsertable into hive (or any container) does not imply Cpp17MoveConstructible. Both the de facto reference implementation for std::hive (Matthew Bentley's plf::hive) and the ongoing implementation for VS use std::sort directly on T.

Strictly speaking, the lack of this precondition can be salvaged by invoking std::sort on a range of values of type X, where X is an ad-hoc type wrapping a T value and an allocator object which implements move construction in terms of std::allocator_traits<A>::construct. So, even if this issue is resolved as NAD, that would be informative for standard library implementors so that they can potentially fix their code.

Proposed resolution:

This wording is relative to N5046.

1. Modify 23.3.9.5 [hive.operations] as indicated:

   template<class Compare = less<T>>
     void sort(Compare comp = Compare());
   

   -13- Preconditions: T is Cpp17MoveInsertable into hive, Cpp17MoveConstructible, Cpp17MoveAssignable, and Cpp17Swappable.

   -14- Effects: Sorts *this according to the comp function object. If an exception is thrown, the order of the elements in *this is unspecified.

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