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### 3521. Overly strict requirements on `qsort` and `bsearch`

Section: 25.12 [alg.c.library] Status: WP Submitter: Richard Smith Opened: 2021-02-02 Last modified: 2021-06-07

Priority: Not Prioritized

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Discussion:

Per 25.12 [alg.c.library]/2, for `qsort` and `bsearch`, we have:

> Preconditions: The objects in the array pointed to by `base` are of trivial type.

This seems like an unnecessarily strict requirement. `qsort` only needs the objects to be of a trivially-copyable type (because it will use `memcpy` or equivalent to relocate them), and `bsearch` doesn't need any particular properties of the array element type. Presumably it would be in improvement to specify the more-precise requirements instead.

We should also reconsider the other uses of the notion of a trivial type. It's really not a useful or meaningful type property by itself, because it doesn't actually require that any operations on the type are valid (due to the possibility of them being ambiguous or only some of them being available) and the places that consider it very likely actually mean `is_trivially_copyable` plus `is_trivially_default_constructible` instead, or perhaps `is_trivially_copy_constructible` and `is_trivially_move_constructible` and so on.

Other than `qsort` and `bsearch`, the only uses of this type property in the standard are to constrain `max_align_t`, `aligned_storage`, `aligned_union`, and the element type of `basic_string` (and in the definition of the deprecated `is_pod` trait), all of which (other than `is_pod`) I think really mean "is trivially default constructible", not "has at least one eligible default constructor and all eligible default constructors are trivial". And in fact I think the alignment types are underspecified — we don't want to require merely that they be trivially-copyable, since that doesn't require any particular operation on them to actually be valid — we also want to require that they actually model `semiregular`.

[2021-02-23; Casey Carter provides concrete wording]

[2021-03-12; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4878.

1. Modify 25.12 [alg.c.library] as indicated:

   >     void* bsearch(const void* key, const void* base, size_t nmemb, size_t size,
   >                  c-compare-pred* compar);
   >     void* bsearch(const void* key, const void* base, size_t nmemb, size_t size,
   >                   compare-pred* compar);
   >     void qsort(void* base, size_t nmemb, size_t size, c-compare-pred* compar);
   >     void qsort(void* base, size_t nmemb, size_t size, compare-pred* compar);

   > > -2- Preconditions: For `qsort`, t The objects in the array pointed to by base are of trivial trivially copyable type.