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3237. LWG 3038 and 3190 have inconsistent PRs

Section: 20.5.3.3 [mem.poly.allocator.mem] Status: C++20 Submitter: Casey Carter Opened: 2019-07-18 Last modified: 2021-02-25

Priority: 2

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Discussion:

Both LWG 3038⁽ⁱ⁾ and LWG 3190⁽ⁱ⁾ deal with how to respond to requests to allocate "n * sizeof(T)" bytes of memory when n * sizeof(T) is not sufficient storage for n objects of type T, i.e., when n > SIZE_MAX / sizeof(T). LWG 3038⁽ⁱ⁾ changed polymorphic_allocator::allocate to throw length_error upon detecting this condition, whereas LWG 3190⁽ⁱ⁾ changed allocator::allocate to throw bad_array_new_length. It's peculiar that two standard library components which allocate memory both detect this condition but handle it by throwing different exception types; for consistency, the two should be harmonized.

Reflector discussion of 3190 seemed to achieve consensus that bad_array_new_length was the better option. Unlike length_error, bad_array_new_length derives from bad_alloc so we can make this change without altering the invariant that allocation functions either succeed or throw an exception derived from bad_alloc.

Further, P0339R6 "polymorphic_allocator<> as a vocabulary type" recently added the function template "template<class T> T* allocate_object(size_t n = 1);" to std::pmr::polymorphic_allocator, which is another instance of the "allocate memory for n objects of type T" pattern. 20.5.3.3 [mem.poly.allocator.mem] paragraph 8.1 specifies that allocate_object throws length_error when SIZE_MAX / sizeof(T) < n, presumably for consistency with std::pmr::polymorphic_allocator::allocate specified in paragraph 1. allocate_object's behavior should be consistent with allocator::allocate and polymorphic_allocator::allocate so we have a single means of communicating "request for allocation of unrepresentable size" errors in the Standard Library.

[2020-02 Moved to Immediate on Thursday afternoon in Prague.]

Proposed resolution:

This wording is relative to N4820.

1. Modify 20.5.3.3 [mem.poly.allocator.mem] as indicated:

   [[nodiscard]] Tp* allocate(size_t n);
   

   -1- Effects: If SIZE_MAX / sizeof(Tp) < n, throws length_errorbad_array_new_length. Otherwise equivalent to:

   return static_cast<Tp*>(memory_rsrc->allocate(n * sizeof(Tp), alignof(Tp)));
   

   […]

   template<class T>
     T* allocate_object(size_t n = 1);
   

   -8- Effects: Allocates memory suitable for holding an array of n objects of type T, as follows:

   1. (8.1) — if SIZE_MAX / sizeof(T) < n, throws length_errorbad_array_new_length,

   2. (8.2) — otherwise equivalent to:

      return static_cast<T*>(allocate_bytes(n*sizeof(T), alignof(T)));