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### 3036. `polymorphic_allocator::destroy` is extraneous

Section: 20.12.3 [mem.poly.allocator.class] Status: WP Submitter: Casey Carter Opened: 2017-11-15 Last modified: 2020-11-09

Priority: 3

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Discussion:

`polymorphic_allocator`'s member function `destroy` is exactly equivalent to the default implementation of `destroy` in `allocator_traits` (20.10.8.3 [allocator.traits.members] para 6). It should be struck from `polymorphic_allocator` as it provides no value.

[28-Nov-2017 Mailing list discussion - set priority to P3]

PJ says that Dinkumware is shipping an implementation of `polymorphic_allocator` with `destroy`, so removing it would be a breaking change for him.

[2019-02; Kona Wednesday night issue processing]

Status to Open; revisit once P0339 lands. Poll taken was 5-3-2 in favor of removal.

[2020-10-05; Jonathan provides new wording]

Previous resolution [SUPERSEDED]:

> Wording relative to N4700.

> 1. Strike the declaration of `destroy` from the synopsis of class `polymorphic_allocator` in 20.12.3 [mem.poly.allocator.class]:

>    >     template <class T1, class T2, class U, class V>
>    >       void construct(pair<T1,T2>* p, pair<U, V>&& pr);

>    >     template <class T>
>    >       void destroy(T* p);

>    >     polymorphic_allocator select_on_container_copy_construction() const;

> 2. Strike the specification of `destroy` in 20.12.3.3 [mem.poly.allocator.mem]:

>    > […]

>    >     template <class T>
>    >       void destroy(T* p);

>    > 14 Effects: As if by `p->~T()`.

>    > […]

[2020-10-11; Reflector poll]

Moved to Tentatively Ready after seven votes in favour.

[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]

Proposed resolution:

Wording relative to N4861.

1. Strike the declaration of `destroy` from the synopsis of class `polymorphic_allocator` in 20.12.3 [mem.poly.allocator.class]:

   >     template <class T1, class T2, class U, class V>
   >       void construct(pair<T1,T2>* p, pair<U, V>&& pr);

   >     template <class T>
   >       void destroy(T* p);

   >     polymorphic_allocator select_on_container_copy_construction() const;

2. Adjust the specification of `delete_object` in 20.12.3.3 [mem.poly.allocator.mem]:

   >     template <class T>
   >       void delete_object(T* p);

   > -13- Effects: Equivalent to:

   > >       allocator_traits<polymorphic_allocator>::destroy(*this, p);
   > >       deallocate_object(p);

3. Strike the specification of `destroy` in 20.12.3.3 [mem.poly.allocator.mem]:

   > […]

   >     template <class T>
   >       void destroy(T* p);

   > -17- Effects: As if by `p->~T()`.

   > […]

4. Add a new subclause to Annex D:

   > D.?? Deprecated `polymorphic_allocator` member function

   > -1- The following member is declared in addition to those members specified in 20.12.3.3 [mem.poly.allocator.mem]:

   > >     namespace std::pmr {
   > >       template<class Tp = byte>
   > >       class polymorphic_allocator {
   > >       public:
   > >         template <class T>
   > >           void destroy(T* p);
   > >       };
   > >     }

   >     template <class T>
   >       void destroy(T* p);

   > -1- Effects: As if by `p->~T()`.