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3036. polymorphic_allocator::destroy is extraneous

Section: 20.4.3 [mem.poly.allocator.class] Status: C++23 Submitter: Casey Carter Opened: 2017-11-15 Last modified: 2023-11-22

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 (20.2.9.3 [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Previous resolution [SUPERSEDED]: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 20.4.3 [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 20.4.3.3 [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 20.4.3 [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 20.4.3.3 [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 20.4.3.3 [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 20.4.3.3 [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().