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3683. operator== for polymorphic_allocator cannot deduce template argument in common cases

Section: 20.4.3 [mem.poly.allocator.class] Status: C++23 Submitter: Pablo Halpern Opened: 2022-03-18 Last modified: 2023-11-22

Priority: Not Prioritized

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Discussion:

In <memory_resource>, the equality comparison operator for pmr::polymorphic_allocator is declared in namespace scope as:

template<class T1, class T2>
  bool operator==(const polymorphic_allocator<T1>& a,
                  const polymorphic_allocator<T2>& b) noexcept;

Since polymorphic_allocator is implicitly convertible from memory_resource*, one would naively expect — and the author of polymorphic_allocator intended — the following code to work:

std::pmr::unsynchronized_pool_resource pool_rsrc;
std::pmr::vector<int> vec(&pool_rsrc); // Converts to std::pmr::polymorphic_allocator<int>
[…]
assert(vec.get_allocator() == &pool_rsrc);  // (1) Compare polymorphic_allocator to memory_resource*

Unfortunately, the line labeled (1) is ill-formed because the type T2 in operator== cannot be deduced.

Possible resolution 1 (PR1) is to supply a second operator==, overloaded for comparison to memory_resource*:

template<class T1, class T2>
  bool operator==(const polymorphic_allocator<T1>& a,
                  const polymorphic_allocator<T2>& b) noexcept;
template<class T>
  bool operator==(const polymorphic_allocator<T>& a,
                  memory_resource* b) noexcept;

The rules for implicitly defined spaceship and comparison operators obviates defining operator!= or operator==(b, a). This PR would allow polymorphic_allocator to be compared for equality with memory_resource*, but not with any other type that is convertible to polymorphic_allocator.

Possible resolution 2 (PR2) is to replace operator== with a homogeneous version where type deduction occurs only for one template parameter:

template<class T1, class T2>
  bool operator==(const polymorphic_allocator<T1>& a,
                  const polymorphic_allocator<T2>& b) noexcept;
template<class T>
  bool operator==(const polymorphic_allocator<T>& a,
                  const type_identity_t<polymorphic_allocator<T>>& b) noexcept;

This version will work with any type that is convertible to polymorphic_allocator.

Possible resolution 3 (PR3), the proposed resolution, below, is to add a homogeneous equality operator as a "hidden friend", such that it is found by ADL only if one argument is a polymorphic_allocator and the other argument is convertible to polymorphic_allocator. As with PR2, this PR will work with any type that is convertible to polymorphic_allocator.

Note to reader: Proof of concept for the three possible resolutions can be seen at this godbolt link. Uncomment one of PR1, PR2, or PR3 macros to see the effects of each PR.

[2022-05-17; Reflector poll]

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

[2022-07-15; LWG telecon: move to Ready]

[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready → WP.]

Proposed resolution:

This wording is relative to N4901.

1. Modify 20.4.3 [mem.poly.allocator.class], class template polymorphic_allocator synopsis, as indicated:

   namespace std::pmr {
     template<class Tp = byte> class polymorphic_allocator {
       memory_resource* memory_rsrc; // exposition only
   
     public:
       using value_type = Tp;
   
       […]
       memory_resource* resource() const;
       
       // friends
       friend bool operator==(const polymorphic_allocator& a,
                              const polymorphic_allocator& b) noexcept {
         return *a.resource() == *b.resource();
       }
     };
   }