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3454. pointer_traits::pointer_to should be constexpr

Section: 20.2.3 [pointer.traits] Status: LEWG Submitter: Alisdair Meredith Opened: 2020-06-21 Last modified: 2022-07-21

Priority: Not Prioritized

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Discussion:

Trying to implement a constexpr std::list (inspired by Tim Song's note on using variant members in the node) as part of evaluating the constexpr container and adapters proposals, I hit problems I could not code around in pointer_traits, as only the specialization for native pointers has a constexpr pointer_to function.

This means that containers of my custom allocator, that delegates all allocation behavior to std::allocator<T> but adds extra telemetry and uses a fancy pointer, does not work with the approach I tried for implementing list (common link type, shared between nodes, and stored as end sentinel directly in the list object).

[2020-07-17; Forwarded to LEWG after review in telecon]

[2022-07-19; Casey Carter comments]

This is no longer simply a theoretical problem that impedes implementing constexpr std::list, but an actual defect affecting current implementations of constexpr std::string. More specifically, it makes it impossible to support so-called "fancy pointers" in a constexpr basic_string that performs the small string optimization (SSO). (pointer_traits::pointer_to is critically necessary to get a pointer that designates the SSO buffer.) As things currently stand, constexpr basic_string can support fancy pointers or SSO, but not both.

Proposed resolution:

This wording is relative to N4861.

1. Modify 20.2.3 [pointer.traits] as indicated:

   -1- The class template pointer_traits supplies a uniform interface to certain attributes of pointer-like types.

   namespace std {
     template<class Ptr> struct pointer_traits {
       using pointer         = Ptr;
       using element_type    = see below;
       using difference_type = see below;
       
       template<class U> using rebind = see below;
       
       static constexpr pointer pointer_to(see below r);
     };
     […]
   }
   

2. Modify 20.2.3.3 [pointer.traits.functions] as indicated:

   static constexpr pointer pointer_traits::pointer_to(see below r);
   static constexpr pointer pointer_traits<T*>::pointer_to(see below r) noexcept;
   

   -1- Mandates: For the first member function, Ptr::pointer_to(r) is well-formed.

   -2- Preconditions: For the first member function, Ptr::pointer_to(r) returns a pointer to r through which indirection is valid.

   -3- Returns: The first member function returns Ptr::pointer_to(r). The second member function returns addressof(r).

   -4- Remarks: If element_type is cv void, the type of r is unspecified; otherwise, it is element_type&.