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3267. Rebound allocators and is_always_equal

Section: 16.4.4.6 [allocator.requirements] Status: New Submitter: FrankHB1989 Opened: 2019-08-27 Last modified: 2022-04-24

Priority: 4

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Discussion:

[allocator.requirements] does not mention the interaction between is_always_equal and allocator rebinding. As the result, a rebound allocator may have different is_always_equal::value to the original allocator.

Further, for an allocator type X satisfying std::allocator_type<X>::is_always_equal::value == true, rebound allocators of X with same type are not guaranteed equal.

Consider:

1. X is used as an allocator for value_type used in a node-based container;

2. Y is the rebound allocator type for the node type used in the implementation;

3. b1 and b2 are values of Y from different allocator objects.

Then, std::allocator_type<X>::is_always_equal::value == true does not necessarily imply b1 == b2.

Since some of containers in the standard have already explicitly relied on is_always_equal of allocators for their value_type (notably, in the exception specification of the move assignment), this can cause subtle problems.

In general, the implementation of the move assignment operator of such a container can not avoid allocation for new nodes when !std::allocator_traits<Y>::propagate_on_container_move_assignment::value && b1 != b2. This can throw, and it can clash with the required exception specification based on std::allocator_traits<value_type>::is_always_equal:

#include <utility>
#include <memory>
#include <new>
#include <map>
#include <functional> 
#include <type_traits> 

using K = int;
using V = int;
using P = std::pair<const K, V>; 

bool stop_alloc; 

template<typename T>
struct AT
{
  using value_type = T; 

  std::shared_ptr<void> sp = {}; 

  template<typename U>
  struct rebind
  {
    using other = AT<U>;
  }; 

  using is_always_equal = std::is_same<T, P>; 

  AT() : sp(is_always_equal::value ? nullptr : new T*()) {}

  AT(const AT& a) = default;

  template<typename U>
  AT(const AT<U>& a) noexcept : sp(a.sp) {} 

  T* allocate(std::size_t size)
  {
    if (stop_alloc)
      throw std::bad_alloc();
    return static_cast<T*>(::operator new(size * sizeof(T)));
  } 

  void deallocate(T* p, std::size_t)
  {
    ::operator delete(p);
  }

  friend bool operator==(const AT& x, const AT& y) noexcept
  {
    return !x.sp.owner_before(y.sp) && !y.sp.owner_before(x.sp);
  } 

  friend bool operator!=(const AT& x, const AT& y) noexcept 
  {
    return !(x == y);
  }

};

using A = AT<P>; 

int main()
{
  // Some sanity checks:
  static_assert(std::is_same_v<A::template rebind<A::value_type>::other, A>);
  // For any U:
  using U = int;
  static_assert(std::is_same_v<A::template rebind<U>::other::template rebind<A::value_type>::other, A>); 

  using C = std::less<>;
  using M = std::map<K, V, C, A>; 

  // As required by the current wording of the container move operator:
  using always_equal = std::allocator_traits<A>::is_always_equal;
  constexpr bool std_nothrow = always_equal::value && std::is_nothrow_move_assignable_v<C>;
  static_assert(std_nothrow);

  // For conforming implementations:
  // static_assert(!(std_nothrow && !std::is_nothrow_move_assignable<M>::value)); 

  M m{{K(), V()}}, m2;
  auto a = m.get_allocator(); 

  a.sp = std::make_shared<int>(42);
  stop_alloc = true;

  try
  {
    // Call terminate with conforming implementations. This does not work on libstdc++.
    m2 = std::move(m);
    // For libstdc++, terminate on allocator-extended move constructor call.
    //    M m3(std::move(m), a);
  }
  catch(...)
  {}
}

[2019-10 Priority set to 4 after reflector discussion]

Previous resolution [SUPERSEDED]:

This wording is relative to N4830.

[Drafting note: Additional questions: Is it necessary to ensure that
XX::propagate_on_container_copy_assignment::value == YY::propagate_on_container_copy_assignment::value is true as well?]

1. Modify 16.4.4.6 [allocator.requirements], Table [tab:cpp17.allocator] "Cpp17Allocator requirements" as indicated:

   Table 34 — Cpp17Allocator requirements [tab:cpp17.allocator]

   Expression	Return type	Assertion/note pre-/post-condition	Default
   …
   typename X::template rebind<U>::other	Y	For all U (including T), Y::template rebind<T>::other is X. XX::is_always_equal::value == YY::is_always_equal::value is true.	See Note A, below.
   …

[2022-04-24; Daniel rebases wording on N4910]

Proposed resolution:

This wording is relative to N4910.

[Drafting note: Additional questions: Is it necessary to ensure that
XX::propagate_on_container_copy_assignment::value == YY::propagate_on_container_copy_assignment::value is true as well?]

1. Modify 16.4.4.6 [allocator.requirements] as indicated:

   typename X::template rebind<U>::other
   

   -16- Result: Y

   -17- Postconditions: For all U (including T), Y::template rebind<T>::other is X. XX::is_always_equal::value == YY::is_always_equal::value is true.

   -18- Remarks: If Allocator is a class template instantiation of the form SomeAllocator<T, Args>, where Args is zero or more type arguments, and Allocator does not supply a rebind member template, the standard allocator_traits template uses SomeAllocator<U, Args> in place of Allocator::rebind<U>::other by default. For allocator types that are not template instantiations of the above form, no default is provided.

   -19- [Note 1: The member class template rebind of X is effectively a typedef template. In general, if the name Allocator is bound to SomeAllocator<T>, then Allocator::rebind<U>::other is the same type as SomeAllocator<U>, where SomeAllocator<T>::value_type is T and SomeAllocator<U>::value_type is U. — end note]