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3267. Rebound allocators and is_always_equal

Section: [allocator.requirements] Status: New Submitter: FrankHB1989 Opened: 2019-08-27 Last modified: 2020-09-06

Priority: 4

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[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.


  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>;

  // 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;

    // 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);

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

Proposed resolution:

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

    Table 34 — Cpp17Allocator requirements [tab:cpp17.allocator]
    Expression Return type Assertion/note
    Y For all U (including T),
    is X.
    XX::is_always_equal::value == YY::is_always_equal::value
    is true.
    See Note A,