2311. Allocator requirements should be further minimized

Section: 16.4.4.6 [allocator.requirements] Status: NAD Submitter: Stephan T. Lavavej Opened: 2013-09-21 Last modified: 2016-01-28

Priority: 2

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Discussion:

C++11's minimized allocator requirements are great, but they're still requiring more things from users than absolutely necessary.

• They require X::value_type, but that can be deduced from SomeAllocator<T, Args>.

• They require a1 == a2, but that could default to true as most allocators are stateless.

• They require a1 != a2, but if we start requiring STL implementations to go through allocator_traits to provide an op== default, we won't need to require op!= from users at all. (std::allocator, of course, would continue to provide op!=. Note that this is analogous to reference/const_reference — std::allocator still provides them, but we don't require them from users, and in fact we don't require them to be consistent or meaningful if present.)

• They require a == b and a != b. This requirement was not present in C++98/03, it is not necessary (a == b is always required to be equivalent to rebind-then-compare), and STL implementations don't even need to compare allocators of different types directly.

[2014-02-14 Issaquah: Close as NAD]

Different vendors rely on each of the different elements suggested to be removed.

While value_type my be deduced as suggested, far too much wording relies on it being available, and the standard churn is likely to be much harder than presented here.

Proposed resolution:

This wording is relative to N3691.

1. Change in 16.4.4.6 [allocator.requirements], Table 28 — "Allocator requirements" as indicated:

   Table 28 — Allocator requirements (continued)

   Expression	Return type	Assertion/note pre-/post-condition	Default
   …
   X::value_type	Identical to T		See Note B, below.
   …
   a1 == a2	bool	returns true only if storage allocated from each can be deallocated via the other. operator== shall be reflexive, symmetric, and transitive, and shall not exit via an exception.	true
   a1 != a2	bool	same as !(a1 == a2)
   a == b	bool	same as a == Y::rebind<T>::other(b)
   a != b	bool	same as !(a == b)
   …
   X a(b);		Shall not exit via an exception. post: Y(a) == b, a == X(b)
   …
   X a(move(b));		Shall not exit via an exception. post: a equals the prior value of X(b).
   …

2. After 16.4.4.6 [allocator.requirements] p3, add a new paragraph:

   Note B: 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 nested type named value_type, the standard allocator_traits template uses T in place of Allocator::value_type by default. For allocator types that are not template instantiations of the above form, no default is provided.

3. In the example provided in 16.4.4.6 [allocator.requirements]/5, delete as indicated:

   template <class Tp>
   struct SimpleAllocator {
     typedef Tp value_type;
     SimpleAllocator(ctor args);
     template <class T> SimpleAllocator(const SimpleAllocator<T>& other);
     Tp *allocate(std::size_t n);
     void deallocate(Tp *p, std::size_t n);
   };
   
   template <class T, class U>
   bool operator==(const SimpleAllocator<T>&, const SimpleAllocator<U>&);
   template <class T, class U>
   bool operator!=(const SimpleAllocator<T>&, const SimpleAllocator<U>&);
   

4. Edit 20.2.9 [allocator.traits]p1, class template allocator_traits synopsis, as indicated:

   namespace std {
     template <class Alloc> struct allocator_traits {
       typedef Alloc allocator_type;
   
       typedef typename Alloc::value_typesee below value_type;
   
       […]
   
       static Alloc select_on_container_copy_construction(const Alloc& rhs);
   
       static bool equal(const Alloc& a1, const Alloc& a2) noexcept;
     };
   }
   

5. At the beginning of 20.2.9.2 [allocator.traits.types], add a new paragraph:

   typedef see below value_type;
   

   Type: Alloc::value_type if such a type exists; otherwise, T if Alloc is a class template instantiation of the form Alloc<T, Args>, where Args is zero or more type arguments; otherwise, the program is ill-formed.

6. At the end of 20.2.9.3 [allocator.traits.members], add a new paragraph:

   static bool equal(const Alloc& a1, const Alloc& a2) noexcept;
   

   -?- Returns: a1 == a2 if that expression is well-formed; otherwise, true.