23 General utilities library [utilities]

23.10 Memory [memory]

23.10.8 Allocator traits [allocator.traits]

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The class template allocator_­traits supplies a uniform interface to all allocator types. An allocator cannot be a non-class type, however, even if allocator_­traits supplies the entire required interface. [ Note: Thus, it is always possible to create a derived class from an allocator.  — end note ] 

namespace std {
  template <class Alloc> struct allocator_traits {
    using allocator_type     = Alloc;

    using value_type         = typename Alloc::value_type;

    using pointer            = see below;
    using const_pointer      = see below;
    using void_pointer       = see below;
    using const_void_pointer = see below;

    using difference_type    = see below;
    using size_type          = see below;

    using propagate_on_container_copy_assignment = see below;
    using propagate_on_container_move_assignment = see below;
    using propagate_on_container_swap            = see below;
    using is_always_equal                        = see below;

    template <class T> using rebind_alloc = see below;
    template <class T> using rebind_traits = allocator_traits<rebind_alloc<T>>;

    static pointer allocate(Alloc& a, size_type n);
    static pointer allocate(Alloc& a, size_type n, const_void_pointer hint);

    static void deallocate(Alloc& a, pointer p, size_type n);

    template <class T, class... Args>
      static void construct(Alloc& a, T* p, Args&&... args);

    template <class T>
      static void destroy(Alloc& a, T* p);

    static size_type max_size(const Alloc& a) noexcept;

    static Alloc select_on_container_copy_construction(const Alloc& rhs);
  };
}

23.10.8.1 Allocator traits member types [allocator.traits.types]

using pointer = see below;

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Type: Alloc​::​pointer if the qualified-id Alloc​::​pointer is valid and denotes a type ([temp.deduct]); otherwise, value_­type*.

using const_pointer = see below;

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Type: Alloc​::​const_­pointer if the qualified-id Alloc​::​const_­pointer is valid and denotes a type ([temp.deduct]); otherwise, pointer_­traits<pointer>​::​rebind<​const value_­type>.

using void_pointer = see below;

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Type: Alloc​::​void_­pointer if the qualified-id Alloc​::​void_­pointer is valid and denotes a type ([temp.deduct]); otherwise, pointer_­traits<pointer>​::​rebind<​void>.

using const_void_pointer = see below;

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Type: Alloc​::​const_­void_­pointer if the qualified-id Alloc​::​const_­void_­pointer is valid and denotes a type ([temp.deduct]); otherwise, pointer_­traits<pointer>​::​​rebind<const void>.

using difference_type = see below;

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Type: Alloc​::​difference_­type if the qualified-id Alloc​::​difference_­type is valid and denotes a type ([temp.deduct]); otherwise, pointer_­traits<pointer>​::​difference_­type.

using size_type = see below;

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Type: Alloc​::​size_­type if the qualified-id Alloc​::​size_­type is valid and denotes a type ([temp.deduct]); otherwise, make_­unsigned_­t<difference_­type>.

using propagate_on_container_copy_assignment = see below;

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Type: Alloc​::​propagate_­on_­container_­copy_­assignment if the qualified-id Alloc​::​propagate_­on_­container_­copy_­assignment is valid and denotes a type ([temp.deduct]); otherwise false_­type.

using propagate_on_container_move_assignment = see below;

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Type: Alloc​::​propagate_­on_­container_­move_­assignment if the qualified-id Alloc​::​propagate_­on_­container_­move_­assignment is valid and denotes a type ([temp.deduct]); otherwise false_­type.

using propagate_on_container_swap = see below;

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Type: Alloc​::​propagate_­on_­container_­swap if the qualified-id Alloc​::​propagate_­on_­container_­swap is valid and denotes a type ([temp.deduct]); otherwise false_­type.

using is_always_equal = see below;

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Type: Alloc​::​is_­always_­equal if the qualified-id Alloc​::​is_­always_­equal is valid and denotes a type ([temp.deduct]); otherwise is_­empty<Alloc>​::​type.

template <class T> using rebind_alloc = see below;

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Alias template: Alloc​::​rebind<T>​::​other if the qualified-id Alloc​::​rebind<T>​::​other is valid and denotes a type ([temp.deduct]); otherwise, Alloc<T, Args> if Alloc is a class template instantiation of the form Alloc<U, Args>, where Args is zero or more type arguments; otherwise, the instantiation of rebind_­alloc is ill-formed.

23.10.8.2 Allocator traits static member functions [allocator.traits.members]

static pointer allocate(Alloc& a, size_type n);

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Returns: a.allocate(n).

static pointer allocate(Alloc& a, size_type n, const_void_pointer hint);

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Returns: a.allocate(n, hint) if that expression is well-formed; otherwise, a.allocate(n).

static void deallocate(Alloc& a, pointer p, size_type n);

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Effects: Calls a.deallocate(p, n).

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Throws: Nothing.

template <class T, class... Args> static void construct(Alloc& a, T* p, Args&&... args);
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Effects: Calls a.construct(p, std​::​forward<Args>(args)...) if that call is well-formed; otherwise, invokes ​::​new (static_­cast<void*>(p)) T(std​::​forward<Args>(args)...).

template <class T> static void destroy(Alloc& a, T* p);
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Effects: Calls a.destroy(p) if that call is well-formed; otherwise, invokes p->~T().

static size_type max_size(const Alloc& a) noexcept;

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Returns: a.max_­size() if that expression is well-formed; otherwise, numeric_­limits<size_­type>​::​​max()/sizeof(value_­type).

static Alloc select_on_container_copy_construction(const Alloc& rhs);

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Returns: rhs.select_­on_­container_­copy_­construction() if that expression is well-formed; otherwise, rhs.