19 General utilities library [utilities]

19.10 Memory [memory]

19.10.9 Allocator traits [allocator.traits]

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>>;

    [[nodiscard]] static pointer allocate(Alloc& a, size_type n);
    [[nodiscard]] 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);
  };
}

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

using pointer = see below;
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;
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;
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;
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;
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;
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;
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;
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;
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;
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;
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.

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

[[nodiscard]] static pointer allocate(Alloc& a, size_type n);
Returns: a.allocate(n).
[[nodiscard]] static pointer allocate(Alloc& a, size_type n, const_void_pointer hint);
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);
Effects: Calls a.deallocate(p, n).
Throws: Nothing.
template<class T, class... Args> static void construct(Alloc& a, T* p, Args&&... args);
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);
Effects: Calls a.destroy(p) if that call is well-formed; otherwise, invokes p->~T().
static size_type max_size(const Alloc& a) noexcept;
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);
Returns: rhs.select_­on_­container_­copy_­construction() if that expression is well-formed; otherwise, rhs.