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.

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]

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using pointer = see below;

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

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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>.

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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>.

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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>.

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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.

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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>.

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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.

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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.

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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.

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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.

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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]

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

Returns: a.allocate(n).

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[[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).

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

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

Throws: Nothing.

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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)...).

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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().

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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).

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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.