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# C++ concepts: Allocator

From cppreference.com
< cpp‎ | concept

Encapsulates a memory allocation and deallocation strategy.

Every standard library component that may need to allocate or release storage, from `std::string`, `std::vector`, and every container except `std::array`, to `std::shared_ptr` and `std::function`, does so through an `Allocator`: an object of a class type that satisfies the following requirements.

Some requirements are optional: the template std::allocator_traits supplies the default implementations for all optional requirements, and all standard library containers and other allocator-aware classes access the allocator through `std::allocator_traits`, not directly.

### [edit] Requirements

Given

- `A`, an Allocator type for type `T`
- `a`, an object of type `A`
- `B`, the corresponding Allocator type for type `U` (as obtained by rebinding `A`)
- `ptr`, a value of type `allocator_traits<A>::pointer`, obtained by calling allocator_traits<A>::allocate()
- `cptr`, a value of type `allocator_traits<A>::const_pointer`, obtained by conversion from `ptr`
- `vptr`, a value of type `allocator_traits<A>::void_pointer`, obtained by conversion from `ptr`
- `cvptr`, a value of type `allocator_traits<A>::const_void_pointer`, obtained by conversion from `cptr` or from `vptr`
- `xptr`, a dereferencable pointer to some type `X`

Expression | Requirements | Return type
--- | --- | ---
A::pointer (optional) | Satisfies `NullablePointer` and `RandomAccessIterator` |
A::const_pointer (optional) | `A::pointer` is convertible to `A::const_pointer`. Satisfies `NullablePointer` and `RandomAccessIterator` |
A::void_pointer (optional) | `A::pointer` is convertible to `A::void_pointer` `B::void_pointer` and `A::void_pointer` are the same type. Satisfies `NullablePointer` |
A::const_void_pointer (optional) | `A::pointer`, `A::const_pointer`, and `A::void_pointer` are convertible to `A::const_void_pointer` `B::const_void_pointer` and `A::const_void_pointer` are the same type. Satisfies `NullablePointer` |
A::value_type |  | the type `T`
A::size_type (optional) | `A::size_type` can represent the size of the largest object `A` can allocate | unsigned integer type
A::difference_type (optional) | `A::difference_type` can represent the difference of any two pointers to the objects allocated by `A` | signed integer type
A::template rebind<U>::other (optional[1]) | for any `U`, `B::template rebind<T>::other` is `A` | the type `B`
*ptr |  | `T&`
*cptr | *cptr and *ptr identify the same object | `const T&`
ptr->m | same as (*ptr).m, if (*ptr).m is well-defined | the type of `T::m`
cptr->m | same as (*cptr).m, if (*cptr).m is well-defined | the type of `T::m`
static_cast<A::pointer>(vptr) | static_cast<A::pointer>(vptr) == ptr | `A::pointer`
static_cast<A::const_pointer>(cvptr) | static_cast<A::const_pointer>(vptr) == cptr | `A::const_pointer`
a.allocate(n) | allocates storage suitable for `n` objects of type `T`, but does not construct them. May throw exceptions. | `A::pointer`
a.allocate(n, cvptr) (optional) | same as `a.allocate(n)`, but may use `cvptr` (a pointer obtained from `a.allocate()` or `nullptr_t`) in unspecified manner to aid locality | `A::pointer`
a.deallocate(ptr, n) | deallocates storage previously allocated by a call to a.allocate(n). Does not call destructors, if any objects were constructed, they must be destroyed before calling a.deallocate(). Does not throw exceptions. | (not used)
a.max_size() (optional) | the largest value that can be passed to `A::allocate()` | `A::size_type`
a1 == a2 | returns true only if the storage allocated by the allocator `a1` can be deallocated through `a2`. Establishes reflexive, symmetric, and transitive relationship. Does not throw exceptions. | bool
a1 != a2 | same as !(a1==a2) | bool
A a1(a) A a1 = a | Copy-constructs `a1` such that a1 == a. Does not throw exceptions. (Note: every `Allocator` also satisfies `CopyConstructible`) |
A a(b) | Constructs `a` such that B(a)==b and A(b)==a. Does not throw exceptions. (Note: this implies that all allocators related by rebind maintain each other's resources, such as memory pools) |
A a1(std::move(a)) A a1 = std::move(a) | Constructs `a1` such that it equals the prior value of `a`. Does not throw exceptions. |
A a(std::move(b)) | Constructs `a` such that it equals the prior value of `A(b)`. Does not throw exceptions. |
a.construct(xptr, args) (optional) | Constructs an object of type `X` in previously-allocated storage at the address pointed to by `xptr`, using args as the constructor arguments |
a.destroy(xptr) (optional) | Destructs an object of type `X` pointed to by `xptr`, but does not deallocate any storage. |
a.select_on_container_copy_construction() (optional) | Provides an instance of `A` to be used by the container that is copy-constructed from the one that uses `a` currently. Usually returns either a copy of `a` or a default-constructed `A()`. | `A`
a.propagate_on_container_copy_assignment (optional) | true if the allocator of type `A` needs to be copied when the container that uses it is copy-assigned. Note that if the allocators of the source and the target containers do not compare equal, copy assignment has to deallocate the target's memory using the old allocator and then allocate it using the new allocator before copying the elements (and the allocator). | std::true_type or std::false_type or derived from such
a.propagate_on_container_move_assignment (optional) | true if the allocator of type `A` needs to be copied when the container that uses it is move-assigned. If this member is false and the allocators of the source and the target containers do not compare equal, move assignment cannot take ownership of the source memory and must move-assign or move-construct the elements individually, resizing its own memory as needed. | std::true_type or std::false_type or derived from such
a.propagate_on_container_swap (optional) | true if the allocators of type `A` need to be swapped when two containers that use them are swapped. If this member is false and the allocators of the two containers do not compare equal, the behavior of container swap is undefined. | std::true_type or std::false_type or derived from such

Notes: [1] rebind is only optional (provided by std::allocator_traits) if this allocator is a template of the form SomeAllocator<T, Args>, where `Args` is zero or more additional template parameters.

Given `x1` and `x2`, objects of (possibly different) types `X::void_pointer`, `X::const_void_pointer`, `X::pointer`, or `X::const_pointer` Then, x1 and x2 are equivalently-valued pointer values, if and only if both `x1` and `x2` can be explicitly converted to the two corresponding objects `px1` and `px2` of type `X::const_pointer`, using a sequence of `static_casts` using only these four types, and the expression px1 == px2 evaluates to true Given `w1` and `w2`, objects of type `X::void_pointer`. Then for the expression w1 == w2 and w1 != w2 either or both objects may be replaced by an equivalently-valued object of type `X::const_void_pointer` with no change in semantics. Given `p1` and `p2`, objects of type `X::pointer` Then, for the expressions p1 == p2, p1 != p2, p1 < p2 p1 <= p2, p1 >= p2, p1 > p2, p1 - p2} either or both objects may be replaced by an equivalently-valued object of type `X::const_pointer` with no change in semantics. The above requirements make it possible to compare `Container`'s iterators and const_iterators. | (since C++14)

### [edit] Examples

a minimum C++11 allocator

Run this code

    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>&);

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