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C++ concepts: Allocator

From cppreference.com
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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

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>&);