# C++ named requirements: AllocatorAwareContainer (since C++11)

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An AllocatorAwareContainer is a Container that holds an instance of an Allocator and uses that instance in all its member functions to allocate and deallocate memory and to construct and destroy objects in that memory (such objects may be container elements, nodes, or, for unordered containers, bucket arrays), except that std::basic_string specializations do not use the allocators for construction/destruction of their elements (since C++23).

The following rules apply to container construction

- Copy constructors of AllocatorAwareContainers obtain their instances of the allocator by calling std::allocator_traits<allocator_type>::select_on_container_copy_construction on the allocator of the container being copied.
- Move constructors obtain their instances of allocators by move-constructing from the allocator belonging to the old container.
- All other constructors take a const allocator_type& parameter.

The only way to replace an allocator is copy-assignment, move-assignment, and swap:

- Copy-assignment will replace the allocator only if std::allocator_traits<allocator_type>::propagate_on_container_copy_assignment::value is true
- Move-assignment will replace the allocator only if std::allocator_traits<allocator_type>::propagate_on_container_move_assignment::value is true
- Swap will replace the allocator only if std::allocator_traits<allocator_type>::propagate_on_container_swap::value is true. Specifically, it will exchange the allocator instances through an unqualified call to the non-member function swap, see Swappable.

Note: swapping two containers with unequal allocators if `propagate_on_container_swap` is false is undefined behavior.

- The accessor `get_allocator()` obtains a copy of the allocator that was used to construct the container or installed by the most recent allocator replacement operation.

### Requirements

#### Legend

`X` | Container type
`T` | Element type
`A` | Allocator for `T`
a, b | Objects of type `X` (non-const lvalue)
t | Object of type `X` (lvalue or const rvalue)
rv | Object of type `X` (non-const rvalue)
m | Object of type `A`

#### Types

Expression | Return type | Requirements
--- | --- | ---
`allocator_type` | `A` | allocator_type::value_type must be the same as X::value_type

#### Member functions and operators

Expression | Return type | Pre/requirements | Post/effects | Complexity
--- | --- | --- | --- | ---
get_allocator() | `A` |  |  | Constant
X u; |  | `A` is DefaultConstructible | u.empty() == true && u.get_allocator() == A() | Constant
X u(m); |  |  | u.empty() == true && u.get_allocator() == m | Constant
X u(t,m); |  | `T` is CopyInsertable into `X` | u == t && u.get_allocator() == m | Linear
X u(rv); |  | Move constructor of `A` must not throw exceptions | u has the same elements and an equal allocator as rv had before the construction | Constant
X u(rv,m); |  | `T` is MoveInsertable into `X` | The elements of u are the same or copies of those of rv and u.get_allocator() == m | Constant if m == rv.get_allocator(), otherwise linear
a = t | X& | `T` is CopyInsertable into `X` and CopyAssignable | a == t | Linear
a = rv | X& | If the allocator will not be replaced by move-assignment (see above), then `T` is MoveInsertable into `X` and MoveAssignable | All existing elements of a are either move assigned to or destroyed; if a and rv do not refer the same object, a is equal to the value that rv had before the assignment | Linear
a.swap(b) | void |  | Exchanges the contents of a and b | Constant

### Notes

Allocator-aware containers always call std::allocator_traits<A>::construct(m, p, args) to construct an object of type `T` at p using args, with m == get_allocator(). The default `construct` in std::allocator calls ::new((void*)p) T(args) (until C++20)std::allocator has no `construct` member and std::construct_at(p, args) is called when constructing elements (since C++20), but specialized allocators may choose a different definition

### Standard library

All standard library containers except std::array are AllocatorAwareContainers:

- std::basic_string
- std::deque
- std::forward_list
- std::list
- std::vector
- std::map
- std::multimap
- std::set
- std::multiset
- std::unordered_map
- std::unordered_multimap
- std::unordered_set
- std::unordered_multiset

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 2839 | C++11 | self move assignment of standard containers was not allowed | allowed but the result is unspecified

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