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# C++ concepts: AllocatorAwareContainer

From cppreference.com
< cpp‎ | concept

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).

The following rules apply to container construction

- Copy constructors of `AllocatorAwareContainer`s 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 an allocator 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.

### [edit] 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`
`Q` | Allocator type

expression | return type | pre/requirements | post/effects | complexity
--- | --- | --- | --- | ---
allocator_type | `A` | allocator_type::value_type must be the same as X::value_type |  | compile-time
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; `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

### [edit] Concept requirements

A

- `DefaultConstructible`

T

- `CopyInsertable`

X

- `CopyAssignable`

### [edit] Notes

Allocator-aware containers always call std::allocator_traits<A>::construct(m, p, args) to construct an object of type `A` at `p` using `args`, with m == get_allocator(). The default `construct` in std::allocator calls ::new((void*)p) T(args), but specialized allocators may choose a different definition

### [edit] Standard library

All standard library containers except std::array are `AllocatorAwareContainer`s:

- 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

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