class template
<deque>

std::deque

template < class T, class Alloc = allocator<T> > class deque;
Double ended queue
deque (usually pronounced like "deck") is an irregular acronym of double-ended queue. Double-ended queues are sequence containers with dynamic sizes that can be expanded or contracted on both ends (either its front or its back).

Specific libraries may implement deques in different ways, generally as some form of dynamic array. But in any case, they allow for the individual elements to be accessed directly through random access iterators, with storage handled automatically by expanding and contracting the container as needed.

Therefore, they provide a functionality similar to vectors, but with efficient insertion and deletion of elements also at the beginning of the sequence, and not only at its end. But, unlike vectors, deques are not guaranteed to store all its elements in contiguous storage locations: accessing elements in a deque by offsetting a pointer to another element causes undefined behavior.

Both vectors and deques provide a very similar interface and can be used for similar purposes, but internally both work in quite different ways: While vectors use a single array that needs to be occasionally reallocated for growth, the elements of a deque can be scattered in different chunks of storage, with the container keeping the necessary information internally to provide direct access to any of its elements in constant time and with a uniform sequential interface (through iterators). Therefore, deques are a little more complex internally than vectors, but this allows them to grow more efficiently under certain circumstances, especially with very long sequences, where reallocations become more expensive.

For operations that involve frequent insertion or removals of elements at positions other than the beginning or the end, deques perform worse and have less consistent iterators and references than lists and forward lists.

Container properties

Sequence
Elements in sequence containers are ordered in a strict linear sequence. Individual elements are accessed by their position in this sequence.
Dynamic array
Generally implemented as a dynamic array, it allows direct access to any element in the sequence and provides relatively fast addition/removal of elements at the beginning or the end of the sequence.
Allocator-aware
The container uses an allocator object to dynamically handle its storage needs.

Template parameters

T
Type of the elements.
Aliased as member type deque::value_type.
Alloc
Type of the allocator object used to define the storage allocation model. By default, the allocator class template is used, which defines the simplest memory allocation model and is value-independent.
Aliased as member type deque::allocator_type.

Member types

member typedefinitionnotes
value_typeThe first template parameter (T)
allocator_typeThe second template parameter (Alloc)defaults to: allocator<value_type>
referenceallocator_type::referencefor the default allocator: value_type&
const_referenceallocator_type::const_referencefor the default allocator: const value_type&
pointerallocator_type::pointerfor the default allocator: value_type*
const_pointerallocator_type::const_pointerfor the default allocator: const value_type*
iteratora random access iterator to value_typeconvertible to const_iterator
const_iteratora random access iterator to const value_type
reverse_iteratorreverse_iterator<iterator>
const_reverse_iteratorreverse_iterator<const_iterator>
difference_typea signed integral type, identical to: iterator_traits<iterator>::difference_typeusually the same as ptrdiff_t
size_typean unsigned integral type that can represent any non-negative value of difference_typeusually the same as size_t
member typedefinitionnotes
value_typeThe first template parameter (T)
allocator_typeThe second template parameter (Alloc)defaults to: allocator<value_type>
referencevalue_type&
const_referenceconst value_type&
pointerallocator_traits<allocator_type>::pointerfor the default allocator: value_type*
const_pointerallocator_traits<allocator_type>::const_pointerfor the default allocator: const value_type*
iteratora random access iterator to value_typeconvertible to const_iterator
const_iteratora random access iterator to const value_type
reverse_iteratorreverse_iterator<iterator>
const_reverse_iteratorreverse_iterator<const_iterator>
difference_typea signed integral type, identical to:
iterator_traits<iterator>::difference_type
usually the same as ptrdiff_t
size_typean unsigned integral type that can represent any non-negative value of difference_typeusually the same as size_t

Member functions


Iterators:

Capacity:

Element access:

Modifiers:

Allocator:

Non-member functions overloads

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