std::deque

 

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Iterator invalidation

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Non-member function table

 

std::deque

Member types
Member functions
deque::deque deque::~deque deque::operator= deque::assign deque::assign_range (C++23)   deque::get_allocator Element access deque::at deque::operator[] deque::front deque::back Iterators deque::begindeque::cbegin (C++11) deque::enddeque::cend (C++11) deque::rbegindeque::crbegin (C++11) deque::renddeque::crend (C++11)	Capacity deque::empty deque::size deque::max_size deque::shrink_to_fit (DR*) Modifiers deque::clear deque::insert deque::insert_range (C++23) deque::emplace deque::erase deque::push_front deque::emplace_front (C++11) deque::prepend_range (C++23) deque::pop_front deque::push_back deque::emplace_back (C++11) deque::append_range (C++23) deque::pop_back deque::resize deque::swap
Non-member functions
operator==operator<=> (C++20) swap(std::deque) erase(std::deque)erase_if(std::deque) (C++20)(C++20)	operator!=operator<operator>operator<=operator>= (until C++20)(until C++20)(until C++20)(until C++20)(until C++20)
Deduction guides(C++17)

 

Defined in header <deque>
template<     class T,     class Allocator = std::allocator<T> > class deque;	(1)
namespace pmr {     template <class T>     using deque = std::deque<T, std::pmr::polymorphic_allocator<T>>; }	(2)	(since C++17)

std::deque (double-ended queue) is an indexed sequence container that allows fast insertion and deletion at both its beginning and its end. In addition, insertion and deletion at either end of a deque never invalidates pointers or references to the rest of the elements.

As opposed to std::vector, the elements of a deque are not stored contiguously: typical implementations use a sequence of individually allocated fixed-size arrays.

The storage of a deque is automatically expanded and contracted as needed. Expansion of a deque is cheaper than the expansion of a std::vector because it does not involve copying of the existing elements to a new memory location.

The complexity (efficiency) of common operations on deques is as follows:

• Random access - constant O(1)
• Insertion or removal of elements at the end or beginning - constant O(1)
• Insertion or removal of elements - linear O(n)

std::deque meets the requirements of Template:concept, Template:concept, Template:concept and Template:concept.

Template parameters

T	-	The type of the elements. T must meet the requirements of CopyAssignable and CopyConstructible. (until C++11) The requirements that are imposed on the elements depend on the actual operations performed on the container. Generally, it is required that element type is a complete type and meets the requirements of Erasable, but many member functions impose stricter requirements. (since C++11)
Allocator	-	An allocator that is used to acquire/release memory and to construct/destroy the elements in that memory. The type must meet the requirements of Allocator. The behavior is undefined(until C++20)The program is ill-formed(since C++20) if Allocator::value_type is not the same as T.

Iterator invalidation

There are still a few inaccuracies in this section, refer to individual member function pages for more detail

Operations	Invalidated
All read only operations	Never
swap, std::swap	The past-the-end iterator may be invalidated (implementation defined)
shrink_to_fit, clear, insert, emplace, push_front, push_back, emplace_front, emplace_back	Always
erase	If erasing at begin - only erased elements If erasing at end - only erased elements and the past-the-end iterator Otherwise - all iterators are invalidated (including the past-the-end iterator).
resize	If the new size is smaller than the old one : only erased elements and the past-the-end iterator If the new size is bigger than the old one : all iterators are invalidated Otherwise - none iterators are invalidated.
pop_front	Only to the element erased
pop_back	Only to the element erased and the past-the-end iterator

Notes

• When inserting at either end of the deque, references are not invalidated by insert and emplace.
• push_front, push_back, emplace_front and emplace_back do not invalidate any references to elements of the deque.
• When erasing at either end of the deque, references to non-erased elements are not invalidated by erase, pop_front and pop_back.
• A call to resize with a smaller size does not invalidate any references to non-erased elements.
• A call to resize with a bigger size does not invalidate any references to elements of the deque.

Member types

Member type	Definition
value_type	T
allocator_type	Allocator
size_type	Unsigned integer type (usually std::size_t)
difference_type	Signed integer type (usually std::ptrdiff_t)
reference	value_type&
const_reference	const value_type&
pointer	Allocator::pointer (until C++11) std::allocator_traits<Allocator>::pointer (since C++11)
const_pointer	Allocator::const_pointer (until C++11) std::allocator_traits<Allocator>::const_pointer (since C++11)
iterator	LegacyRandomAccessIterator to value_type
const_iterator	LegacyRandomAccessIterator to const value_type
reverse_iterator	std::reverse_iterator<iterator>
const_reverse_iterator	std::reverse_iterator<const_iterator>

Member functions

(constructor)	constructs the deque (public member function)
(destructor)	destructs the deque (public member function)
operator=	assigns values to the container (public member function)
assign	assigns values to the container (public member function)
get_allocator	returns the associated allocator (public member function)
Element access
at	access specified element with bounds checking (public member function)
operator[]	access specified element (public member function)
front	access the first element (public member function)
back	access the last element (public member function)
Iterators
begincbegin (C++11)	returns an iterator to the beginning (public member function)
endcend (C++11)	returns an iterator to the end (public member function)
rbegincrbegin (C++11)	returns a reverse iterator to the beginning (public member function)
rendcrend (C++11)	returns a reverse iterator to the end (public member function)
Capacity
empty	checks whether the container is empty (public member function)
size	returns the number of elements (public member function)
max_size	returns the maximum possible number of elements (public member function)
shrink_to_fit (DR*)	reduces memory usage by freeing unused memory (public member function)
Modifiers
clear	clears the contents (public member function)
insert	inserts elements (public member function)
emplace (C++11)	constructs element in-place (public member function)
erase	erases elements (public member function)
push_back	adds an element to the end (public member function)
emplace_back (C++11)	constructs an element in-place at the end (public member function)
pop_back	removes the last element (public member function)
push_front	inserts an element to the beginning (public member function)
emplace_front (C++11)	constructs an element in-place at the beginning (public member function)
pop_front	removes the first element (public member function)
resize	changes the number of elements stored (public member function)
swap	swaps the contents (public member function)

Non-member functions

operator==operator!=operator<operator<=operator>operator>=operator<=> (removed in C++20)(removed in C++20)(removed in C++20)(removed in C++20)(removed in C++20)(C++20)	lexicographically compares the values of two deques (function template)
std::swap(std::deque)	specializes the std::swap algorithm (function template)

Example

Run this code

#include <iostream>
#include <deque>
 
int main()
{
    // Create a deque containing integers
    std::deque<int> d = {7, 5, 16, 8};
 
    // Add an integer to the beginning and end of the deque
    d.push_front(13);
    d.push_back(25);
 
    // Iterate and print values of deque
    for(int n : d) {
        std::cout << n << '\n';
    }
}

Output:

13
7
5
16
8
25