std::deque

template<
    class T,
    class Allocator = std::allocator<T>
> class deque;

namespace pmr {
    template< class T >
    using deque = std::deque<T, std::pmr::polymorphic_allocator<T>>;
}

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, with additional bookkeeping, which means indexed access to deque must perform two pointer dereferences, compared to vector's indexed access which performs only one.

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. On the other hand, deques typically have large minimal memory cost; a deque holding just one element has to allocate its full internal array (e.g. 8 times the object size on 64-bit libstdc++; 16 times the object size or 4096 bytes, whichever is larger, on 64-bit libc++).

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 Container, AllocatorAwareContainer, SequenceContainer and ReversibleContainer.

Template parameters

Iterator invalidation

Invalidation 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 functions

Non-member functions

Notes

Example

#include <deque>
#include <iostream>

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 << ' ';
    std::cout << '\n';
}

13 7 5 16 8 25

Defect reports

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

See also