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std::deque

From cppreference.com
< cpp‎ | container
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, with additional bookkeeping, which means indexed access to std: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

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 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

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 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
Allocator::reference (until C++11)
value_type& (since C++11)
const_reference
Allocator::const_reference (until C++11)
const value_type& (since C++11)
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 RandomAccessIterator
const_iterator Constant random access iterator
reverse_iterator std::reverse_iterator<iterator>
const_reverse_iterator std::reverse_iterator<const_iterator>

Member functions

constructs the deque
(public member function)
destructs the deque
(public member function)
assigns values to the container
(public member function)
assigns values to the container
(public member function)
returns the associated allocator
(public member function)
Element access
access specified element with bounds checking
(public member function)
access specified element
(public member function)
access the first element
(public member function)
access the last element
(public member function)
Iterators
returns an iterator to the beginning
(public member function)
returns an iterator to the end
(public member function)
returns a reverse iterator to the beginning
(public member function)
returns a reverse iterator to the end
(public member function)
Capacity
checks whether the container is empty
(public member function)
returns the number of elements
(public member function)
returns the maximum possible number of elements
(public member function)
reduces memory usage by freeing unused memory
(public member function)
Modifiers
clears the contents
(public member function)
inserts elements
(public member function)
(C++11)
constructs element in-place
(public member function)
erases elements
(public member function)
adds an element to the end
(public member function)
constructs an element in-place at the end
(public member function)
removes the last element
(public member function)
inserts an element to the beginning
(public member function)
constructs an element in-place at the beginning
(public member function)
removes the first element
(public member function)
changes the number of elements stored
(public member function)
swaps the contents
(public member function)

Non-member functions

lexicographically compares the values in the deque
(function template)
specializes the std::swap algorithm
(function template)

Example

#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