std::deque(3)

NAME
std::deque - std::deque

Synopsis
   Defined in header <deque>
   template<
       class T, (1)
       class Allocator = std::allocator<T>
   > class deque;
   namespace pmr {
       template< class T >
       using deque = std::deque<T, (2) (since C++17)
   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
               The type of the elements.
               T must meet the requirements of CopyAssignable and (until C++11)
               CopyConstructible.
               The requirements that are imposed on the elements depend
   T - on the actual operations performed on the container.
               Generally, it is required that element type is a complete (since C++11)
               type and meets the requirements of Erasable, but many
               member functions impose stricter requirements.
               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.
   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
    This section is incomplete
    Reason: 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, Always.
   push_back, emplace_front,
   emplace_back
                        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.
                        It is unspecified when the past-the-end iterator is
   erase invalidated.
                        (until C++11)
                        The past-the-end iterator is also invalidated unless
                        the erased
                        elements are at the beginning of the container and the
                        last element is not erased.
                        (since C++11)
                        If the new size is smaller than the old one - only
                        erased elements and the
                        past-the-end iterator.
   resize
                        If the new size is bigger than the old one - all
                        iterators are invalidated.
                        Otherwise - none iterators are invalidated.
                        To the element erased.
                        The past-the-end iterator
   pop_front, pop_back may be invalidated (implementation defined)
                        (until C++11)
                        is also invalidated.
                        (since C++11)
     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 value_type&
   const_reference const value_type&
                        Allocator::pointer (until C++11)
   pointer std::allocator_traits<Allocator>::pointer (since C++11)
                        Allocator::const_pointer (until C++11)
   const_pointer 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)
   assign_range assigns a range of values to the container
   (C++23) (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
   begin returns an iterator to the beginning
   cbegin (public member function)
   (C++11)
   end returns an iterator to the end
   cend (public member function)
   (C++11)
   rbegin returns a reverse iterator to the beginning
   crbegin (public member function)
   (C++11)
   rend returns a reverse iterator to the end
   crend (public member function)
   (C++11)

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 reduces memory usage by freeing unused memory
   (DR*) (public member function)

Modifiers
   clear clears the contents
                 (public member function)
   insert inserts elements
                 (public member function)
   insert_range inserts a range of elements
   (C++23) (public member function)
   emplace constructs element in-place
   (C++11) (public member function)
   erase erases elements
                 (public member function)
   push_back adds an element to the end
                 (public member function)
   emplace_back constructs an element in-place at the end
   (C++11) (public member function)
   append_range adds a range of elements to the end
   (C++23) (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 constructs an element in-place at the beginning
   (C++11) (public member function)
   prepend_range adds a range of elements to the beginning
   (C++23) (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>= lexicographically compares the values of two deques
   operator<=> (function template)
   (removed in C++20)
   (removed in C++20)
   (removed in C++20)
   (removed in C++20)
   (removed in C++20)
   (C++20)
   std::swap(std::deque) specializes the std::swap algorithm
                        (function template)
   erase(std::deque) erases all elements satisfying specific criteria
   erase_if(std::deque) (function template)
   (C++20)
     Deduction guides (since C++17)

Notes
       Feature-test macro Value Std Feature
   __cpp_lib_containers_ranges 202202L (C++23) Ranges construction and insertion for
                        containers

Example
// Run this code
 #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';
 }

Output:
 13 7 5 16 8 25
   Defect reports
   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.
     DR Applied to Behavior as published Correct behavior
                      T was not required to be CopyConstructible T is also required to
   LWG 230 C++98 (an element of type T might not be able to be CopyConstructible
                      be constructed)

See also
   queue adapts a container to provide queue (FIFO data structure)
         (class template)

Category:
     * Todo with reason