22 Containers library [containers]

22.3 Sequence containers [sequences]

22.3.8 Class template deque [deque]

22.3.8.1 Overview [deque.overview]

A deque is a sequence container that supports random access iterators.
In addition, it supports constant time insert and erase operations at the beginning or the end; insert and erase in the middle take linear time.
That is, a deque is especially optimized for pushing and popping elements at the beginning and end.
Storage management is handled automatically.
A deque meets all of the requirements of a container, of a reversible container (given in tables in [container.requirements]), of a sequence container, including the optional sequence container requirements ([sequence.reqmts]), and of an allocator-aware container (Table 78).
Descriptions are provided here only for operations on deque that are not described in one of these tables or for operations where there is additional semantic information.
namespace std { template<class T, class Allocator = allocator<T>> class deque { public: // types using value_type = T; using allocator_type = Allocator; using pointer = typename allocator_traits<Allocator>::pointer; using const_pointer = typename allocator_traits<Allocator>::const_pointer; using reference = value_type&; using const_reference = const value_type&; using size_type = implementation-defined; // see [container.requirements] using difference_type = implementation-defined; // see [container.requirements] using iterator = implementation-defined; // see [container.requirements] using const_iterator = implementation-defined; // see [container.requirements] using reverse_iterator = std::reverse_iterator<iterator>; using const_reverse_iterator = std::reverse_iterator<const_iterator>; // [deque.cons], construct/copy/destroy deque() : deque(Allocator()) { } explicit deque(const Allocator&); explicit deque(size_type n, const Allocator& = Allocator()); deque(size_type n, const T& value, const Allocator& = Allocator()); template<class InputIterator> deque(InputIterator first, InputIterator last, const Allocator& = Allocator()); deque(const deque& x); deque(deque&&); deque(const deque&, const Allocator&); deque(deque&&, const Allocator&); deque(initializer_list<T>, const Allocator& = Allocator()); ~deque(); deque& operator=(const deque& x); deque& operator=(deque&& x) noexcept(allocator_traits<Allocator>::is_always_equal::value); deque& operator=(initializer_list<T>); template<class InputIterator> void assign(InputIterator first, InputIterator last); void assign(size_type n, const T& t); void assign(initializer_list<T>); allocator_type get_allocator() const noexcept; // iterators iterator begin() noexcept; const_iterator begin() const noexcept; iterator end() noexcept; const_iterator end() const noexcept; reverse_iterator rbegin() noexcept; const_reverse_iterator rbegin() const noexcept; reverse_iterator rend() noexcept; const_reverse_iterator rend() const noexcept; const_iterator cbegin() const noexcept; const_iterator cend() const noexcept; const_reverse_iterator crbegin() const noexcept; const_reverse_iterator crend() const noexcept; // [deque.capacity], capacity [[nodiscard]] bool empty() const noexcept; size_type size() const noexcept; size_type max_size() const noexcept; void resize(size_type sz); void resize(size_type sz, const T& c); void shrink_to_fit(); // element access reference operator[](size_type n); const_reference operator[](size_type n) const; reference at(size_type n); const_reference at(size_type n) const; reference front(); const_reference front() const; reference back(); const_reference back() const; // [deque.modifiers], modifiers template<class... Args> reference emplace_front(Args&&... args); template<class... Args> reference emplace_back(Args&&... args); template<class... Args> iterator emplace(const_iterator position, Args&&... args); void push_front(const T& x); void push_front(T&& x); void push_back(const T& x); void push_back(T&& x); iterator insert(const_iterator position, const T& x); iterator insert(const_iterator position, T&& x); iterator insert(const_iterator position, size_type n, const T& x); template<class InputIterator> iterator insert(const_iterator position, InputIterator first, InputIterator last); iterator insert(const_iterator position, initializer_list<T>); void pop_front(); void pop_back(); iterator erase(const_iterator position); iterator erase(const_iterator first, const_iterator last); void swap(deque&) noexcept(allocator_traits<Allocator>::is_always_equal::value); void clear() noexcept; }; template<class InputIterator, class Allocator = allocator<iter-value-type<InputIterator>>> deque(InputIterator, InputIterator, Allocator = Allocator()) -> deque<iter-value-type<InputIterator>, Allocator>; }

22.3.8.2 Constructors, copy, and assignment [deque.cons]

explicit deque(const Allocator&);
Effects: Constructs an empty deque, using the specified allocator.
Complexity: Constant.
explicit deque(size_type n, const Allocator& = Allocator());
Preconditions: T is Cpp17DefaultInsertable into *this.
Effects: Constructs a deque with n default-inserted elements using the specified allocator.
Complexity: Linear in n.
deque(size_type n, const T& value, const Allocator& = Allocator());
Preconditions: T is Cpp17CopyInsertable into *this.
Effects: Constructs a deque with n copies of value, using the specified allocator.
Complexity: Linear in n.
template<class InputIterator> deque(InputIterator first, InputIterator last, const Allocator& = Allocator());
Effects: Constructs a deque equal to the range [first, last), using the specified allocator.
Complexity: Linear in distance(first, last).

22.3.8.3 Capacity [deque.capacity]

void resize(size_type sz);
Preconditions: T is Cpp17MoveInsertable and Cpp17DefaultInsertable into *this.
Effects: If sz < size(), erases the last size() - sz elements from the sequence.
Otherwise, appends sz - size() default-inserted elements to the sequence.
void resize(size_type sz, const T& c);
Preconditions: T is Cpp17CopyInsertable into *this.
Effects: If sz < size(), erases the last size() - sz elements from the sequence.
Otherwise, appends sz - size() copies of c to the sequence.
void shrink_to_fit();
Preconditions: T is Cpp17MoveInsertable into *this.
Effects: shrink_­to_­fit is a non-binding request to reduce memory use but does not change the size of the sequence.
[Note 1:
The request is non-binding to allow latitude for implementation-specific optimizations.
— end note]
If the size is equal to the old capacity, or if an exception is thrown other than by the move constructor of a non-Cpp17CopyInsertable T, then there are no effects.
Complexity: If the size is not equal to the old capacity, linear in the size of the sequence; otherwise constant.
Remarks: If the size is not equal to the old capacity, then invalidates all the references, pointers, and iterators referring to the elements in the sequence, as well as the past-the-end iterator.

22.3.8.4 Modifiers [deque.modifiers]

iterator insert(const_iterator position, const T& x); iterator insert(const_iterator position, T&& x); iterator insert(const_iterator position, size_type n, const T& x); template<class InputIterator> iterator insert(const_iterator position, InputIterator first, InputIterator last); iterator insert(const_iterator position, initializer_list<T>); template<class... Args> reference emplace_front(Args&&... args); template<class... Args> reference emplace_back(Args&&... args); template<class... Args> iterator emplace(const_iterator position, Args&&... args); void push_front(const T& x); void push_front(T&& x); void push_back(const T& x); void push_back(T&& x);
Effects: An insertion in the middle of the deque invalidates all the iterators and references to elements of the deque.
An insertion at either end of the deque invalidates all the iterators to the deque, but has no effect on the validity of references to elements of the deque.
Complexity: The complexity is linear in the number of elements inserted plus the lesser of the distances to the beginning and end of the deque.
Inserting a single element at either the beginning or end of a deque always takes constant time and causes a single call to a constructor of T.
Remarks: If an exception is thrown other than by the copy constructor, move constructor, assignment operator, or move assignment operator of T there are no effects.
If an exception is thrown while inserting a single element at either end, there are no effects.
Otherwise, if an exception is thrown by the move constructor of a non-Cpp17CopyInsertable T, the effects are unspecified.
iterator erase(const_iterator position); iterator erase(const_iterator first, const_iterator last); void pop_front(); void pop_back();
Effects: An erase operation that erases the last element of a deque invalidates only the past-the-end iterator and all iterators and references to the erased elements.
An erase operation that erases the first element of a deque but not the last element invalidates only iterators and references to the erased elements.
An erase operation that erases neither the first element nor the last element of a deque invalidates the past-the-end iterator and all iterators and references to all the elements of the deque.
[Note 1:
pop_­front and pop_­back are erase operations.
— end note]
Throws: Nothing unless an exception is thrown by the assignment operator of T.
Complexity: The number of calls to the destructor of T is the same as the number of elements erased, but the number of calls to the assignment operator of T is no more than the lesser of the number of elements before the erased elements and the number of elements after the erased elements.

22.3.8.5 Erasure [deque.erasure]

template<class T, class Allocator, class U> typename deque<T, Allocator>::size_type erase(deque<T, Allocator>& c, const U& value);
Effects: Equivalent to: auto it = remove(c.begin(), c.end(), value); auto r = distance(it, c.end()); c.erase(it, c.end()); return r;
template<class T, class Allocator, class Predicate> typename deque<T, Allocator>::size_type erase_if(deque<T, Allocator>& c, Predicate pred);
Effects: Equivalent to: auto it = remove_if(c.begin(), c.end(), pred); auto r = distance(it, c.end()); c.erase(it, c.end()); return r;