---
name: "std::deque"
section: "3"
platform: "linux"
package: "gcc"
version: "16.2.1+r23+gd564253eb6c8-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/std::deque"
---

# std::deque(3)

A standard container using fixed-size memory allocation and constant-time manipulation of elements at either end.

## NAME

std::deque\< \_Tp, \_Alloc \> - A standard container using fixed-size memory allocation and constant-time manipulation of elements at either end.

## SYNOPSIS

\#include \<deque\>

Inherits **std::\_Deque\_base\< \_Tp, \_Alloc \>**.

### Public Types

typedef \_Alloc **allocator\_type**\
 typedef \_Base::const\_iterator **const\_iterator**\
 typedef \_Alloc\_traits::const\_pointer **const\_pointer**\
 typedef \_Alloc\_traits::const\_reference **const\_reference**\
 typedef **std::reverse\_iterator**\< const\_iterator \> **const\_reverse\_iterator**\
 typedef ptrdiff\_t **difference\_type**\
 typedef \_Base::iterator **iterator**\
 typedef \_Alloc\_traits::pointer **pointer**\
 typedef \_Alloc\_traits::reference **reference**\
 typedef **std::reverse\_iterator**\< iterator \> **reverse\_iterator**\
 typedef size\_t **size\_type**\
 typedef \_Tp **value\_type**\

### Public Member Functions

**deque** ()=default\
 Creates a deque with no elements.  template\<typename \_InputIterator, typename = std::\_RequireInputIter\<\_InputIterator\>\> **deque** (\_InputIterator \_\_first, \_InputIterator \_\_last, const allocator\_type &\_\_a=allocator\_type())\
 Builds a deque from a range.  **deque** (const allocator\_type &\_\_a)\
 Creates a deque with no elements.  **deque** (const deque &\_\_x)\
 Deque copy constructor.  **deque** (const deque &\_\_x, const \_\_type\_identity\_t\< allocator\_type \> &\_\_a)\
 Copy constructor with alternative allocator.  **deque** (deque &&)=default\
 Deque move constructor.  **deque** (deque &&\_\_x, const \_\_type\_identity\_t\< allocator\_type \> &\_\_a)\
 Move constructor with alternative allocator.  **deque** (**initializer\_list**\< value\_type \> \_\_l, const allocator\_type &\_\_a=allocator\_type())\
 Builds a deque from an initializer list.  **deque** (size\_type \_\_n, const allocator\_type &\_\_a=allocator\_type())\
 Creates a deque with default constructed elements.  **deque** (size\_type \_\_n, const value\_type &\_\_value, const allocator\_type &\_\_a=allocator\_type())\
 Creates a deque with copies of an exemplar element.  **~deque** ()\
 template\<typename \_InputIterator, typename = std::\_RequireInputIter\<\_InputIterator\>\> void **assign** (\_InputIterator \_\_first, \_InputIterator \_\_last)\
 Assigns a range to a deque.  void **assign** (**initializer\_list**\< value\_type \> \_\_l)\
 Assigns an initializer list to a deque.  void **assign** (size\_type \_\_n, const value\_type &\_\_val)\
 Assigns a given value to a deque.  reference **at** (size\_type \_\_n)\
 Provides access to the data contained in the deque.  const\_reference **at** (size\_type \_\_n) const\
 Provides access to the data contained in the deque.  const\_reference **back** () const noexcept\
 reference **back** () noexcept\
 const\_iterator **begin** () const noexcept\
 iterator **begin** () noexcept\
 const\_iterator **cbegin** () const noexcept\
 const\_iterator **cend** () const noexcept\
 void **clear** () noexcept\
 const\_reverse\_iterator **crbegin** () const noexcept\
 const\_reverse\_iterator **crend** () const noexcept\
 template\<typename... \_Args\> iterator **emplace** (const\_iterator \_\_position, \_Args &&... \_\_args)\
 Inserts an object in deque before specified iterator.  template\<typename... \_Args\> reference **emplace\_back** (\_Args &&... \_\_args)\
 template\<typename... \_Args\> reference **emplace\_front** (\_Args &&... \_\_args)\
 bool **empty** () const noexcept\
 const\_iterator **end** () const noexcept\
 iterator **end** () noexcept\
 iterator **erase** (const\_iterator \_\_first, const\_iterator \_\_last)\
 Remove a range of elements.  iterator **erase** (const\_iterator \_\_position)\
 Remove element at given position.  const\_reference **front** () const noexcept\
 reference **front** () noexcept\
 allocator\_type **get\_allocator** () const noexcept\
 Get a copy of the memory allocation object.  iterator **insert** (const\_iterator \_\_p, **initializer\_list**\< value\_type \> \_\_l)\
 Inserts an initializer list into the deque.  template\<typename \_InputIterator, typename = std::\_RequireInputIter\<\_InputIterator\>\> iterator **insert** (const\_iterator \_\_position, \_InputIterator \_\_first, \_InputIterator \_\_last)\
 Inserts a range into the deque.  iterator **insert** (const\_iterator \_\_position, const value\_type &\_\_x)\
 Inserts given value into deque before specified iterator.  iterator **insert** (const\_iterator \_\_position, size\_type \_\_n, const value\_type &\_\_x)\
 Inserts a number of copies of given data into the deque.  iterator **insert** (const\_iterator \_\_position, value\_type &&\_\_x)\
 Inserts given rvalue into deque before specified iterator.  size\_type **max\_size** () const noexcept\
 **deque** & **operator=** (const **deque** &\_\_x)\
 Deque assignment operator.  **deque** & **operator=** (**deque** &&\_\_x) noexcept(\_Alloc\_traits::\_S\_always\_equal())\
 Deque move assignment operator.  **deque** & **operator=** (**initializer\_list**\< value\_type \> \_\_l)\
 Assigns an initializer list to a deque.  const\_reference **operator\[\]** (size\_type \_\_n) const noexcept\
 Subscript access to the data contained in the deque.  reference **operator\[\]** (size\_type \_\_n) noexcept\
 Subscript access to the data contained in the deque.  void **pop\_back** () noexcept\
 Removes last element.  void **pop\_front** () noexcept\
 Removes first element.  void **push\_back** (const value\_type &\_\_x)\
 Add data to the end of the deque.  void **push\_back** (value\_type &&\_\_x)\
 void **push\_front** (const value\_type &\_\_x)\
 Add data to the front of the deque.  void **push\_front** (value\_type &&\_\_x)\
 const\_reverse\_iterator **rbegin** () const noexcept\
 reverse\_iterator **rbegin** () noexcept\
 const\_reverse\_iterator **rend** () const noexcept\
 reverse\_iterator **rend** () noexcept\
 void **resize** (size\_type \_\_new\_size)\
 Resizes the deque to the specified number of elements.  void **resize** (size\_type \_\_new\_size, const value\_type &\_\_x)\
 Resizes the deque to the specified number of elements.  void **shrink\_to\_fit** () noexcept\
 size\_type **size** () const noexcept\
 void **swap** (**deque** &\_\_x) noexcept\
 Swaps data with another deque.

### Protected Types

enum { **\_S\_initial\_map\_size** }\
 typedef **\_\_gnu\_cxx::\_\_alloc\_traits**\< \_Map\_alloc\_type \> **\_Map\_alloc\_traits**\
 typedef \_Alloc\_traits::template rebind\< \_Ptr \>::other **\_Map\_alloc\_type**\
 typedef \_Alloc\_traits::pointer **\_Ptr**\
 typedef \_Alloc\_traits::const\_pointer **\_Ptr\_const**\

### Protected Member Functions

template\<typename \_ForwardIterator\> void **\_M\_assign\_aux** (\_ForwardIterator \_\_first, \_ForwardIterator \_\_last, **std::forward\_iterator\_tag**)\
 template\<typename \_InputIterator\> void **\_M\_assign\_aux** (\_InputIterator \_\_first, \_InputIterator \_\_last, **std::input\_iterator\_tag**)\
 void **\_M\_deallocate\_map** (\_Map\_pointer \_\_p, size\_t \_\_n) noexcept\
 void **\_M\_default\_append** (size\_type \_\_n)\
 void **\_M\_default\_initialize** ()\
 template\<typename \_Alloc1\> void **\_M\_destroy\_data** (iterator \_\_first, iterator \_\_last, const \_Alloc1 &)\
 void **\_M\_destroy\_data** (iterator \_\_first, iterator \_\_last, const **std::allocator**\< \_Tp \> &)\
 void **\_M\_destroy\_data\_aux** (iterator \_\_first, iterator \_\_last)\
 template\<typename... \_Args\> iterator **\_M\_emplace\_aux** (iterator \_\_pos, \_Args &&... \_\_args)\
 iterator **\_M\_erase** (iterator \_\_first, iterator \_\_last)\
 iterator **\_M\_erase** (iterator \_\_pos)\
 void **\_M\_erase\_at\_begin** (iterator \_\_pos)\
 void **\_M\_erase\_at\_end** (iterator \_\_pos)\
 void **\_M\_fill\_assign** (size\_type \_\_n, const value\_type &\_\_val)\
 void **\_M\_fill\_initialize** (const value\_type &\_\_value)\
 Fills the deque with copies of value.  void **\_M\_fill\_insert** (iterator \_\_pos, size\_type \_\_n, const value\_type &\_\_x)\
 \_Map\_alloc\_type **\_M\_get\_map\_allocator** () const noexcept\
 template\<typename \_ForwardIterator\> void **\_M\_insert\_aux** (iterator \_\_pos, \_ForwardIterator \_\_first, \_ForwardIterator \_\_last, size\_type \_\_n)\
 iterator **\_M\_insert\_aux** (iterator \_\_pos, const value\_type &\_\_x)\
 void **\_M\_insert\_aux** (iterator \_\_pos, size\_type \_\_n, const value\_type &\_\_x)\
 void **\_M\_move\_assign1** (**deque** &&\_\_x, **false\_type**)\
 void **\_M\_move\_assign1** (**deque** &&\_\_x, **true\_type**) noexcept\
 void **\_M\_move\_assign2** (**deque** &&\_\_x, **false\_type**)\
 void **\_M\_move\_assign2** (**deque** &&\_\_x, **true\_type**)\
 template\<typename \_InputIterator, typename \_Sentinel\> void **\_M\_range\_append** (\_InputIterator \_\_first, \_Sentinel \_\_last, size\_type \_\_n)\
 void **\_M\_range\_check** (size\_type \_\_n) const\
 Safety check used only from at().  template\<typename \_ForwardIterator\> void **\_M\_range\_insert\_aux** (iterator \_\_pos, \_ForwardIterator \_\_first, \_ForwardIterator \_\_last, **std::forward\_iterator\_tag**)\
 template\<typename \_InputIterator\> void **\_M\_range\_insert\_aux** (iterator \_\_pos, \_InputIterator \_\_first, \_InputIterator \_\_last, **std::input\_iterator\_tag**)\
 template\<typename \_InputIterator, typename \_Sentinel\> void **\_M\_range\_prepend** (\_InputIterator \_\_first, \_Sentinel \_\_last, size\_type \_\_n)\
 template\<typename... \_Args\> void **\_M\_replace\_map** (\_Args &&... \_\_args)\
 bool **\_M\_shrink\_to\_fit** ()\

template\<typename \_InputIterator\> void **\_M\_range\_initialize** (\_InputIterator \_\_first, \_InputIterator \_\_last, **std::input\_iterator\_tag**)\
 Fills the deque with whatever is in \[first,last).  template\<typename \_ForwardIterator\> void **\_M\_range\_initialize** (\_ForwardIterator \_\_first, \_ForwardIterator \_\_last, **std::forward\_iterator\_tag**)\
 Fills the deque with whatever is in \[first,last).

template\<typename... \_Args\> void **\_M\_push\_back\_aux** (\_Args &&... \_\_args)\
 Helper functions for push\_\* and pop\_\*.  template\<typename... \_Args\> void **\_M\_push\_front\_aux** (\_Args &&... \_\_args)\
 Helper functions for push\_\* and pop\_\*.  void **\_M\_pop\_back\_aux** ()\
 Helper functions for push\_\* and pop\_\*.  void **\_M\_pop\_front\_aux** ()\
 Helper functions for push\_\* and pop\_\*.

iterator **\_M\_reserve\_elements\_at\_front** (size\_type \_\_n)\
 Memory-handling helpers for the previous internal insert functions.  iterator **\_M\_reserve\_elements\_at\_back** (size\_type \_\_n)\
 Memory-handling helpers for the previous internal insert functions.  void **\_M\_new\_elements\_at\_front** (size\_type \_\_new\_elements)\
 Memory-handling helpers for the previous internal insert functions.  void **\_M\_new\_elements\_at\_back** (size\_type \_\_new\_elements)\
 Memory-handling helpers for the previous internal insert functions.

void **\_M\_reserve\_map\_at\_back** (size\_type \_\_nodes\_to\_add=1)\
 Memory-handling helpers for the major map.  void **\_M\_reserve\_map\_at\_front** (size\_type \_\_nodes\_to\_add=1)\
 Memory-handling helpers for the major map.  void **\_M\_reallocate\_map** (size\_type \_\_nodes\_to\_add, bool \_\_add\_at\_front)\
 Memory-handling helpers for the major map.

### Static Protected Member Functions

static size\_t **\_S\_check\_init\_len** (size\_t \_\_n, const allocator\_type &\_\_a)\
 static size\_type **\_S\_max\_size** (const \_Tp\_alloc\_type &\_\_a) noexcept\

## Detailed Description

### template\<typename \_Tp, typename \_Alloc = std::allocator\<\_Tp\>\>

class std::deque\< \_Tp, \_Alloc \>"A standard container using fixed-size memory allocation and constant-time manipulation of elements at either end.

**Template Parameters**

*\_Tp* Type of element. \
 *\_Alloc* Allocator type, defaults to allocator\<\_Tp\>.

Meets the requirements of a container, a reversible container, and a sequence, including the optional sequence requirements.

In previous HP/SGI versions of deque, there was an extra template parameter so users could control the node size. This extension turned out to violate the C++ standard (it can be detected using template template parameters), and it was removed.

Here's how a deque\<Tp\> manages memory. Each deque has 4 members:

- •

  Tp\*\* \_M\_map

- •

  size\_t \_M\_map\_size

- •

  iterator \_M\_start, \_M\_finish

map\_size is at least 8. map is an array of map\_size pointers-to-*nodes*. (The name map has nothing to do with the std::map class, and **nodes** should not be confused with std::list's usage of *node*.)

A *node* has no specific type name as such, but it is referred to as *node* in this file. It is a simple array-of-Tp. If Tp is very large, there will be one Tp element per node (i.e., an *array* of one). For non-huge Tp's, node size is inversely related to Tp size: the larger the Tp, the fewer Tp's will fit in a node. The goal here is to keep the total size of a node relatively small and constant over different Tp's, to improve allocator efficiency.

Not every pointer in the map array will point to a node. If the initial number of elements in the deque is small, the /middle/ map pointers will be valid, and the ones at the edges will be unused. This same situation will arise as the map grows: available map pointers, if any, will be on the ends. As new nodes are created, only a subset of the map's pointers need to be copied *outward*.

Class invariants:

- •

  For any nonsingular iterator i:

- •

  i.node points to a member of the map array. (Yes, you read that correctly: i.node does not actually point to a node.) The member of the map array is what actually points to the node.

- •

  i.first == \*(i.node) (This points to the node (first Tp element).)

- •

  i.last == i.first + node\_size

- •

  i.cur is a pointer in the range \[i.first, i.last). NOTE: the implication of this is that i.cur is always a dereferenceable pointer, even if i is a past-the-end iterator.

- •

  Start and Finish are always nonsingular iterators. NOTE: this means that an empty deque must have one node, a deque with \<N elements (where N is the node buffer size) must have one node, a deque with N through (2N-1) elements must have two nodes, etc.

- •

  For every node other than start.node and finish.node, every element in the node is an initialized object. If start.node == finish.node, then \[start.cur, finish.cur) are initialized objects, and the elements outside that range are uninitialized storage. Otherwise, \[start.cur, start.last) and \[finish.first, finish.cur) are initialized objects, and \[start.first, start.cur) and \[finish.cur, finish.last) are uninitialized storage.

- •

  \[map, map + map\_size) is a valid, non-empty range.

- •

  \[start.node, finish.node\] is a valid range contained within \[map, map + map\_size).

- •

  A pointer in the range \[map, map + map\_size) points to an allocated node if and only if the pointer is in the range \[start.node, finish.node\].

Here's the magic: nothing in deque is **aware** of the discontiguous storage!

The memory setup and layout occurs in the parent, \_Base, and the iterator class is entirely responsible for *leaping* from one node to the next. All the implementation routines for deque itself work only through the start and finish iterators. This keeps the routines simple and sane, and we can use other standard algorithms as well.

## Constructor & Destructor Documentation

### template\<typename \_Tp, typename \_Alloc = std::allocator\<\_Tp\>\> std::deque\< \_Tp, \_Alloc \>::deque () \[default\]

Creates a deque with no elements.

Referenced by **insert()**, and **operator=()**.

### template\<typename \_Tp, typename \_Alloc = std::allocator\<\_Tp\>\> std::deque\< \_Tp, \_Alloc \>::deque (const allocator\_type & \_\_a) \[inline\], \[explicit\]

Creates a deque with no elements.

**Parameters**

*\_\_a* An allocator object.

### template\<typename \_Tp, typename \_Alloc = std::allocator\<\_Tp\>\> std::deque\< \_Tp, \_Alloc \>::deque (size\_type \_\_n, const allocator\_type & \_\_a = allocator\_type()) \[inline\], \[explicit\]

Creates a deque with default constructed elements.

**Parameters**

*\_\_n* The number of elements to initially create. \
 *\_\_a* An allocator.

This constructor fills the deque with *n* default constructed elements.

### template\<typename \_Tp, typename \_Alloc = std::allocator\<\_Tp\>\> std::deque\< \_Tp, \_Alloc \>::deque (size\_type \_\_n, const value\_type & \_\_value, const allocator\_type & \_\_a = allocator\_type()) \[inline\]

Creates a deque with copies of an exemplar element.

**Parameters**

*\_\_n* The number of elements to initially create. \
 *\_\_value* An element to copy. \
 *\_\_a* An allocator.

This constructor fills the deque with *\_\_n* copies of *\_\_value*.

### template\<typename \_Tp, typename \_Alloc = std::allocator\<\_Tp\>\> std::deque\< \_Tp, \_Alloc \>::deque (const deque\< \_Tp, \_Alloc \> & \_\_x) \[inline\]

Deque copy constructor.

**Parameters**

*\_\_x* A deque of identical element and allocator types.

The newly-created deque uses a copy of the allocator object used by *\_\_x* (unless the allocator traits dictate a different object).

### template\<typename \_Tp, typename \_Alloc = std::allocator\<\_Tp\>\> std::deque\< \_Tp, \_Alloc \>::deque (deque\< \_Tp, \_Alloc \> && ) \[default\]

Deque move constructor. The newly-created deque contains the exact contents of the moved instance. The contents of the moved instance are a valid, but unspecified deque.

### template\<typename \_Tp, typename \_Alloc = std::allocator\<\_Tp\>\> std::deque\< \_Tp, \_Alloc \>::deque (const deque\< \_Tp, \_Alloc \> & \_\_x, const \_\_type\_identity\_t\< allocator\_type \> & \_\_a) \[inline\]

Copy constructor with alternative allocator.

### template\<typename \_Tp, typename \_Alloc = std::allocator\<\_Tp\>\> std::deque\< \_Tp, \_Alloc \>::deque (deque\< \_Tp, \_Alloc \> && \_\_x, const \_\_type\_identity\_t\< allocator\_type \> & \_\_a) \[inline\]

Move constructor with alternative allocator.

### template\<typename \_Tp, typename \_Alloc = std::allocator\<\_Tp\>\> std::deque\< \_Tp, \_Alloc \>::deque (initializer\_list\< value\_type \> \_\_l, const allocator\_type & \_\_a = allocator\_type()) \[inline\]

Builds a deque from an initializer list.

**Parameters**

*\_\_l* An initializer\_list. \
 *\_\_a* An allocator object.

Create a deque consisting of copies of the elements in the initializer\_list *\_\_l*.

This will call the element type's copy constructor N times (where N is \_\_l.size()) and do no memory reallocation.

### template\<typename \_Tp, typename \_Alloc = std::allocator\<\_Tp\>\> template\<typename \_InputIterator, typename = std::\_RequireInputIter\<\_InputIterator\>\> std::deque\< \_Tp, \_Alloc \>::deque (\_InputIterator \_\_first, \_InputIterator \_\_last, const allocator\_type & \_\_a = allocator\_type()) \[inline\]

Builds a deque from a range.

**Parameters**

*\_\_first* An input iterator. \
 *\_\_last* An input iterator. \
 *\_\_a* An allocator object.

Create a deque consisting of copies of the elements from \[\_\_first, \_\_last).

If the iterators are forward, bidirectional, or random-access, then this will call the elements' copy constructor N times (where N is distance(\_\_first,\_\_last)) and do no memory reallocation. But if only input iterators are used, then this will do at most 2N calls to the copy constructor, and logN memory reallocations.

### template\<typename \_Tp, typename \_Alloc = std::allocator\<\_Tp\>\> std::deque\< \_Tp, \_Alloc \>::~deque () \[inline\]

The dtor only erases the elements, and note that if the elements themselves are pointers, the pointed-to memory is not touched in any way. Managing the pointer is the user's responsibility.

## Member Function Documentation

### template\<typename \_Tp, typename \_Alloc\> void deque::\_M\_fill\_initialize (const value\_type & \_\_value) \[protected\]

Fills the deque with copies of value.

**Parameters**

*\_\_value* Initial value.

**Precondition**

\_M\_start and \_M\_finish have already been initialized, but none of the deque's elements have yet been constructed.

This function is called only when the user provides an explicit size (with or without an explicit exemplar value).

References **std::\_Destroy()**.

Referenced by **std::deque\< \_Tp, polymorphic\_allocator\< \_Tp \> \>::deque()**.

### template\<typename \_Tp, typename \_Alloc\> void deque::\_M\_new\_elements\_at\_back (size\_type \_\_new\_elements) \[protected\]

Memory-handling helpers for the previous internal insert functions.

References **\_M\_reserve\_map\_at\_back()**, **max\_size()**, and **std::size()**.

Referenced by **std::deque\< \_Tp, polymorphic\_allocator\< \_Tp \> \>::\_M\_reserve\_elements\_at\_back()**.

### template\<typename \_Tp, typename \_Alloc\> void deque::\_M\_new\_elements\_at\_front (size\_type \_\_new\_elements) \[protected\]

Memory-handling helpers for the previous internal insert functions.

References **\_M\_reserve\_map\_at\_front()**, **max\_size()**, and **std::size()**.

Referenced by **std::deque\< \_Tp, polymorphic\_allocator\< \_Tp \> \>::\_M\_reserve\_elements\_at\_front()**.

### template\<typename \_Tp, typename \_Alloc\> void deque::\_M\_pop\_back\_aux () \[protected\]

Helper functions for push\_\* and pop\_\*.

Referenced by **std::deque\< \_Tp, polymorphic\_allocator\< \_Tp \> \>::pop\_back()**.

### template\<typename \_Tp, typename \_Alloc\> void deque::\_M\_pop\_front\_aux () \[protected\]

Helper functions for push\_\* and pop\_\*.

Referenced by **std::deque\< \_Tp, polymorphic\_allocator\< \_Tp \> \>::pop\_front()**.

### template\<typename \_Tp, typename \_Alloc\> template\<typename... \_Args\> void deque::\_M\_push\_back\_aux (\_Args &&... \_\_args) \[protected\]

Helper functions for push\_\* and pop\_\*.

References **\_M\_reserve\_map\_at\_back()**, **std::forward()**, **max\_size()**, and **std::size()**.

Referenced by **std::deque\< \_Tp, polymorphic\_allocator\< \_Tp \> \>::push\_back()**.

### template\<typename \_Tp, typename \_Alloc\> template\<typename... \_Args\> void deque::\_M\_push\_front\_aux (\_Args &&... \_\_args) \[protected\]

Helper functions for push\_\* and pop\_\*.

References **\_M\_reserve\_map\_at\_front()**, **std::forward()**, **max\_size()**, and **std::size()**.

Referenced by **std::deque\< \_Tp, polymorphic\_allocator\< \_Tp \> \>::push\_front()**.

### template\<typename \_Tp, typename \_Alloc = std::allocator\<\_Tp\>\> void std::deque\< \_Tp, \_Alloc \>::\_M\_range\_check (size\_type \_\_n) const \[inline\], \[protected\]

Safety check used only from at().

Referenced by **std::deque\< \_Tp, polymorphic\_allocator\< \_Tp \> \>::at()**, and **std::deque\< \_Tp, polymorphic\_allocator\< \_Tp \> \>::at()**.

### template\<typename \_Tp, typename \_Alloc\> template\<typename \_ForwardIterator\> void deque::\_M\_range\_initialize (\_ForwardIterator \_\_first, \_ForwardIterator \_\_last, std::forward\_iterator\_tag ) \[protected\]

Fills the deque with whatever is in \[first,last).

**Parameters**

*\_\_first* An input iterator. \
 *\_\_last* An input iterator.

If the iterators are actually forward iterators (or better), then the memory layout can be done all at once. Else we move forward using push\_back on each value from the iterator.

References **std::\_Destroy()**, **std::advance()**, and **std::distance()**.

### template\<typename \_Tp, typename \_Alloc\> template\<typename \_InputIterator\> void deque::\_M\_range\_initialize (\_InputIterator \_\_first, \_InputIterator \_\_last, std::input\_iterator\_tag ) \[protected\]

Fills the deque with whatever is in \[first,last).

**Parameters**

*\_\_first* An input iterator. \
 *\_\_last* An input iterator.

If the iterators are actually forward iterators (or better), then the memory layout can be done all at once. Else we move forward using push\_back on each value from the iterator.

References **clear()**, and **push\_back()**.

Referenced by **std::deque\< \_Tp, polymorphic\_allocator\< \_Tp \> \>::deque()**, and **std::deque\< \_Tp, polymorphic\_allocator\< \_Tp \> \>::deque()**.

### template\<typename \_Tp, typename \_Alloc\> void deque::\_M\_reallocate\_map (size\_type \_\_nodes\_to\_add, bool \_\_add\_at\_front) \[protected\]

Memory-handling helpers for the major map. Makes sure the \_M\_map has space for new nodes. Does not actually add the nodes. Can invalidate \_M\_map pointers. (And consequently, deque iterators.)

References **std::max()**, and **max\_size()**.

Referenced by **std::deque\< \_Tp, polymorphic\_allocator\< \_Tp \> \>::\_M\_reserve\_map\_at\_back()**, and **std::deque\< \_Tp, polymorphic\_allocator\< \_Tp \> \>::\_M\_reserve\_map\_at\_front()**.

### template\<typename \_Tp, typename \_Alloc = std::allocator\<\_Tp\>\> iterator std::deque\< \_Tp, \_Alloc \>::\_M\_reserve\_elements\_at\_back (size\_type \_\_n) \[inline\], \[protected\]

Memory-handling helpers for the previous internal insert functions.

### template\<typename \_Tp, typename \_Alloc = std::allocator\<\_Tp\>\> iterator std::deque\< \_Tp, \_Alloc \>::\_M\_reserve\_elements\_at\_front (size\_type \_\_n) \[inline\], \[protected\]

Memory-handling helpers for the previous internal insert functions.

### template\<typename \_Tp, typename \_Alloc = std::allocator\<\_Tp\>\> void std::deque\< \_Tp, \_Alloc \>::\_M\_reserve\_map\_at\_back (size\_type \_\_nodes\_to\_add = 1) \[inline\], \[protected\]

Memory-handling helpers for the major map. Makes sure the \_M\_map has space for new nodes. Does not actually add the nodes. Can invalidate \_M\_map pointers. (And consequently, deque iterators.)

Referenced by **\_M\_new\_elements\_at\_back()**, and **\_M\_push\_back\_aux()**.

### template\<typename \_Tp, typename \_Alloc = std::allocator\<\_Tp\>\> void std::deque\< \_Tp, \_Alloc \>::\_M\_reserve\_map\_at\_front (size\_type \_\_nodes\_to\_add = 1) \[inline\], \[protected\]

Memory-handling helpers for the major map. Makes sure the \_M\_map has space for new nodes. Does not actually add the nodes. Can invalidate \_M\_map pointers. (And consequently, deque iterators.)

Referenced by **\_M\_new\_elements\_at\_front()**, and **\_M\_push\_front\_aux()**.

### template\<typename \_Tp, typename \_Alloc = std::allocator\<\_Tp\>\> template\<typename \_InputIterator, typename = std::\_RequireInputIter\<\_InputIterator\>\> void std::deque\< \_Tp, \_Alloc \>::assign (\_InputIterator \_\_first, \_InputIterator \_\_last) \[inline\]

Assigns a range to a deque.

**Parameters**

*\_\_first* An input iterator. \
 *\_\_last* An input iterator.

This function fills a deque with copies of the elements in the range \[\_\_first,\_\_last).

Note that the assignment completely changes the deque and that the resulting deque's size is the same as the number of elements assigned.

### template\<typename \_Tp, typename \_Alloc = std::allocator\<\_Tp\>\> void std::deque\< \_Tp, \_Alloc \>::assign (initializer\_list\< value\_type \> \_\_l) \[inline\]

Assigns an initializer list to a deque.

**Parameters**

*\_\_l* An initializer\_list.

This function fills a deque with copies of the elements in the initializer\_list *\_\_l*.

Note that the assignment completely changes the deque and that the resulting deque's size is the same as the number of elements assigned.

### template\<typename \_Tp, typename \_Alloc = std::allocator\<\_Tp\>\> void std::deque\< \_Tp, \_Alloc \>::assign (size\_type \_\_n, const value\_type & \_\_val) \[inline\]

Assigns a given value to a deque.

**Parameters**

*\_\_n* Number of elements to be assigned. \
 *\_\_val* Value to be assigned.

This function fills a deque with *n* copies of the given value. Note that the assignment completely changes the deque and that the resulting deque's size is the same as the number of elements assigned.

### template\<typename \_Tp, typename \_Alloc = std::allocator\<\_Tp\>\> reference std::deque\< \_Tp, \_Alloc \>::at (size\_type \_\_n) \[inline\]

Provides access to the data contained in the deque.

**Parameters**

*\_\_n* The index of the element for which data should be accessed.

**Returns**

Read/write reference to data.

**Exceptions**

**std::out\_of\_range If** *\_\_n* **is an invalid index.**

**This function provides for safer data access. The parameter is first checked that it is in the range of the deque. The function throws out\_of\_range if the check fails.**

### template\<typename \_Tp, typename \_Alloc = std::allocator\<\_Tp\>\> const\_reference std::deque\< \_Tp, \_Alloc \>::at (size\_type \_\_n) const \[inline\]

**Provides access to the data contained in the deque.**

**Parameters**

*\_\_n* **The index of the element for which data should be accessed.**

**Returns**

**Read-only (constant) reference to data.**

**Exceptions**

**std::out\_of\_range If** *\_\_n* **is an invalid index.**

**This function provides for safer data access. The parameter is first checked that it is in the range of the deque. The function throws out\_of\_range if the check fails.**

### template\<typename \_Tp, typename \_Alloc = std::allocator\<\_Tp\>\> const\_reference std::deque\< \_Tp, \_Alloc \>::back () const \[inline\], \[nodiscard\], \[noexcept\]

**Returns a read-only (constant) reference to the data at the last element of the deque.**

### template\<typename \_Tp, typename \_Alloc = std::allocator\<\_Tp\>\> reference std::deque\< \_Tp, \_Alloc \>::back () \[inline\], \[nodiscard\], \[noexcept\]

**Returns a read/write reference to the data at the last element of the deque.**

### template\<typename \_Tp, typename \_Alloc = std::allocator\<\_Tp\>\> const\_iterator std::deque\< \_Tp, \_Alloc \>::begin () const \[inline\], \[nodiscard\], \[noexcept\]

**Returns a read-only (constant) iterator that points to the first element in the deque. Iteration is done in ordinary element order.**

### template\<typename \_Tp, typename \_Alloc = std::allocator\<\_Tp\>\> iterator std::deque\< \_Tp, \_Alloc \>::begin () \[inline\], \[nodiscard\], \[noexcept\]

**Returns a read/write iterator that points to the first element in the deque. Iteration is done in ordinary element order.**

**Referenced by std::deque\< \_Tp, polymorphic\_allocator\< \_Tp \> \>::deque(), std::deque\< \_Tp, polymorphic\_allocator\< \_Tp \> \>::deque(), std::deque\< \_Tp, polymorphic\_allocator\< \_Tp \> \>::~deque(), std::deque\< \_Tp, polymorphic\_allocator\< \_Tp \> \>::clear(), std::deque\< \_Tp, polymorphic\_allocator\< \_Tp \> \>::front(), std::deque\< \_Tp, polymorphic\_allocator\< \_Tp \> \>::front(), std::deque\< \_Tp, polymorphic\_allocator\< \_Tp \> \>::insert(), std::deque\< \_Tp, polymorphic\_allocator\< \_Tp \> \>::insert(), std::deque\< \_Tp, polymorphic\_allocator\< \_Tp \> \>::insert(), std::operator\<=\>(), and operator=().**

### template\<typename \_Tp, typename \_Alloc = std::allocator\<\_Tp\>\> const\_iterator std::deque\< \_Tp, \_Alloc \>::cbegin () const \[inline\], \[noexcept\]

**Returns a read-only (constant) iterator that points to the first element in the deque. Iteration is done in ordinary element order.**

**Referenced by std::deque\< \_Tp, polymorphic\_allocator\< \_Tp \> \>::insert(), std::deque\< \_Tp, polymorphic\_allocator\< \_Tp \> \>::insert(), and std::deque\< \_Tp, polymorphic\_allocator\< \_Tp \> \>::insert().**

### template\<typename \_Tp, typename \_Alloc = std::allocator\<\_Tp\>\> const\_iterator std::deque\< \_Tp, \_Alloc \>::cend () const \[inline\], \[noexcept\]

**Returns a read-only (constant) iterator that points one past the last element in the deque. Iteration is done in ordinary element order.**

### template\<typename \_Tp, typename \_Alloc = std::allocator\<\_Tp\>\> void std::deque\< \_Tp, \_Alloc \>::clear () \[inline\], \[noexcept\]

**Erases all the elements. Note that this function only erases the elements, and that if the elements themselves are pointers, the pointed-to memory is not touched in any way. Managing the pointer is the user's responsibility.**

**Referenced by \_M\_range\_initialize().**

### template\<typename \_Tp, typename \_Alloc = std::allocator\<\_Tp\>\> const\_reverse\_iterator std::deque\< \_Tp, \_Alloc \>::crbegin () const \[inline\], \[noexcept\]

**Returns a read-only (constant) reverse iterator that points to the last element in the deque. Iteration is done in reverse element order.**

### template\<typename \_Tp, typename \_Alloc = std::allocator\<\_Tp\>\> const\_reverse\_iterator std::deque\< \_Tp, \_Alloc \>::crend () const \[inline\], \[noexcept\]

**Returns a read-only (constant) reverse iterator that points to one before the first element in the deque. Iteration is done in reverse element order.**

### template\<typename \_Tp, typename \_Alloc\> template\<typename... \_Args\> deque\< \_Tp, \_Alloc \>::iterator deque::emplace (const\_iterator \_\_position, \_Args &&... \_\_args)

**Inserts an object in deque before specified iterator.**

**Parameters**

*\_\_position* **A const\_iterator into the deque.** \
 *\_\_args* **Arguments.**

**Returns**

**An iterator that points to the inserted data.**

**This function will insert an object of type T constructed with T(std::forward\<Args\>(args)...) before the specified location.**

**References std::forward().**

**Referenced by std::deque\< \_Tp, polymorphic\_allocator\< \_Tp \> \>::insert().**

### template\<typename \_Tp, typename \_Alloc = std::allocator\<\_Tp\>\> bool std::deque\< \_Tp, \_Alloc \>::empty () const \[inline\], \[nodiscard\], \[noexcept\]

**Returns true if the deque is empty. (Thus begin() would equal end().)**

### template\<typename \_Tp, typename \_Alloc = std::allocator\<\_Tp\>\> const\_iterator std::deque\< \_Tp, \_Alloc \>::end () const \[inline\], \[nodiscard\], \[noexcept\]

**Returns a read-only (constant) iterator that points one past the last element in the deque. Iteration is done in ordinary element order.**

### template\<typename \_Tp, typename \_Alloc = std::allocator\<\_Tp\>\> iterator std::deque\< \_Tp, \_Alloc \>::end () \[inline\], \[nodiscard\], \[noexcept\]

**Returns a read/write iterator that points one past the last element in the deque. Iteration is done in ordinary element order.**

**Referenced by std::deque\< \_Tp, polymorphic\_allocator\< \_Tp \> \>::deque(), std::deque\< \_Tp, polymorphic\_allocator\< \_Tp \> \>::deque(), std::deque\< \_Tp, polymorphic\_allocator\< \_Tp \> \>::~deque(), std::deque\< \_Tp, polymorphic\_allocator\< \_Tp \> \>::back(), std::deque\< \_Tp, polymorphic\_allocator\< \_Tp \> \>::back(), std::operator\<=\>(), and operator=().**

### template\<typename \_Tp, typename \_Alloc = std::allocator\<\_Tp\>\> iterator std::deque\< \_Tp, \_Alloc \>::erase (const\_iterator \_\_first, const\_iterator \_\_last) \[inline\]

**Remove a range of elements.**

**Parameters**

*\_\_first* **Iterator pointing to the first element to be erased.** \
 *\_\_last* **Iterator pointing to one past the last element to be erased.**

**Returns**

**An iterator pointing to the element pointed to by** *last* **prior to erasing (or end()).**

**This function will erase the elements in the range \[\_\_first,\_\_last) and shorten the deque accordingly.**

**The user is cautioned that this function only erases the elements, and that if the elements themselves are pointers, the pointed-to memory is not touched in any way. Managing the pointer is the user's responsibility.**

### template\<typename \_Tp, typename \_Alloc = std::allocator\<\_Tp\>\> iterator std::deque\< \_Tp, \_Alloc \>::erase (const\_iterator \_\_position) \[inline\]

**Remove element at given position.**

**Parameters**

*\_\_position* **Iterator pointing to element to be erased.**

**Returns**

**An iterator pointing to the next element (or end()).**

**This function will erase the element at the given position and thus shorten the deque by one.**

**The user is cautioned that this function only erases the element, and that if the element is itself a pointer, the pointed-to memory is not touched in any way. Managing the pointer is the user's responsibility.**

### template\<typename \_Tp, typename \_Alloc = std::allocator\<\_Tp\>\> const\_reference std::deque\< \_Tp, \_Alloc \>::front () const \[inline\], \[nodiscard\], \[noexcept\]

**Returns a read-only (constant) reference to the data at the first element of the deque.**

### template\<typename \_Tp, typename \_Alloc = std::allocator\<\_Tp\>\> reference std::deque\< \_Tp, \_Alloc \>::front () \[inline\], \[nodiscard\], \[noexcept\]

**Returns a read/write reference to the data at the first element of the deque.**

### template\<typename \_Tp, typename \_Alloc = std::allocator\<\_Tp\>\> allocator\_type std::deque\< \_Tp, \_Alloc \>::get\_allocator () const \[inline\], \[nodiscard\], \[noexcept\]

**Get a copy of the memory allocation object.**

**Referenced by operator=().**

### template\<typename \_Tp, typename \_Alloc = std::allocator\<\_Tp\>\> iterator std::deque\< \_Tp, \_Alloc \>::insert (const\_iterator \_\_p, initializer\_list\< value\_type \> \_\_l) \[inline\]

**Inserts an initializer list into the deque.**

**Parameters**

*\_\_p* **An iterator into the deque.** \
 *\_\_l* **An initializer\_list.**

**Returns**

**An iterator that points to the inserted data.**

**This function will insert copies of the data in the initializer\_list** *\_\_l* **into the deque before the location specified by** *\_\_p***. This is known as** *list insert***.**

### template\<typename \_Tp, typename \_Alloc = std::allocator\<\_Tp\>\> template\<typename \_InputIterator, typename = std::\_RequireInputIter\<\_InputIterator\>\> iterator std::deque\< \_Tp, \_Alloc \>::insert (const\_iterator \_\_position, \_InputIterator \_\_first, \_InputIterator \_\_last) \[inline\]

**Inserts a range into the deque.**

**Parameters**

*\_\_position* **A const\_iterator into the deque.** \
 *\_\_first* **An input iterator.** \
 *\_\_last* **An input iterator.**

**Returns**

**An iterator that points to the inserted data.**

**This function will insert copies of the data in the range \[\_\_first,\_\_last) into the deque before the location specified by** *\_\_position***. This is known as** *range insert***.**

### template\<typename \_Tp, typename \_Alloc\> deque\< \_Tp, \_Alloc \>::iterator deque::insert (const\_iterator \_\_position, const value\_type & \_\_x)

**Inserts given value into deque before specified iterator.**

**Parameters**

*\_\_position* **A const\_iterator into the deque.** \
 *\_\_x* **Data to be inserted.**

**Returns**

**An iterator that points to the inserted data.**

**This function will insert a copy of the given value before the specified location.**

**References deque(), insert(), push\_back(), and push\_front().**

**Referenced by insert().**

### template\<typename \_Tp, typename \_Alloc = std::allocator\<\_Tp\>\> iterator std::deque\< \_Tp, \_Alloc \>::insert (const\_iterator \_\_position, size\_type \_\_n, const value\_type & \_\_x) \[inline\]

**Inserts a number of copies of given data into the deque.**

**Parameters**

*\_\_position* **A const\_iterator into the deque.** \
 *\_\_n* **Number of elements to be inserted.** \
 *\_\_x* **Data to be inserted.**

**Returns**

**An iterator that points to the inserted data.**

**This function will insert a specified number of copies of the given data before the location specified by** *\_\_position***.**

### template\<typename \_Tp, typename \_Alloc = std::allocator\<\_Tp\>\> iterator std::deque\< \_Tp, \_Alloc \>::insert (const\_iterator \_\_position, value\_type && \_\_x) \[inline\]

**Inserts given rvalue into deque before specified iterator.**

**Parameters**

*\_\_position* **A const\_iterator into the deque.** \
 *\_\_x* **Data to be inserted.**

**Returns**

**An iterator that points to the inserted data.**

**This function will insert a copy of the given rvalue before the specified location.**

### template\<typename \_Tp, typename \_Alloc = std::allocator\<\_Tp\>\> size\_type std::deque\< \_Tp, \_Alloc \>::max\_size () const \[inline\], \[nodiscard\], \[noexcept\]

**Returns the size() of the largest possible deque.**

**Referenced by \_M\_new\_elements\_at\_back(), \_M\_new\_elements\_at\_front(), \_M\_push\_back\_aux(), \_M\_push\_front\_aux(), and \_M\_reallocate\_map().**

### template\<typename \_Tp, typename \_Alloc\> deque\< \_Tp, \_Alloc \> & deque::operator= (const deque\< \_Tp, \_Alloc \> & \_\_x)

**Deque assignment operator.**

**Parameters**

*\_\_x* **A deque of identical element and allocator types.**

**All the elements of** *x* **are copied.**

**The newly-created deque uses a copy of the allocator object used by** *\_\_x* **(unless the allocator traits dictate a different object).**

**References deque(), std::\_\_addressof(), begin(), end(), get\_allocator(), size(), and std::size().**

### template\<typename \_Tp, typename \_Alloc = std::allocator\<\_Tp\>\> deque & std::deque\< \_Tp, \_Alloc \>::operator= (deque\< \_Tp, \_Alloc \> && \_\_x) \[inline\], \[noexcept\]

**Deque move assignment operator.**

**Parameters**

*\_\_x* **A deque of identical element and allocator types.**

**The contents of** *\_\_x* **are moved into this deque (without copying, if the allocators permit it).** *\_\_x* **is a valid, but unspecified deque.**

### template\<typename \_Tp, typename \_Alloc = std::allocator\<\_Tp\>\> deque & std::deque\< \_Tp, \_Alloc \>::operator= (initializer\_list\< value\_type \> \_\_l) \[inline\]

**Assigns an initializer list to a deque.**

**Parameters**

*\_\_l* **An initializer\_list.**

**This function fills a deque with copies of the elements in the initializer\_list** *\_\_l***.**

**Note that the assignment completely changes the deque and that the resulting deque's size is the same as the number of elements assigned.**

### template\<typename \_Tp, typename \_Alloc = std::allocator\<\_Tp\>\> const\_reference std::deque\< \_Tp, \_Alloc \>::operator\[\] (size\_type \_\_n) const \[inline\], \[nodiscard\], \[noexcept\]

**Subscript access to the data contained in the deque.**

**Parameters**

*\_\_n* **The index of the element for which data should be accessed.**

**Returns**

**Read-only (constant) reference to data.**

**This operator allows for easy, array-style, data access. Note that data access with this operator is unchecked and out\_of\_range lookups are not defined. (For checked lookups see at().)**

### template\<typename \_Tp, typename \_Alloc = std::allocator\<\_Tp\>\> reference std::deque\< \_Tp, \_Alloc \>::operator\[\] (size\_type \_\_n) \[inline\], \[nodiscard\], \[noexcept\]

**Subscript access to the data contained in the deque.**

**Parameters**

*\_\_n* **The index of the element for which data should be accessed.**

**Returns**

**Read/write reference to data.**

**This operator allows for easy, array-style, data access. Note that data access with this operator is unchecked and out\_of\_range lookups are not defined. (For checked lookups see at().)**

### template\<typename \_Tp, typename \_Alloc = std::allocator\<\_Tp\>\> void std::deque\< \_Tp, \_Alloc \>::pop\_back () \[inline\], \[noexcept\]

**Removes last element. This is a typical stack operation. It shrinks the deque by one.**

**Note that no data is returned, and if the last element's data is needed, it should be retrieved before pop\_back() is called.**

### template\<typename \_Tp, typename \_Alloc = std::allocator\<\_Tp\>\> void std::deque\< \_Tp, \_Alloc \>::pop\_front () \[inline\], \[noexcept\]

**Removes first element. This is a typical stack operation. It shrinks the deque by one.**

**Note that no data is returned, and if the first element's data is needed, it should be retrieved before pop\_front() is called.**

### template\<typename \_Tp, typename \_Alloc = std::allocator\<\_Tp\>\> void std::deque\< \_Tp, \_Alloc \>::push\_back (const value\_type & \_\_x) \[inline\]

**Add data to the end of the deque.**

**Parameters**

*\_\_x* **Data to be added.**

**This is a typical stack operation. The function creates an element at the end of the deque and assigns the given data to it. Due to the nature of a deque this operation can be done in constant time.**

**Referenced by \_M\_range\_initialize(), and insert().**

### template\<typename \_Tp, typename \_Alloc = std::allocator\<\_Tp\>\> void std::deque\< \_Tp, \_Alloc \>::push\_front (const value\_type & \_\_x) \[inline\]

**Add data to the front of the deque.**

**Parameters**

*\_\_x* **Data to be added.**

**This is a typical stack operation. The function creates an element at the front of the deque and assigns the given data to it. Due to the nature of a deque this operation can be done in constant time.**

**Referenced by insert().**

### template\<typename \_Tp, typename \_Alloc = std::allocator\<\_Tp\>\> const\_reverse\_iterator std::deque\< \_Tp, \_Alloc \>::rbegin () const \[inline\], \[nodiscard\], \[noexcept\]

**Returns a read-only (constant) reverse iterator that points to the last element in the deque. Iteration is done in reverse element order.**

### template\<typename \_Tp, typename \_Alloc = std::allocator\<\_Tp\>\> reverse\_iterator std::deque\< \_Tp, \_Alloc \>::rbegin () \[inline\], \[nodiscard\], \[noexcept\]

**Returns a read/write reverse iterator that points to the last element in the deque. Iteration is done in reverse element order.**

### template\<typename \_Tp, typename \_Alloc = std::allocator\<\_Tp\>\> const\_reverse\_iterator std::deque\< \_Tp, \_Alloc \>::rend () const \[inline\], \[nodiscard\], \[noexcept\]

**Returns a read-only (constant) reverse iterator that points to one before the first element in the deque. Iteration is done in reverse element order.**

### template\<typename \_Tp, typename \_Alloc = std::allocator\<\_Tp\>\> reverse\_iterator std::deque\< \_Tp, \_Alloc \>::rend () \[inline\], \[nodiscard\], \[noexcept\]

**Returns a read/write reverse iterator that points to one before the first element in the deque. Iteration is done in reverse element order.**

### template\<typename \_Tp, typename \_Alloc = std::allocator\<\_Tp\>\> void std::deque\< \_Tp, \_Alloc \>::resize (size\_type \_\_new\_size) \[inline\]

**Resizes the deque to the specified number of elements.**

**Parameters**

*\_\_new\_size* **Number of elements the deque should contain.**

**This function will resize the deque to the specified number of elements. If the number is smaller than the deque's current size the deque is truncated, otherwise default constructed elements are appended.**

### template\<typename \_Tp, typename \_Alloc = std::allocator\<\_Tp\>\> void std::deque\< \_Tp, \_Alloc \>::resize (size\_type \_\_new\_size, const value\_type & \_\_x) \[inline\]

**Resizes the deque to the specified number of elements.**

**Parameters**

*\_\_new\_size* **Number of elements the deque should contain.** \
 *\_\_x* **Data with which new elements should be populated.**

**This function will resize the deque to the specified number of elements. If the number is smaller than the deque's current size the deque is truncated, otherwise the deque is extended and new elements are populated with given data.**

### template\<typename \_Tp, typename \_Alloc = std::allocator\<\_Tp\>\> void std::deque\< \_Tp, \_Alloc \>::shrink\_to\_fit () \[inline\], \[noexcept\]

**A non-binding request to reduce memory use.**

### template\<typename \_Tp, typename \_Alloc = std::allocator\<\_Tp\>\> size\_type std::deque\< \_Tp, \_Alloc \>::size () const \[inline\], \[nodiscard\], \[noexcept\]

**Returns the number of elements in the deque.**

**Referenced by std::deque\< \_Tp, polymorphic\_allocator\< \_Tp \> \>::\_M\_range\_check(), std::operator\<=\>(), operator=(), std::deque\< \_Tp, polymorphic\_allocator\< \_Tp \> \>::resize(), and std::deque\< \_Tp, polymorphic\_allocator\< \_Tp \> \>::resize().**

### template\<typename \_Tp, typename \_Alloc = std::allocator\<\_Tp\>\> void std::deque\< \_Tp, \_Alloc \>::swap (deque\< \_Tp, \_Alloc \> & \_\_x) \[inline\], \[noexcept\]

**Swaps data with another deque.**

**Parameters**

*\_\_x* **A deque of the same element and allocator types.**

**This exchanges the elements between two deques in constant time. (Four pointers, so it should be quite fast.) Note that the global std::swap() function is specialized such that std::swap(d1,d2) will feed to this function.**

**Whether the allocators are swapped depends on the allocator traits.**

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