# Sequence container (C++)

In computing, sequence containers refer to a group of container class templates in the standard library of the C++ programming language that implement storage of data elements. Being templates, they can be used to store arbitrary elements, such as integers or custom classes. One common property of all sequential containers is that the elements can be accessed sequentially. Like all other standard library components, they reside in namespace std.

The following containers are defined in the current revision of the C++ standard: `array`, `vector`, `list`, `forward_list`, `deque`. Each of these containers implements different algorithms for data storage, which means that they have different speed guarantees for different operations:[1]

• `array` implements a compile-time non-resizable array.
• `vector` implements an array with fast random access and an ability to automatically resize when appending elements.
• `deque` implements a double-ended queue with comparatively fast random access.
• `list` implements a doubly linked list.
• `forward_list` implements a singly linked list.

Since each of the containers needs to be able to copy its elements in order to function properly, the type of the elements must fulfill `CopyConstructible` and `Assignable` requirements.[2] For a given container, all elements must belong to the same type. For instance, one cannot store data in the form of both char and int within the same container instance.

## History

Originally, only `vector`, `list` and `deque` were defined. Until the standardization of the C++ language in 1998, they were part of the Standard Template Library (STL), published by SGI.

The `array` container at first appeared in several books under various names. Later it was incorporated into a Boost library, and was proposed for inclusion in the standard C++ library. The motivation for inclusion of `array` was that it solves two problems of the C-style array: the lack of an STL-like interface, and an inability to be copied like any other object. It firstly appeared in C++ TR1 and later was incorporated into C++11.

The `forward_list` container was added to C++11 as a space-efficient alternative to `list` when reverse iteration is not needed.

## Properties

`array`, `vector` and `deque` all support fast random access to the elements. `list` supports bidirectional iteration, whereas `forward_list` supports only unidirectional iteration.

`array` does not support element insertion or removal. `vector` supports fast element insertion or removal at the end. Any insertion or removal of an element not at the end of the vector needs elements between the insertion position and the end of the vector to be copied. The iterators to the affected elements are thus invalidated. In fact, any insertion can potentially invalidate all iterators. Also, if the allocated storage in the `vector` is too small to insert elements, a new array is allocated, all elements are copied or moved to the new array, and the old array is freed. `deque`, `list` and `forward_list` all support fast insertion or removal of elements anywhere in the container. `list` and `forward_list` preserves validity of iterators on such operation, whereas `deque` invalidates all of them.

### Vector

The elements of a `vector` are stored contiguously.[3] Like all dynamic array implementations, vectors have low memory usage and good locality of reference and data cache utilization. Unlike other STL containers, such as deques and lists, vectors allow the user to denote an initial capacity for the container.

Vectors allow random access; that is, an element of a vector may be referenced in the same manner as elements of arrays (by array indices). Linked-lists and sets, on the other hand, do not support random access or pointer arithmetic.

The vector data structure is able to quickly and easily allocate the necessary memory needed for specific data storage, and it is able to do so in amortized constant time. This is particularly useful for storing data in lists whose length may not be known prior to setting up the list but where removal (other than, perhaps, at the end) is rare. Erasing elements from a vector or even clearing the vector entirely does not necessarily free any of the memory associated with that element.

#### Capacity and reallocation

A typical vector implementation consists, internally, of a pointer to a dynamically allocated array,[1] and possibly data members holding the capacity and size of the vector. The size of the vector refers to the actual number of elements, while the capacity refers to the size of the internal array.

When new elements are inserted, if the new size of the vector becomes larger than its capacity, reallocation occurs.[1][4] This typically causes the vector to allocate a new region of storage, move the previously held elements to the new region of storage, and free the old region.

Because the addresses of the elements change during this process, any references or iterators to elements in the vector become invalidated.[5] Using an invalidated reference causes undefined behaviour.

The reserve() operation may be used to prevent unnecessary reallocations. After a call to reserve(n), the vector's capacity is guaranteed to be at least n.[6]

The vector maintains a certain order of its elements, so that when a new element is inserted at the beginning or in the middle of the vector, subsequent elements are moved backwards in terms of their assignment operator or copy constructor. Consequently, references and iterators to elements after the insertion point become invalidated.[7]

C++ vectors do not support in-place reallocation of memory, by design; i.e., upon reallocation of a vector, the memory it held will always be copied to a new block of memory using its elements' copy constructor, and then released. This is inefficient for cases where the vector holds plain old data and additional contiguous space beyond the held block of memory is available for allocation.

#### Specialization for bool

The Standard Library defines a specialization of the `vector` template for `bool`. The description of this specialization indicates that the implementation should pack the elements so that every `bool` only uses one bit of memory.[8] This is widely considered a mistake.[9][10] `vector<bool>` does not meet the requirements for a C++ Standard Library container. For instance, a `container<T>::reference` must be a true lvalue of type `T`. This is not the case with `vector<bool>::reference`, which is a proxy class convertible to `bool`.[11] Similarly, the `vector<bool>::iterator` does not yield a `bool&` when dereferenced. There is a general consensus among the C++ Standard Committee and the Library Working Group that `vector<bool>` should be deprecated and subsequently removed from the standard library, while the functionality will be reintroduced under a different name.[12]

### List

The `list` data structure implements a doubly linked list. Data is stored non-contiguously in memory which allows the list data structure to avoid the reallocation of memory that can be necessary with vectors when new elements are inserted into the list.

The list data structure allocates and deallocates memory as needed; therefore, it does not allocate memory that it is not currently using. Memory is freed when an element is removed from the list.

Lists are efficient when inserting new elements in the list; this is an ${\displaystyle O(1)}$ operation. No shifting is required like with vectors.

Lists do not have random access ability like vectors (${\displaystyle O(1)}$ operation). Accessing a node in a list is an ${\displaystyle O(n)}$ operation that requires a list traversal to find the node that needs to be accessed.

With small data types (such as ints) the memory overhead is much more significant than that of a vector. Each node takes up `sizeof(type) + 2 * sizeof(type*)`. Pointers are typically one word (usually four bytes under 32-bit operating systems), which means that a list of four byte integers takes up approximately three times as much memory as a vector of integers.

### Forward list

The `forward_list` data structure implements a singly linked list.

### Deque

`deque` is a container class template that implements a double-ended queue. It provides similar computational complexity to `vector` for most operations, with the notable exception that it provides amortized constant-time insertion and removal from both ends of the element sequence. Unlike `vector`, `deque` uses discontiguous blocks of memory, and provides no means to control the capacity of the container and the moment of reallocation of memory. Like `vector`, `deque` offers support for random access iterators, and insertion and removal of elements invalidates all iterators to the deque.

### Array

`array` implements a compile-time non-resizable array. The size is determined at compile-time by a template parameter. By design, the container does not support allocators. Unlike the other standard containers, `array` does not provide constant-time swap.

## Overview of functions

The containers are defined in headers named after the names of the containers, e.g. `vector` is defined in header `<vector>`. All containers satisfy the requirements of the Container concept, which means they have `begin()`, `end()`, `size()`, `max_size()`, `empty()`, and `swap()` methods.

### Member functions

Functions `array`
(C++11)
`vector`
`deque`
`list`
`forward_list`
(C++11)
Description
Basics (implicit) (constructor) (constructor) (constructor) (constructor) Constructs the container from variety of sources
(destructor) (destructor) (destructor) (destructor) Destructs the container and the contained elements
`operator=` `operator=` `operator=` `operator=` Assigns values to the container
N/A `assign` `assign` `assign` `assign` Assigns values to the container
Allocators `get_allocator` `get_allocator` `get_allocator` `get_allocator` Returns the allocator used to allocate memory for the elements
Element
access
`at` `at` `at` N/A N/A Accesses specified element with bounds checking.
`operator[]` `operator[]` `operator[]` Accesses specified element without bounds checking.
`front` `front` `front` `front` `front` Accesses the first element
`back` `back` `back` `back` N/A Accesses the last element
`data` `data` N/A N/A Accesses the underlying array
Iterators `begincbegin` `begincbegin` `begincbegin` `begincbegin` `begincbegin` Returns an iterator to the beginning of the container
`endcend` `endcend` `endcend` `endcend` `endcend` Returns an iterator to the end of the container
`rbegincrbegin` `rbegincrbegin` `rbegincrbegin` `rbegincrbegin` N/A Returns a reverse iterator to the reverse beginning of the container
`rendcrend` `rendcrend` `rendcrend` `rendcrend` Returns a reverse iterator to the reverse end of the container
Capacity `empty` `empty` `empty` `empty` `empty` Checks whether the container is empty
`size` `size` `size` `size` N/A Returns the number of elements in the container.
`max_size` `max_size` `max_size` `max_size` `max_size` Returns the maximum possible number of elements in the container.
N/A `reserve` N/A N/A N/A Reserves storage in the container
`capacity` Returns the number of elements that can be held in currently allocated storage
`shrink_to_fit` `shrink_to_fit` Reduces memory usage by freeing unused memory (C++11)
Modifiers `clear` `clear` `clear` `clear` Clears the contents
`insert` `insert` `insert` N/A Inserts elements
`emplace` `emplace` `emplace` Constructs elements in-place (C++11)
`erase` `erase` `erase` Erases elements
N/A `push_front` `push_front` `push_front` Inserts elements to the beginning
`emplace_front` `emplace_front` `emplace_front` Constructs elements in-place at the beginning (C++11)
`pop_front` `pop_front` `pop_front` Removes the first element
`push_back` `push_back` `push_back` N/A Inserts elements to the end
`emplace_back` `emplace_back` `emplace_back` Constructs elements in-place at the end (C++11)
`pop_back` `pop_back` `pop_back` Removes the last element
N/A N/A N/A `insert_after` Inserts elements after specified position (C++11)
`emplace_after` Constructs elements in-place after specified position (C++11)
`erase_after` Erases elements in-place after specified position (C++11)
`resize` `resize` `resize` `resize` Changes the number of stored elements
`swap` `swap` `swap` `swap` `swap` Swaps the contents with another container of the same type
`fill` N/A N/A N/A N/A Fills the array with the given value

There are other operations that are available as a part of the list class and there are algorithms that are part of the C++ STL (Algorithm (C++)) that can be used with the `list` and `forward_list` class:

#### Operations

• `list::merge` and `forward_list::merge` - Merges two sorted lists
• `list::splice` and `forward_list::splice_after` - Moves elements from another list
• `list::remove` and `forward_list::remove` - Removes elements equal to the given value
• `list::remove_if` and `forward_list::remove_if` - Removes elements satisfying specific criteria
• `list::reverse` and `forward_list::reverse` - Reverses the order of the elements
• `list::unique` and `forward_list::unique` - Removes consecutive duplicate elements
• `list::sort` and `forward_list::sort` - Sorts the elements

## Usage example

The following example demonstrates various techniques involving a vector and C++ Standard Library algorithms, notably shuffling, sorting, finding the largest element, and erasing from a vector using the erase-remove idiom.

```#include <iostream>
#include <vector>
#include <array>
#include <algorithm> // sort, max_element, random_shuffle, remove_if, lower_bound
#include <functional> // greater
#include <iterator> //begin, end, cbegin, cend, distance

// used here for convenience, use judiciously in real programs.
using namespace std;
using namespace std::placeholders;

auto main(int, char**)
-> int
{
array<int,4> arr{ 1, 2, 3, 4 };

// initialize a vector from an array
vector<int> numbers( cbegin(arr), cend(arr) );

// insert more numbers into the vector
numbers.push_back(5);
numbers.push_back(6);
numbers.push_back(7);
numbers.push_back(8);
// the vector currently holds { 1, 2, 3, 4, 5, 6, 7, 8 }

// randomly shuffle the elements
random_shuffle( begin(numbers), end(numbers) );

// locate the largest element, O(n)
auto largest = max_element( cbegin(numbers), cend(numbers) );

cout << "The largest number is " << *largest << "\n";
cout << "It is located at index " << distance(largest, cbegin(numbers)) << "\n";

// sort the elements
sort( begin(numbers), end(numbers) );

// find the position of the number 5 in the vector
auto five = lower_bound( cbegin(numbers), cend(numbers), 5 );

cout << "The number 5 is located at index " << distance(five, cbegin(numbers)) << "\n";

// erase all the elements greater than 4
numbers.erase( remove_if(begin(numbers), end(numbers),
bind(greater<>{}, _1, 4) ), end(numbers) );

// print all the remaining numbers
for(const auto& element : numbers)
cout << element << " ";

return 0;
}
```

The output will be the following:

```The largest number is 8
It is located at index 6 (implementation-dependent)
The number 5 is located at index 4
1 2 3 4
```

## References

• William Ford, William Topp. Data Structures with C++ and STL, Second Edition. Prentice Hall, 2002. ISBN 0-13-085850-1. Chapter 4: The Vector Class, pp. 195–203.
• Josuttis, Nicolai M. (1999). The C++ Standard Library. Addison-Wesley. ISBN 0-201-37926-0.

## Notes

1. ^ a b c Josuttis, Nicolai (1999). C++ Standard Library - A Tutorial and Reference. Addison-Wesley.
2. ^ ISO/IEC (2003). ISO/IEC 14882:2003(E): Programming Languages - C++ §23.1 Container requirements [lib.container.requirements] para. 4
3. ^ ISO/IEC (2003). ISO/IEC 14882:2003(E): Programming Languages - C++ §23.2.4 Class template vector [lib.vector] para. 1
4. ^ ISO/IEC (2003). ISO/IEC 14882:2003(E): Programming Languages - C++ §23.2.4.3 vector modifiers [lib.vector.modifiers] para. 1
5. ^ ISO/IEC (2003). ISO/IEC 14882:2003(E): Programming Languages - C++ §23.2.4.2 vector capacity [lib.vector.capacity] para. 5
6. ^ ISO/IEC (2003). ISO/IEC 14882:2003(E): Programming Languages - C++ §23.2.4.2 vector capacity [lib.vector.capacity] para. 2
7. ^ ISO/IEC (2003). ISO/IEC 14882:2003(E): Programming Languages - C++ §23.2.4.3 vector modifiers [lib.vector.modifiers] para. 3
8. ^ ISO/IEC (2003). ISO/IEC 14882:2003(E): Programming Languages - C++ §23.2.5 Class vector<bool> [lib.vector.bool] para. 1
9. ^ "vector<bool>: More Problems, Better Solutions" (PDF). August 1999. Retrieved 28 November 2017.
10. ^ "A Specification to deprecate vector<bool>". March 2007. Retrieved 28 November 2017.
11. ^ ISO/IEC (2003). ISO/IEC 14882:2003(E): Programming Languages - C++ §23.2.5 Class vector<bool> [lib.vector.bool] para. 2
12. ^ "96. Vector<bool> is not a container". Retrieved 28 June 2018.