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, inplace_vector, list, forward_list, deque, and hive. 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.
• inplace_vector implements a resizable array with contiguous in-place storage.
• deque implements a double-ended queue with comparatively fast random access.
• list implements a doubly linked list.
• forward_list implements a singly linked list.
• hive implements an object pool.

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. Alexander Stepanov, the primary designer of the STL, bemoans the choice of the name vector, saying that it comes from the older programming languages Scheme and Lisp but is inconsistent with the mathematical meaning of the term.^[3]

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.

In C++26, two new sequence containers were added: inplace_vector and hive.

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.

The collections vector, deque, list and forward_list also have a template parameter Allocator which is default-specified as std::allocator<T> (where T is the stored type).

Array

std::array<T, N> implements a non-resizable array. The size is determined at compile-time by a template parameter. By design, the container does not support allocators because it is basically a C-style array wrapper. C++ does not support variable-length arrays.

It is essentially equivalent to core-language arrays such as T[] in Java or [T; N] in Rust. Traditional C arrays (T[]) do not store information such as length. array, unlike T[], must always specify its size at declaration, be inferred by the compiler. This is similar to another collection, std::bitset, which acts as a bit array whose size must be known at compile time.

using std::array;

array<int, 3> a = {1, 2, 3}; // OK
array<int> a = {1, 2, 3}; // not OK

// compare with C-style arrays:
int a[3] = {1, 2, 3}; // OK
int a[] = {1, 2, 3}; // OK

std::span is a close relative of array, for non-owning array views. std::mdspan is a multi-dimensional version of span, however these are view containers.

Vectors

Vector

The elements of a std::vector<T> are stored contiguously.^[4] 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.

It is essentially equivalent to java.util.ArrayList in Java or std::vec::Vec in Rust.

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][5] 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.^[6] 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.^[7]

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.^[8]

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.^[9] This is widely considered a mistake.^[10][11] 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.^[12] 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.^[13]

If looking to store a list of bool whose size is known at compile time, a std::bitset<N> (bit array) is the more reasonable collection to use, however bitset unlike vector is not dynamic.

In-place vector

std::inplace_vector<T, N> is a dynamically-resizable array with contiguous in-place storage.

Deque

std::deque<T> 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.

It is essentially equivalent to java.util.Deque in Java or std::collections::VecDeque in Rust.

Linked lists

Doubly linked list

The std::list<T> 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.

It is essentially equivalent to java.util.LinkedList in Java or std::collections::LinkedList in Rust.

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 (of type T takes up sizeof(T) + 2 * sizeof(T*). 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.

Singly linked list

The std::forward_list<T> data structure implements a singly linked list.

Few languages have a distinct singly linked list type like forward_list, however external libraries such as fwdlist for Rust exist.^[14]

Hive

std::hive<T> is a collection that reuses erased elements' memory. It can be seen as similar to an object pool. It uses another class, std::hive_limits to store layout information on block capacity limits.

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
	—	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	—	—	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	—	Accesses the last element
	data	data	—	—		Accesses the underlying array
Iterators	begin cbegin	begin cbegin	begin cbegin	begin cbegin	begin cbegin	Returns an iterator to the beginning of the container
	end cend	end cend	end cend	end cend	end cend	Returns an iterator to the end of the container
	rbegin crbegin	rbegin crbegin	rbegin crbegin	rbegin crbegin	—	Returns a reverse iterator to the reverse beginning of the container
	rend crend	rend crend	rend crend	rend crend		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	—	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.
	—	reserve	—	—	—	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	—	Inserts elements
		emplace	emplace	emplace		Constructs elements in-place (C++11)
		erase	erase	erase		Erases elements
		—	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	—	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
		—	—	—	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	—	—	—	—	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

Non-member functions

Usage example

The following example demonstrates various techniques involving a vector and C++ Standard Library algorithms (with C++20 std::ranges), notably shuffling, sorting, finding the largest element, and erasing from a vector using the erase-remove idiom.

import std;

using std::array;
using std::mt19937;
using std::random_device;
using std::vector;

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

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

    // 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_device rd; // Seed for the random number generator
    mt19937 g(rd()); // Mersenne Twister random number engine
    std::ranges::shuffle(numbers, g);

    // locate the largest element, O(n)
    int largest = std::ranges::max_element(numbers);
    int indexOfLargest = std::ranges::distance(numbers.cbegin(), largest);

    std::println("The largest number is {}, located at index {}", largest, indexOfLargest);

    // sort the elements
    std::ranges::sort(numbers);

    // find the position of the number 5 in the vector 
    int five = std::ranges::lower_bound(numbers, 5);
    int indexOfFive = std::ranges::distance(numbers.cbegin(), five);

    std::println("The number 5 is located at index {}", indexOfFive);

    // erase all the elements greater than 4   
    numbers.erase(
        std::ranges::remove_if(
            numbers,
            [](int n) constexpr -> bool { return n > 4; }
        ),
        numbers.end()
    );

    // print all the remaining numbers
    for (int element : numbers) {
        std::print("{}", element);
    }
}

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.