Overview

template <typename T, std::size_t N>
class array

std::array is a container that encapsulates fixed size arrays. It is a safer alternative to C-style arrays - T arr[N].

Example usage

The examples in this section are very simple ones. Navigate to examples section at the bottom for more.

Create

C++20

To create an array with an automatically deduced size or/and type, we can use std::to_array (since C++20).
Note: type and size deduction (at the same time) was possible since C++17.

#include <array>

int main() {
	// type: std::array<int, 4>
	auto ints		= std::to_array({1, 5, 3, 7});
	// type: std::array<float, 4>
	auto floats		= std::to_array<float>({1, 5, 3, 7});
	// type: std::array<char, 6>
	auto hello		= std::to_array("Hello");
	// this is different to std::array{"Hello"}, which creates std::array<char const*, 1>
}

C++17

Using CTAD

#include <array>

int main() {
	std::array numbers{1, 2, 4, 8};
	// Content: 1,2,4,8

	// ❌ Error, cannot deduce size only (look C++20 to_array)
	std::array<int> numbers{1, 2, 4, 8};
}

until C++17

Unfortunately no deduction.

#include <array>

int main() {
	std::array<int, 8> numbers = {1, 2, 4, 8};
	// Content: 1,2,4,8,0,0,0,0
}

String literal

Note: this is only recommended if you don't have C++20, otherwise use std::to_array.

#include <array>

int main() {
	std::array<char, 6> str = { "Hello" };
	// Content: 'H','e','l','l','o', <nul>
}

Fill and print

#include <iostream>
#include <array>

int main()
{
	std::array<int, 4> playerScores{}; // initialize with zeros

	for (int score : playerScores)
		std::cout << score << ", ";

	playerScores.fill(123);
	playerScores.back() = 30;

	std::cout << '\n';
	for (int score : playerScores)
		std::cout << score << ", ";
}

Sort

C++20

Note: ranges are available since C++20.

#include <array>
#include <algorithm>

int main() {
	std::array<int, 4> numbers = {13, 2, 7, 4};
	// Content: 13,2,7,4
	std::ranges::sort(numbers);
	// Content: 2,4,7,13
}

until C++20

#include <array>
#include <algorithm>

int main() {
	std::array<int, 4> numbers = {13, 2, 7, 4};
	// Content: 13,2,7,4
	std::sort(numbers.begin(), numbers.end());
	// Content: 2,4,7,13
}

Memory

The elements of an array are stored contiguously in memory.

Technical details

Technical definition of an array

This container is an aggregate type with the same semantics as a struct holding a C-style array T[N] as its only non-static data member. Unlike a C-style array, it doesn't decay to T* automatically. As an aggregate type, it can be initialized with aggregate-initialization given at most N initializers that are convertible to T:

std::array<int, 3> a = {1,2,3};

The struct combines the performance and accessibility of a C-style array with the benefits of a standard container, such as knowing its own size, supporting assignment, random access iterators, etc.

An array can also be used as a tuple of N elements of the same type.

Zero-length array (N == 0)

There is a special case for a zero-length array (N == 0). In that case, array.begin() == array.end(), which is some unique value. The effect of calling front() or back() on a zero-sized array is undefined.

Named requirements

std::array satisfies the requirements of:

• Container
• ReversibleContainer

except that default-constructed array is not empty and that the complexity of swapping is linear

Satisfies the requirements of:

• ContiguousContainer (since C++17)
• SequenceContainer (partially)

std::array

Defined in

array

Template parameters

pub	T	Type of the elements
pub	N	Number of elements (a compile-time constant)

Type names

pub	value_type	T
pub	size_type	std::size_t
pub	difference_type	std::ptrdiff_t
pub	reference	value_type&
pub	const_reference	value_type const&
pub	pointer	value_type*
pub	const_pointer	value_type const*
pub	iterator	LegacyRandomAccessIterator to value_type
pub	const_iterator	LegacyRandomAccessIterator to value_type const
pub	reverse_iterator	std::reverse_iterator<iterator>
pub	const_reverse_iterator	std::reverse_iterator<const_iterator>

Member functions

pub	(constructor)	Constructs an array following the rules of aggregate initialization.
pub	(destructor)	Destroys every element of the array.
pub	operator=	Overwrites every element of the array with the corresponding element of another array.

Element access

pub	at	Accesses the specified element with bounds checking.
pub	operator[]	Accesses the element.
pub	front	Returns the first element.
pub	back	Returns the last element.
pub	data	Returns a pointer to the first element of the underlying array.

Iterators

pub	begin cbegin	Returns an iterator/const_iterator to the beginning.
pub	end cend	Returns an iterator/const_iterator to the end.
pub	rbegin crbegin	Returns a reverse iterator/const_iterator to the beginning.
pub	rend crend	Returns a reverse iterator/const_iterator to the end.

Capacity

pub	empty	Returns true if the container is empty, otherwise false.
pub	size	Returns the number of elements.
pub	max_size	Returns the maximum possible number of elements.

Operations

pub	fill	Fills the container with specified value.
pub	swap	Swaps the contents.

Non-member functions

pub	operator== operator!= operator< operator> operator<= operator>= operator<=>	Lexicographically compares the values in the array.
pub	std::get (std::array)	Accesses an element of the array.
pub	std::swap (std::array)	Specializes the std::swap algorithm.
pub	std::to_array (C++20)	Creates a std::array object from a built-in array.

Helper classes

pub	std::tuple_size<std::array> (C++11)	Obtains the size of an array.
pub	std::tuple_element<std::array> (C++11)	Obtains the type of the elements of an array.

Deduction guides (since C++17)

Click to expand

Deduction guides

// (1)
template< class T, class... U >
array( T, U... ) -> array<T, 1 + sizeof...(U)>;

(1) allows deduction from a variadic parameter pack

It's purpose is to provide an equivalent of std::experimental::make_array.

The program is ill-formed if any U is not the same as T (std::is_same_v<T, U> && ...).

An empty parameter pack is allowed, the type of std::array is std::array<T, 1> then.

More examples

Using algorithms

Find minimal value (C++20)

#include <iostream>
#include <array>
#include <algorithm>

int main()
{
	namespace rg = std::ranges;

	auto numbers = std::to_array({ 12, 3, 18, 9});
	
	// Find minimal element
	auto minIt = rg::min_element(numbers);

	// obtain value:
	auto value = *minIt;
	// find position:
	auto index = std::distance(numbers.begin(), minIt);

	
	std::cout	<< "Min: "			<< value
				<< ", at index: "	<< index
				<< std::endl;
}

Result

Min: 3, at index: 1

Advanced

Using array as a temporary buffer

#include <fstream>
#include <array>
#include <string>

std::string readFile(std::istream& file)
{
	constexpr size_t BUFFER_SIZE	= 16 * 1024;		// 16 KB
	constexpr size_t RESERVE_SIZE	= 1 * 1024 * 1024;	// 1 MB
	std::string result;
	result.reserve(RESERVE_SIZE); // 1 MB

	std::array<char, BUFFER_SIZE> buf;

	while(file.read(buf.data(), buf.size()))
		result.append(buf.data(), buf.data() + file.gcount());

	result.append(buf.data(), buf.data() + file.gcount());
	return result;
}

// Example usage:
int main()
{
	std::ifstream file("hello.txt");
	if (file.is_open())
	{
		auto fileContents = readFile(file);
		// ...
	}
}

Concat two arrays (C++20)

#include <iostream>
#include <array>
#include <algorithm>

template <typename T, size_t N1, size_t N2>
auto concat(
		std::array<T, N1> const& lhs,
		std::array<T, N2> const& rhs
	)
{
	namespace rg = std::ranges;

	std::array<T, N1 + N2> result;

	rg::copy(lhs, result.begin());
	rg::copy_backward(rhs, result.end());

	return result;
}

int main()
{
	auto left	= std::to_array({1, 2, 3});
	auto right	= std::to_array({4, 5, 6});
	
	auto both	= concat(left, right);

	for (int elem : both)
		std::cout << elem << ' ';
}

Result

1 2 3 4 5 6