Difference between revisions of "cpp/container/array"

Revision as of 18:21, 1 September 2022

std::array is a container that encapsulates fixed size arrays.

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.

std::array satisfies the requirements of Container and 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) and partially satisfies the requirements of SequenceContainer.

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.

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

Iterator invalidation

As a rule, iterators to an array are never invalidated throughout the lifetime of the array. One should take note, however, that during swap, the iterator will continue to point to the same array element, and will thus change its value.

Member types

Member functions

Non-member functions

Helper classes

Deduction guides(since C++17)

Example

#include <string>
#include <iterator>
#include <iostream>
#include <algorithm>
#include <array>
 
int main()
{
    // construction uses aggregate initialization
    std::array<int, 3> a1{ {1, 2, 3} }; // double-braces required in C++11 prior to
                                        // the CWG 1270 revision (not needed in C++11
                                        // after the revision and in C++14 and beyond)
 
    std::array<int, 3> a2 = {1, 2, 3};  // double braces never required after =
 
    std::array<std::string, 2> a3 = { std::string("a"), "b" };
 
    // container operations are supported
    std::sort(a1.begin(), a1.end());
    std::reverse_copy(a2.begin(), a2.end(), 
                      std::ostream_iterator<int>(std::cout, " "));
 
    std::cout << '\n';
 
    // ranged for loop is supported
    for(const auto& s : a3)
        std::cout << s << ' ';
    std::cout << '\n';
 
    // Empty initialization
    std::array<int, 3> z1 = {};         // = {0, 0, 0}
    std::array<double, 3> z2 = {1.1};   // = {1.1, 0.0, 0.0}
 
    for(auto n : z1)
        std::cout << n << ' ';
    std::cout << '\n';
    for(auto n : z2)
        std::cout << n << ' ';
 
    // deduction guide for array creation (since C++17)
    [[maybe_unused]] std::array a4{3.0, 1.0, 4.0};  // -> std::array<double, 3>
}

3 2 1 
a b 
0 0 0 
1.1 0 0

See also