C++ | std::array

Initializing an std::array

Initializing std::array<T, N>, where T is a scalar type and N is the number of elements of type T

If T is a scalar type, std::array can be initialized in the following ways:

// 1) Using aggregate-initialization
std::array<int, 3> a{ 0, 1, 2 };
// or equivalently
std::array<int, 3> a = { 0, 1, 2 };

// 2) Using the copy constructor
std::array<int, 3> a{ 0, 1, 2 };
std::array<int, 3> a2(a);
// or equivalently
std::array<int, 3> a2 = a;

// 3) Using the move constructor
std::array<int, 3> a = std::array<int, 3>{ 0, 1, 2 };

Initializing std::array<T, N>, where T is a non-scalar type and N is the number of elements of type T

If T is a non-scalar type std::array can be initialized in the following ways:

struct A { int values[3]; }; // An aggregate type

// 1) Using aggregate initialization with brace elision
// It works only if T is an aggregate type!
std::array<A, 2> a{ 0, 1, 2, 3, 4, 5 };
// or equivalently
std::array<A, 2> a = { 0, 1, 2, 3, 4, 5 };

// 2) Using aggregate initialization with brace initialization of sub-elements
std::array<A, 2> a{ A{ 0, 1, 2 }, A{ 3, 4, 5 } };
// or equivalently
std::array<A, 2> a = { A{ 0, 1, 2 }, A{ 3, 4, 5 } };

// 3)
std::array<A, 2> a{{ { 0, 1, 2 }, { 3, 4, 5 } }};
// or equivalently
std::array<A, 2> a = {{ { 0, 1, 2 }, { 3, 4, 5 } }};

// 4) Using the copy constructor
std::array<A, 2> a{ 1, 2, 3 };
std::array<A, 2> a2(a);
// or equivalently
std::array<A, 2> a2 = a;

// 5) Using the move constructor
std::array<A, 2> a = std::array<A, 2>{ 0, 1, 2, 3, 4, 5 };

Element access

1. at(pos)

Returns a reference to the element at position pos with bounds checking. If pos is not within the range of the container, an exception of type std::out_of_range is thrown.

The complexity is constant O(1).

#include <array>

int main()
{
    std::array<int, 3> arr;

    // write values
    arr.at(0) = 2;
    arr.at(1) = 4;
    arr.at(2) = 6;

    // read values
    int a = arr.at(0); // a is now 2
    int b = arr.at(1); // b is now 4
    int c = arr.at(2); // c is now 6

    return 0;
}

2) operator[pos]

Returns a reference to the element at position pos without bounds checking. If pos is not within the range of the container, a runtime segmentation violation error can occur. This method provides element access equivalent to classic arrays and thereof more efficient than at(pos).

The complexity is constant O(1).

#include <array>

int main()
{
    std::array<int, 3> arr;

    // write values
    arr[0] = 2;
    arr[1] = 4;
    arr[2] = 6;

    // read values
    int a = arr[0]; // a is now 2
    int b = arr[1]; // b is now 4
    int c = arr[2]; // c is now 6

    return 0;
}

3) std::get<pos>

This non-member function returns a reference to the element at compile-time constant position pos without bounds checking. If pos is not within the range of the container, a runtime segmentation violation error can occur.

The complexity is constant O(1).

#include <array>

int main()
{
    std::array<int, 3> arr;

    // write values
    std::get<0>(arr) = 2;
    std::get<1>(arr) = 4;
    std::get<2>(arr) = 6;

    // read values
    int a = std::get<0>(arr); // a is now 2
    int b = std::get<1>(arr); // b is now 4
    int c = std::get<2>(arr); // c is now 6

    return 0;
}

4) front()

Returns a reference to the first element in container. Calling front() on an empty container is undefined.

The complexity is constant O(1).

Note: For a container c, the expression c.front() is equivalent to *c.begin().

#include <array>

int main()
{
    std::array<int, 3> arr{ 2, 4, 6 };

    int a = arr.front(); // a is now 2

    return 0;
}

5) back()

Returns reference to the last element in the container. Calling back() on an empty container is undefined.

The complexity is constant O(1).

#include <array>

int main()
{
    std::array<int, 3> arr{ 2, 4, 6 };

    int a = arr.back(); // a is now 6

    return 0;
}

6) data()

Returns pointer to the underlying array serving as element storage. The pointer is such that range [data(); data() + size()) is always a valid range, even if the container is empty (data() is not dereferenceable in that case).

The complexity is constant O(1).

#include <iostream>
#include <cstring>
#include <array>

int main ()
{
    const char* cstr = "Test string";
    std::array<char, 12> arr;

    std::memcpy(arr.data(), cstr, 12); // copy cstr to arr

    std::cout << arr.data(); // outputs: Test string

    return 0;
}

Iterating through the Array

std::array being a STL container, can use range-based for loop similar to other containers like vector

int main() {
     std::array<int, 3> arr = { 1, 2, 3 };
     for (auto i : arr)
         cout << i << '\n';
}

Checking size of the Array

One of the main advantage of std::array as compared to C style array is that we can check the size of the array using size() member function

int main() {
    std::array<int, 3> arr = { 1, 2, 3 };
    cout << arr.size() << endl;
}

Changing all array elements at once

The member function fill() can be used on std::array for changing the values at once post initialization

int main() {

    std::array<int, 3> arr = { 1, 2, 3 };
    // change all elements of the array to 100
    arr.fill(100);

}

Parameters

|Parameter|Definition |---|---|---|---|---|---|---|---|---|--- |class T|Specifies the data type of array members |std::size_t N|Specifies the number of members in the array

Remarks

Use of a std::array requires the inclusion of the <array> header using #include <array>.