C++ Array::operator[] Function

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The C++ std::array::operator[] function provides a access to elements of a array by index. It allows both read and write access, similar to the traditional arrays. This operator takes an index as its argument and returns a reference to the element at that position.

Unlike at() method, operator[] does not perform bounds checking, so accessing a out of range element results in undefined behaviour.

Syntax

Following is the syntax for std::array::operator[]() function.

reference operator[] (size_type n);
const_reference operator[] (size_type n) const;

Parameters

• n − It indicates the position of an element in the array.

Return Value

It return a reference to the element at the specified location in the array.

Exceptions

This function never throws exception if value of n is valid array index, otherwise behaviour is undefined.

Time complexity

Constant i.e. O(1)

Example 1

Let's look at the following example, where we are going to access and modify the element.

#include <iostream>
#include <array>
int main() {
   std::array < int, 5 > a = {1,22,23,34,45};
   std::cout << "Element at given index : " << a[3] << std::endl;
   a[3] = 33;
   std::cout << "After modification : " << a[3] << std::endl;
   return 0;
}

Output

Output of the above code is as follows −

Element at given index : 34
After modification : 33

Example 2

Consider the following example, where we are going to use the operator[] in the loop.

#include <iostream>
#include <array>
int main() {
   std::array < int, 4 > a = {10,11,12,23};
   for (int x = 0; x < a.size(); ++x) {
      std::cout << "Element at index " << x << ": " << a[x] << std::endl;
   }
   return 0;
}

Output

Following is the output of the above code −

Element at index 0: 10
Element at index 1: 11
Element at index 2: 12
Element at index 3: 23

Example 3

In the following example, we are going to access the element that is out of range and observing the output.

#include <iostream>
#include <array>
int main() {
   std::array < int, 3 > x = {12,23,34};
   std::cout << "Element at given index: " << x[5] << std::endl;
   return 0;
}

Output

If we run the above code it will generate the following output −

Element at given index: -1233708736