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C++ Deque Operator Subscript

The C++ std::deque::operator[] function allows to access or modify the element at a specific index, similar to how arrays and vetors work. The function takes a single argument, the index, and returns a reference to the element at that position.

Unlike at(), it does not perform bounds checking, so accessing an out-of-range index results in undefined behaviour.

Following is the syntax for std::deque::operator[] function.

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

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

It returns the element at the specified position in the container.

It throws undefined behaviour if the index is out of range.

The time complexity of this function is constant i.e. O(1)

In the following example, we are going to consider the basic usage of the operator[] function.

#include <iostream>
#include <deque>
int main()
{
    std::deque<char> a = {'A', 'B', 'C', 'D'};
    std::cout << "Element at given index : " << a[1] << std::endl;
    return 0;
}

Output of the above code is as follows −

Element at given index : B

Consider the following example, where we are going to modify the element.

#include <iostream>
#include <deque>
int main()
{
    std::deque<char> a = {'A', 'B', 'C', 'D'};
    a[2] = 'Z';
    for (int x = 0; x < a.size(); ++x) {
        std::cout << a[x] << " ";
    }
    std::cout << std::endl;
    return 0;
}

Following is the output of the above code −

A B Z D

Let's look at the following example, where we are going to access the element that is out of range and observing the output.

#include <iostream>
#include <deque>
int main()
{
    std::deque<int> a = {01,12,23,34};
    std::cout << "Element at given index : " << a[5] << std::endl;
    return 0;
}

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

Element at given index : 0

deque.htm

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