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Showing content from https://www.tutorialspoint.com/cpp_standard_library/cpp_deque_pop_front.htm below:

C++ Deque: Pop Front Method

The C++ std::deque::pop_front() function is used to remove the first element from the deque, reducing its size by one. It does not return the removed element. After calling this function, all the remaining elements are shifted one position towards the front.

When we try to invoke the pop_front() function on empty deque, it results in undefined behaviour.

Following is the syntax for std::deque::pop_front() function.

void pop_front();

It does not accept any parameter.

This function does not return anything.

Calling this function on empty deque causes undefined behavior.

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 pop_front() function.

#include <iostream>
#include <deque>
int main()
{
    std::deque<char> A = {'A', 'B', 'C', 'D'};
    A.pop_front();
    std::cout << "After pop_front(): ";
    for (char n : A) std::cout << n << " ";
    std::cout << std::endl;
    return 0;
}

Output of the above code is as follows −

After pop_front(): B C D 

Consider the following example, where we are going to handle the empty deque.

#include <iostream>
#include <deque>
int main()
{
    std::deque<int> a;
    if (!a.empty()) {
        a.pop_front();
    } else {
        std::cout << "Deque is empty, cannot perform pop_front()." << std::endl;
    }
    return 0;
}

Following is the output of the above code −

Deque is empty, cannot perform pop_front().

In the following example, we are going to perform the conditional removal of the elements.

#include <iostream>
#include <deque>
int main()
{
    std::deque<int> a = {01,12,23,34,45};
    while (!a.empty() && a.front() < 23) {
        a.pop_front();
    }
    for (int n : a) {
        std::cout << n << ' ';
    }
    return 0;
}

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

23 34 45 

deque.htm

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