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C++ Deque Push Front Function

The C++ std::deque::push_front() function is used to insert the element at the beginning of the deque. It efficiently adds the element to the front, moving the existing elements to insert the new element. Unlike vector, deque supports efficient insertion and deletion at both the ends.

This function has 2 polymorphic variants: with using the default version and the move version (you can find the syntaxes of all the variants below).

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

void push_front (const value_type& val);
or
void push_front (value_type&& val);

• val − It indicates the value of the element to be inserted into the deque.

It does not return anything.

This function never throws exception.

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 push_front() function.

#include <iostream>
#include <deque>
int main()
{
    std::deque<char> a = {'B', 'C', 'D'};
    a.push_front('A');
    for (auto& elem : a) {
        std::cout << elem << " ";
    }
    std::cout << std::endl;
    return 0;
}

Output of the above code is as follows −

A B C D 

Consider the following example, where we are going to insert the elements in the loop.

#include <iostream>
#include <deque>
int main()
{
    std::deque<int> a;
    for (int x = 0; x <= 4; ++x) {
        a.push_front(x * 2);
    }
    for (auto y = a.begin(); y != a.end(); ++y) {
        std::cout << *y << " ";
    }
    std::cout << std::endl;
    return 0;
}

Following is the output of the above code −

8 6 4 2 0 

Let's look at the following example, where we are going to use push_front() with strings.

#include <iostream>
#include <deque>
#include <string>
int main()
{
    std::deque<std::string> a;
    a.push_front("Hi");
    a.push_front("Hello");
    for (const auto& str : a) {
        std::cout << str << " ";
    }
    std::cout << std::endl;
    return 0;
}

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

Hello Hi

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