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).

Syntax

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

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

Parameters

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

Return value

It does not return anything.

Exceptions

This function never throws exception.

Time complexity

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

Example

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

Output of the above code is as follows −

A B C D 

Example

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;
}

Output

Following is the output of the above code −

8 6 4 2 0 

Example

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;
}

Output

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

Hello Hi