std::front_inserter in C++

std::front_inserter() is a utility function in the C++ Standard Library that creates a special output iterator for inserting elements at the beginning of a container. It is defined in the <iterator> header file.

• Inserts elements at the front of a container using push_front().
• Allows STL algorithms to insert elements instead of overwriting existing ones.

#include <iostream>
#include <algorithm>
#include <deque>
#include <iterator>
using namespace std;

int main() {
    deque<int> v1 = {1, 2, 3};
    deque<int> v2 = {4, 5, 6};

    copy(v1.begin(), v1.end(),
         front_inserter(v2));

    for (int x : v2)
        cout << x << " ";
}

3 2 1 4 5 6 

Explanation: Since elements are inserted at the front, the copied elements appear in reverse order.

Syntax

std::front_inserter(Container& container);

Parameters: container - The container in which elements will be inserted at the beginning.
Return Value: Returns a std::front_insert_iterator object associated with the given container.

Working of std::front_inserter()

std::front_inserter() creates a special output iterator that inserts elements using the container's push_front() member function.

• Creates a front_insert_iterator for the specified container.
• Every assignment to the iterator internally calls push_front().
• Allows STL algorithms to insert elements at the beginning of a container.

Internally:

*iterator = value;

is equivalent to:

container.push_front(value);

For example:

auto it = front_inserter(dq);
*it = 10; // dq.push_front(10)
*it = 20; // dq.push_front(20)

After these operations 20 and 10 is stored in the container.

Example: Reversing a Container

Since every element is inserted at the front, std::front_inserter() naturally reverses the order of copied elements.

#include <iostream>
#include <algorithm>
#include <deque>
#include <iterator>
using namespace std;

int main() {
    deque<int> v1 = {1, 2, 3, 4, 5};
    deque<int> v2;

    copy(v1.begin(), v1.end(),
         front_inserter(v2));

    for (int x : v2)
        cout << x << " ";
}

5 4 3 2 1 

Containers Supported by std::front_inserter()

std::front_inserter() can only be used with containers that provide the push_front() member function. Supported Containers are:

• std::deque
• std::list

Unsupported Containers are:

• std::vector
• std::array
• std::set
• std::map

Advantages of Using std::front_inserter()

std::front_inserter() provides several benefits when working with STL algorithms and containers:

• Automatically inserts elements at the beginning of a container.
• Eliminates the need to manually call push_front().
• Allows algorithms such as std::copy() to work with empty containers.
• Can be used to efficiently create reversed sequences.

push_front() vs std::front_inserter()