C++ std::list::push_front

The std::list::push_front function adds a new element to the beginning of a std::list. The new element is copied or moved into the list. This operation is efficient because it does not require reallocation of elements like in some other containers (e.g., std::vector).

Syntax of std::list::push_front

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

Parameters

Parameter	Description
`val`	The value of the element to be added to the front of the list. It can be passed as a constant reference or an rvalue.

Return Value

This function does not return a value. It modifies the list by adding a new element at the beginning.

Exceptions

The std::list::push_front function may throw exceptions if memory allocation fails or if the copy/move constructor of the element being added throws an exception.

The std::list::push_front function ensures strong exception safety. If an exception is thrown (e.g., due to memory allocation failure or the copy/move constructor of the element throwing an exception), the state of the list remains unchanged.

Examples for std::list::push_front

Example 1: Adding a Single Element to the Front

This example demonstrates how to use push_front to add an element to the front of a list:

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#include <iostream>
#include <list>

int main() {
    std::list<int> myList = {20, 30, 40};

    myList.push_front(10); // Add 10 to the front of the list

    std::cout << "List contents: ";
    for (const auto& elem : myList) {
        std::cout << elem << " ";
    }
    std::cout << std::endl;

    return 0;
}

Explanation:

A std::list named myList is initialized with the elements {20, 30, 40}.
The function myList.emplace_front(10) is used to add the element 10 to the front of the list. This modifies the list by inserting 10 as the first element.
The program uses a range-based for loop to iterate over all elements of the list, and print them to output.

Output:

List contents: 10 20 30 40

Example 2: Adding Complex Objects

This example demonstrates how to use push_front to add a complex object to the front of a list:

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#include <iostream>
#include <list>
#include <string>

struct Person {
    std::string name;
    int age;

    Person(const std::string& name, int age) : name(name), age(age) {}
};

int main() {
    std::list<Person> people;

    people.push_front(Person("Arjun", 25)); // Add a Person object to the front

    for (const auto& person : people) {
        std::cout << person.name << " (" << person.age << " years old)" << std::endl;
    }

    return 0;
}

Explanation:

A Person struct is defined with two members:
- name: A string representing the person’s name.
- age: An integer representing the person’s age.
- constructor that initializes its members using parameters:
A std::list of Person objects named people is created and initially empty.
The function push_front is used to construct and add a Person object to the front of the list:
- people.push_front("Arjun", 25): Constructs a Person object with the name "Arjun" and age 25 directly in the list without creating a temporary object.
A range-based for loop is used to iterate over the list:

Output:

Arjun (25 years old)

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C++ std::list::push_front