C++ std::list::emplace_front

The std::list::emplace_front function inserts a new element at the beginning of the list. This function constructs the element in place, which avoids unnecessary copies or moves. It is a more efficient alternative to push_front when constructing elements directly.

Syntax of std::list::emplace_front

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template <class... Args>
void emplace_front(Args&&... args);

Parameters

Parameter	Description
`args...`	Arguments to forward to the constructor of the element being emplaced.

Return Value

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

Exceptions

The function may throw exceptions if memory allocation fails or if the constructor of the element being emplaced throws an exception.

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

Examples for std::list::emplace_front

Example 1: Adding a Single Element to the Front

This example demonstrates how to use emplace_front to add a single 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.emplace_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.
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: Emplacing Complex Objects

This example demonstrates how to use emplace_front to construct and 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.emplace_front("Arjun", 25); // Emplace a Person object at 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 emplace_front is used to construct and add a Person object to the front of the list:
- people.emplace_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::emplace_front