C++ Lambda Expressions: Practical Examples

C++ lambda expressions provide a concise way to define anonymous functions. They are particularly useful for passing functions as arguments, especially in algorithms or when working with asynchronous tasks. Below are three diverse examples that illustrate the utility of lambda expressions in C++ programming.

Example 1: Sorting a Vector of Custom Objects

Context

When working with collections of objects, you may need to sort them based on a specific attribute. Lambda expressions simplify this process without requiring a separate function.

#include <iostream>
#include <vector>
#include <algorithm>

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

int main() {
    std::vector<Person> people = {{"Alice", 30}, {"Bob", 25}, {"Charlie", 35}};

    std::sort(people.begin(), people.end(), [](const Person &a, const Person &b) {
        return a.age < b.age;
    });

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

Notes

• This example sorts a vector of Person objects by age using a lambda function as the comparator.
• Lambda expressions can capture variables from the surrounding scope, but in this case, we don’t need to capture any variables.

Example 2: Filtering a List of Integers

Context

Suppose you want to filter out even numbers from a list of integers. Using a lambda expression with the std::remove_if function allows for efficient filtering.

#include <iostream>
#include <vector>
#include <algorithm>

int main() {
    std::vector<int> numbers = {1, 2, 3, 4, 5, 6, 7, 8, 9};

    numbers.erase(std::remove_if(numbers.begin(), numbers.end(), [](int n) {
        return n % 2 == 0;
    }), numbers.end());

    std::cout << "Odd numbers: ";
    for (const auto &num : numbers) {
        std::cout << num << " ";
    }
}

Notes

• The lambda function checks if a number is even and helps in removing it from the vector.
• std::remove_if rearranges the elements, and erase is used to eliminate the unwanted elements from the vector.

Example 3: Capturing by Reference to Modify Variables

Context

In scenarios where you want to modify a variable outside the lambda’s scope, capturing by reference is useful. This example demonstrates how to increase a counter based on a condition.

#include <iostream>
#include <vector>

int main() {
    std::vector<int> scores = {85, 92, 78, 90, 88};
    int count = 0;

    std::for_each(scores.begin(), scores.end(), [&count](int score) {
        if (score > 80) {
            count++;
        }
    });

    std::cout << "Number of scores above 80: " << count << std::endl;
}

Notes

• The lambda captures the count variable by reference (&count), allowing it to be modified directly.
• std::for_each applies the lambda to each element in the scores vector, incrementing count for each qualifying score.

These examples of C++ Lambda Expressions demonstrate how they can be utilized effectively for various programming tasks, enhancing code readability and maintainability.