What are Lambda Functions in C++

In C++ a lambda function is a compact syntax for defining anonymous functions, without the need to declare it explicitly as a traditional function.

These functions are especially useful for simple and quick operations. For example, when it comes to operations on collections, such as filtering, mapping, or reducing.

Syntax of lambda functions

The basic syntax of a lambda function in C++ is as follows:

[capture](parameters) -> return_type { body }

Basic example

Here is a basic example of a lambda function that adds two numbers:

#include <iostream>

int main() {
    auto add = [](int x, int y) -> int {
        return x + y;
    };

    std::cout << "Sum: " << add(5, 3) << std::endl; // Output: 8

    return 0;
}

Variable capture

Lambda functions can capture variables from the context in which they are defined. There are different capture modes:

#include <iostream>

int main() {
    int value = 10;

    auto lambda = [value]() {
        std::cout << "Value: " << value << std::endl;
    };

    lambda(); // Output: Value: 10

    return 0;
}

#include <iostream>

int main() {
    int value = 10;

    auto lambda = [&value]() {
        value++;
        std::cout << "Value: " << value << std::endl;
    };

    lambda(); // Output: Value: 11
    std::cout << "Value after lambda: " << value << std::endl; // Output: Value after lambda: 11

    return 0;
}

Use in higher-order functions

Lambda functions can be passed as arguments to other functions that accept functions or functional objects.

Example with std::function:

#include <iostream>
#include <functional>

void Execute(std::function<void()> func) {
    func();
}

int main() {
    auto message = []() {
        std::cout << "Hello from a lambda function" << std::endl;
    };

    Execute(message);

    return 0;
}

Lambdas capturing this

Lambdas can also capture this to access class members from within the lambda.

#include <iostream>

class MyClass {
private:
    int value;
public:
    MyClass(int v) : value(v) {}
    void printValue() {
        auto lambda = [this]() {
            std::cout << "Value: " << value << std::endl;
        };
        lambda();
    }
};

int main() {
    MyClass obj(42);
    obj.printValue(); // Prints 42
    return 0;
}

Generic Lambdas

Since C++14 lambdas can be generic, allowing them to work with different data types.

#include <iostream>

int main() {
    auto print = [](auto data) {
        std::cout << data << std::endl;
    };
    print(5);        // Prints an integer
    print(3.14);     // Prints a float
    print("Hello");   // Prints a string
    return 0;
}

Practical examples

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

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

    // Sort using a lambda function
    std::sort(numbers.begin(), numbers.end(), [](int a, int b) {
        return a < b;
    });

    for (int num : numbers) {
        std::cout << num << " ";
    }
    std::cout << std::endl;

    return 0;
}

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

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

    // Filter even numbers
    std::vector<int> evens;
    std::copy_if(numbers.begin(), numbers.end(), std::back_inserter(evens), [](int num) {
        return num % 2 == 0;
    });

    for (int num : evens) {
        std::cout << num << " ";
    }
    std::cout << std::endl;

    return 0;
}

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

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

    // Apply a lambda function to each element
    std::for_each(numbers.begin(), numbers.end(), [](int num) {
        std::cout << num << " ";
    });
    std::cout << std::endl;

    return 0;
}

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