C++20 Lambdas with Template Parameters (Concept Lambdas)

 C++20 Lambdas with Template Parameters (Concept Lambdas)

C++20 introduced the concept of lambdas with template parameters, also known as concept lambdas. Concept lambdas allow you to define a lambda function that accepts template arguments and enforces constraints on those arguments using concepts. This provides a more flexible and expressive way to create generic lambda functions. Here's an example demonstrating the usage of concept lambdas:

#include <iostream>

#include <concepts>

template <typename T>

concept Addable = requires(T a, T b) {

    { a + b } -> std::convertible_to<T>;

};

int main() {

    auto add = [](Addable auto a, Addable auto b) {

        return a + b;

    };

    int result = add(3, 4);

    std::cout << "Result: " << result << std::endl;

    double result2 = add(2.5, 3.7);

    std::cout << "Result2: " << result2 << std::endl;

    // Error: The arguments are not addable

    // char result3 = add('a', 'b');

    return 0;

}

In the above code, we define a concept Addable that specifies the requirement for types to support the addition operation. It uses the requires clause to express the constraint.

We then define a lambda function add using the syntax [](Addable auto a, Addable auto b). This lambda function takes two arguments of any type that satisfies the Addable concept. The function performs the addition operation a + b and returns the result.

In the main() function, we call the add lambda function with different arguments. We first call it with two integers and print the result. Then we call it with two doubles and print the result. If we try to call the add function with arguments that are not addable (e.g., characters), it won't compile due to the constraints imposed by the Addable concept.

Concept lambdas provide a powerful way to define generic lambda functions with compile-time constraints, enabling more flexible and type-safe programming.