Functors are objects that can be used as if they were functions. They are commonly used in C++ to encapsulate state and behavior together. In this tutorial, we will learn how to create functors that maintain state in C++.

What is a Functor?

In C++, a functor is an object that can be treated as if it were a function. It overloads the operator() and can be invoked like a regular function. Functors can store state and modify it between invocations, making them useful for maintaining context or stateful behavior.

Creating a Functor

To create a functor, we will define a class and overload the operator() inside it. Let’s start with a simple example of a counter functor that increments a counter each time it’s called:

class Counter {
public:
    Counter() : count(0) {}

    void operator()() {
        count++;
        std::cout << "Count: " << count << std::endl;
    }

private:
    int count;
};

In the above code, we define a Counter class that has an integer member variable count. The default constructor initializes count to 0. The operator() function increments count and prints its value.

Using the Functor

Once we have defined the functor, we can use it just like a regular function. Here’s how we can use the Counter functor:

int main() {
    Counter counter;

    counter(); // Count: 1
    counter(); // Count: 2
    counter(); // Count: 3

    return 0;
}

In the main() function, we create an instance of the Counter functor called counter. We can invoke the counter object just like a function using the () operator. Each time we call it, the count variable is incremented and its value is printed.

Conclusion

In this tutorial, we learned how to create functors that maintain state in C++. Functors are powerful constructs that allow us to encapsulate state and behavior together, providing a convenient way to write reusable and stateful code. By overloading the operator() inside a class, we can define our own functors and utilize them in our programs.

#C++ #Functors