Functors in C++

1. Introduction to Functors in C++
2. Functors with Parameters in C++
3. Pre-defined Functors in C++

This trivial tutorial introduces functors and their use in the C++ programming language. First, this article will briefly introduce functors and then move toward its syntax and usability.

Introduction to Functors in C++

Functors are the class objects that can behave like functions. There are numerous functors available in the C++ Standard Template Library (STL), and you can make your own also.

The main advantage of functors is that although they are not functions, they can still save the state as they are class objects.

Functors are made by overloading the operator () in any class so that the object of that class can be called, as we call a simple function.

Let’s look at the example below to have a clear understanding.

class Hello {
 public:
  void operator()() { cout << "Hello from Functor" << endl; }
};

Now to use this, we will create an object of the Hello class and call it like this:

int main() {
  Hello hello;
  hello();
  return 0;
}

Output:

Hello from Functor

We can see from the output that the function call operator () is called when we use the object with the operator.

Functors with Parameters in C++

Functors can also be defined as having some parameters and return-type, just like functions. We can amend the previous functor to include the parameter in it.

class Hello {
 public:
  void operator()(string name) {
    cout << "Hello " << name << " from Functor" << endl;
  }
};

To call this, we need to pass the parameter as well.

int main() {
  Hello hello;
  hello("david");
  return 0;
}

Output:

Hello david from Functor

Pre-defined Functors in C++

Numerous functors are included in functional header files which are packaged in the C++ Standard Template Library. Some of them are discussed here.