Example of 'functors'

What are 'functors?'

Depending on what you prefer to read, there are many definitions and explanations of Functors.

From the function pointer tutorial:

Functors are functions with a state. In C++ you can realize them as a class with one or more private members to store the state and with an overloaded operator () to execute the function. Functors can encapsulate C and C++ function pointers employing the concepts templates and polymorphism. You can build up a list of pointers to member functions of arbitrary classes and call them all through the same interface without bothering about their class or the need of a pointer to an instance. All the functions just have got to have the same return-type and calling parameters. Sometimes functors are also known as closures. You can also use functors to implement callbacks.

From StackOverflow:

• A functor is pretty much just a class which defines the operator(). That makes it "look like" a function.
• Another advantage to a functor over a pointer to a function is that the call can be inlined in more cases.
• You can use boost::function, to create functors from functions and methods
  

Here is a simple example of functors that I have created using mishmash from various places:

//Program tested on Microsoft Visual Studio 2008 - Zahid Ghadialy
//Program to demonstrate the use of functors

#include<iostream>

using namespace std;

class someClass
{
public:
  someClass(int x) : someVariable(x) {}
  int operator()(int y) {return (someVariable + y);}
  int internalStateValue(){return someVariable;}

private:
  int someVariable;
};

int main()
{
  someClass class1(50);
  someClass class2(75);

  cout<<"Class1 state variable value is : "<<class1.internalStateValue()<<endl;
  cout<<"Class2 state variable value is : "<<class2.internalStateValue()<<endl;

  int test1 = class1(22);
  cout<<"Test 1 value is : "<< test1<<endl;
  int test2 = class2(22);
  cout<<"Test 2 value is : "<< test2<<endl;

  cout<<"Class1 final state variable value is : "<<class1.internalStateValue()<<endl;
  cout<<"Class2 final state variable value is : "<<class2.internalStateValue()<<endl;

  return 0;
}

The output is as follows: