Functors

Functors are objects that behave like regular functions. The beauty is that functors can be substituted in the place of function pointers. Functors are handy objects that let you extend or complement the behavior of an STL function without compromising the object-oriented coding principles.

Functors are easy to implement; all you need to do is overload the function operator. Functors are also referred to as functionoids.

The following code will demonstrate the way a simple functor can be implemented:

#include <iostream>
#include <vector>
#include <iterator>
#include <algorithm>
using namespace std;

template <typename T>
class Printer {
public:
  void operator() ( const T& element ) {
    cout << element << "\t";
  }
};

int main () {
  vector<int> v = { 10, 20, 30, 40, 50 };

  cout << "\nPrint the vector entries using Functor" << endl;

  for_each ( v.begin(), v.end(), Printer<int>() );

  cout << endl;

  return 0;
}

Let's quickly compile the program using the following command:

g++ main.cpp -std=c++17
./a.out

Let's check the output of the program:

Print the vector entries using Functor
10  20  30  40  50

We hope you realize how easy and cool a functor is.