Functors in C++

The functors are the function objects in C++. The functor allows an instance object of some class to be called as if it were an ordinary function. Let us consider a function that takes one argument. We can use this function as function object to do some task on a set of data.

The Functors are widely used in STL algorithms like transform(), sort(), etc.

Functor vs Regular Function: Need of Functor?

Imagine we have a function that takes only one argument, like this:

int increment(int x) {
    return x + 1;
}

Now, what if we want to add different numbers, not just 1. For example, add 5 instead of 1. Then we have to write a new function every time.

int addFive(int x) {
    return x + 5;
}
int addTen(int x) {
    return x + 10;
}

This will be tough to write and manage the functions. So, when we need to pass extra data (like "how much to add") into the function. Functors helps to resolve and work with this in a good way.

Let us go through an examples for better understanding with and without using Functors.

Using a Functor

Using a Functor, we can customize the value to add by storing it inside the object which will overcome the problem that we were facing in traditional functions (as discussed above).

Example

In this example, we define a functor class that adds a fixed number to any given value. In the main() function, we use the transform() function to add 5 to each element of the array by passing an object of the functor:

#include<bits/stdc++.h>
using namespace std;
// Define a class with operator()
class increment {
   int num;
public:
   increment(int n) : num(n) {}
   int operator() (int x) const {
      return x + num;
   }
};
int main() {
   int arr[] = {1, 2, 3, 4, 5};
   int n = sizeof(arr) / sizeof(arr[0]);
   int to_add = 5;
   // Pass functor to transform
   transform(arr, arr + n, arr, increment(to_add));
   for (int i = 0; i < n; i++)
      cout<<arr[i]<<" ";
}

Following is the output to the above program:

6 7 8 9 10