Functors in C++

Please note that the title is Functors (Not Functions)!!

Consider a function that takes only one argument. However, while calling this function we have a lot more information that we would like to pass to this function, but we cannot as it accepts only one parameter. What can be done?

One obvious answer might be global variables. However, good coding practices do not advocate the use of global variables and say they must be used only when there is no other alternative.

Functors are objects that can be treated as though they are a function or function pointer. Functors are most commonly used along with STLs in a scenario like following:

Below program uses transform() in STL to add 1 to all elements of arr[]. 

// A C++ program uses transform() in STL to add 
// 1 to all elements of arr[]
#include <bits/stdc++.h>
using namespace std;
 
int increment(int x) {  return (x+1); }
 
int main()
{
    int arr[] = {1, 2, 3, 4, 5};
    int n = sizeof(arr)/sizeof(arr[0]);
 
    // Apply increment to all elements of
    // arr[] and store the modified elements
    // back in arr[]
    transform(arr, arr+n, arr, increment);
 
    for (int i=0; i<n; i++)
        cout << arr[i] <<" ";
 
    return 0;
}

Output:

2 3 4 5 6

Time Complexity: O(n)

Space Complexity: O(n) where n is the size of the array.

This code snippet adds only one value to the contents of the arr[]. Now suppose, that we want to add 5 to contents of arr[].

See what’s happening? As transform requires a unary function(a function taking only one argument) for an array, we cannot pass a number to increment(). And this would, in effect, make us write several different functions to add each number. What a mess. This is where functors come into use.

A functor (or function object) is a C++ class that acts like a function. Functors are called using the same old function call syntax. To create a functor, we create a object that overloads the operator().

The line,
MyFunctor(10);
Is same as
MyFunctor.operator()(10);

Let’s delve deeper and understand how this can actually be used in conjunction with STLs. 

// C++ program to demonstrate working of
// functors.
#include <bits/stdc++.h>
using namespace std;
// A Functor
class increment
{
private:
    int num;
public:
    increment(int n) : num(n) {  }
    // This operator overloading enables calling
    // operator function () on objects of increment
    int operator () (int arr_num) const {
        return num + arr_num;
    }
};
// Driver code
int main()
{
    int arr[] = {1, 2, 3, 4, 5};
    int n = sizeof(arr)/sizeof(arr[0]);
    int to_add = 5;
    transform(arr, arr+n, arr, increment(to_add));
    for (int i=0; i<n; i++)
        cout << arr[i] << " ";
}

Output:

6 7 8 9 10

Time Complexity: O(n)

Space Complexity: O(n) where n is the size of the array.

Thus, here, Increment is a functor, a c++ class that acts as a function.

The line,
transform(arr, arr+n, arr, increment(to_add));
is the same as writing below two lines,
// Creating object of increment
increment obj(to_add); 
// Calling () on object
transform(arr, arr+n, arr, obj); 

Thus, an object a is created that overloads the operator(). Hence, functors can be used effectively in conjunction with C++ STLs.