Functors

When do we use functors?

Suppose there is an array, and you want to add some value to its elements. Let’s say, in one instance, you want to add 1 to all the elements. And after this, in another instance, you want to add 4 to all the elements. How can you do it using the transform() function in STL? 

Well, it’s very easy to increment with one fixed value. Let’s say the value is 1. Then, we can do it as:

#include <bits/stdc++.h>
using namespace std;
   
int increment(int x) {  
return (x+1); 
}
   
int main()
{
    /* declare and initialise the array */
    int a[] = {6,7,8,9};
    int n = sizeof(a)/sizeof(a[0]);
    
/* using the transform() function to increment */
    transform(a, a+n, a, increment);
   /* Print the array */
    for (int i=0; i<n; i++)
        cout << a[i] <<" ";
   
    return 0;
}

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But, now, how will we increment each element by 4?

Using the above increment function, we can’t. Also, we can’t add another parameter to the increment function, as the transform function doesn’t allow any other parameter to be passed. 

Therefore, here is when we will need functors. We can do this easily using functors. Let’s see how.

#include <bits/stdc++.h>
using namespace std;
  
/* Declaring a functor */
class increment
{
private:
    int number;
public:
    increment(int n) : number(n) { }
  
    /* Operator overloading */
    int operator () (int arr_num) const {
        return num + arr_num;
    }
};
  
 
int main()
{
    /* declaring and initialising an array */
    int a[] = {1, 2, 3, 4, 5};
    int n = sizeof(a)/sizeof(a[0]);
    int amount_of_increment= 5;
  
    /* incrementing using transform function. Also, sending the amount of increment as a parameter easily. */
    transform(a, a+n, a, increment(amount_of_increment));
  
    for (int i=0; i<n; i++)
        cout << a[i] << " ";
}

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Try and compile with online c++ compiler.

Also See - Difference between argument and parameter

FAQs

1. What is a function?
   A function is a set of all statements put together to perform a selected task. It could be statements doing some repeated duties or statements doing a little specialty task like printing etc.
    
2. What is a class?
   A class is a user-defined data type that acts as a blueprint from which objects are created. A class has data members, which are data variables, and member functions which are functions used to manipulate these variables.
    
3. What is an object?
   An object is defined as an entity that has a state and behavior. It is an instance of the Class. When a class is defined, initially, no memory is allocated to it, but when an object is created of that class, memory is allocated for it.
    
4. What is operator overloading?
   Operator overloading provides us a flexible option for the creation of new definitions for most C++ operators. In C++, we can make an operator work for user-defined classes. In simple words, C++ has the ability to provide the operators with special meaning for a data type. This mechanism of giving such special meaning to an operator is known as operator overloading. Operator overloading is a compile-time polymorphism.
    
5. When are functors useful?
   Functors are used in the case when we need to use an in-built STL function, but we’re restricted in terms of sending more than one parameter. Here, we can use functors and send the required parameters.

Key Takeaways

In this article, we learned about functors. We often used the concepts of functions and, objects & classes. To learn more about operator overloading, refer to this. We hope this article helped you a lot! 

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