In C++, functors are objects that can be invoked as if they were functions. They are often used in combination with generic algorithms to provide custom behavior. However, to be used with generic algorithms, functors need to adhere to a specific interface.

The Function Call Operator

To make a functor compatible with generic algorithms, it must implement the function call operator operator() as a member function. The function call operator allows the object to be called like a function. Here’s an example of a simple functor that adds two numbers:

class AddFunctor {
public:
    int operator()(int a, int b) const {
        return a + b;
    }
};

Now, an instance of the AddFunctor can be called with parentheses like a function:

AddFunctor add;
int result = add(5, 3); // result is 8

Adhering to the Functor Interface

To be compatible with generic algorithms, a functor needs to adhere to an interface known as the “Functor Concept”. This concept requires the functor to have the following properties:

• The function call operator must be const so that it can be called on a const object.
• The function call operator must be defined as a member function, not a free-standing function.
• The argument types and return type of the function call operator must match the requirements of the algorithm being used.

class MyFunctor {
public:
    void operator()(int value) const {
        // ...
    }
};

Using Functors with Generic Algorithms

With a functor that adheres to the Functor Concept, you can now use it with generic algorithms that expect a callable object. For example, you can use the standard for_each algorithm to apply the functor to each element in a container:

std::vector<int> numbers = {1, 2, 3, 4, 5};

MyFunctor myFunctor;

std::for_each(numbers.begin(), numbers.end(), myFunctor);

By adhering to the Functor Concept, your functors can be used interchangeably with other functors and functions in generic algorithms, providing a flexible and reusable way to customize behavior.

#C++ #functors #genericalgorithms