Why Overload the `operator()` in C ? Functors and the Power of Generic Programming

Exploring Operator() Overloading in C : Functors and Generic Programming

Overloading the () operator in C , as seen in the Boost Signals library, raises questions about its purpose and conventionality. Let's delve into the reasons behind this practice.

Functors: Bridging the Function and Object Worlds

Operator() overloading plays a crucial role in creating functors, a unique construct that combines function-like behavior with statefulness. Functors maintain their internal state between calls, allowing them to remember past operations.

Examples of Functors

Consider the following Accumulator functor:

struct Accumulator
{
    int counter = 0;
    int operator()(int i) { return counter += i; }
};

Generic Programming: Plugging In Your Logic

Functors find wide application in generic programming, where algorithms operate on a range of elements using a user-supplied function or functor. This flexibility is demonstrated in the std::for_each algorithm:

template <typename InputIterator, typename Functor>
void for_each(InputIterator first, InputIterator last, Functor f)
{
    while (first != last) f(*first++);
}

Operator()'s Convenience

The operator() overload allows both functors and function pointers to plug into generic algorithms. For instance, consider this example:

void print(int i) { std::cout << i << std::endl; }
...
std::vector<int> vec;
// Fill vec

// Using a functor
Accumulator acc;
std::for_each(vec.begin(), vec.end(), acc); // acc.counter contains the vector sum

// Using a function pointer
std::for_each(vec.begin(), vec.end(), print); // prints all elements

Multiple () Operators: A Possibility

Contrary to popular belief, it is indeed possible to overload operator() multiple times in a single class. Adhering to method overloading rules, the return types must differ.

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