Question

In the Boost Signals library, they are overloading the () operator.

Is this a convention in C++? For callbacks, etc.?

I have seen this in code of a co-worker (who happens to be a big Boost fan). Of all the Boost goodness out there, this has only led to confusion for me.

Any insight as to the reason for this overload?

Answer 1

One of the primary goal when overloading operator() is to create a functor. A functor acts just like a function, but it has the advantages that it is stateful, meaning it can keep data reflecting its state between calls.

Here is a simple functor example :

struct Accumulator {     int counter = 0;     int operator()(int i) { return counter += i; } } ... Accumulator acc; cout << acc(10) << endl; //prints '10' cout << acc(20) << endl; //prints '30' 

Functors are heavily used with generic programming. Many STL algorithms are written in a very general way, so that you can plug-in your own function/functor into the algorithm. For example, the algorithm std::for_each allows you to apply an operation on each element of a range. It could be implemented something like that :

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

You see that this algorithm is very generic since it is parametrized by a function. By using the operator(), this function lets you use either a functor or a function pointer. Here’s an example showing both possibilities :

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

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Concerning your question about operator() overloading, well yes it is possible. You can perfectly write a functor that has several parentheses operator, as long as you respect the basic rules of method overloading (e.g. overloading only on the return type is not possible).