Does a virtual keyword with operator()() make sense? (functors)

Question

Consider I have a hierarchy defined as below

class Strategy
{
public:
    virtual void Run();
};
class StrategyA : public Strategy
{
public:
    virtual void Run();
};
class StrategyB : public Strategy
{
public:
    virtual void Run();
};

I was wondering if I replace the Run() with operator() makes any sense and if there are any advantages from a design and efficiency perspective.

class Strategy
{
public:
    virtual void operator()();
};
class StrategyA : public Strategy
{
public:
    virtual void operator()();
};
class StrategyB : public Strategy
{
public:
    virtual void operator()();
};

Thanks

CV.

Answer 1

Yes. Its fully makes sense.

Any operator overload is a function, after all. It adds syntactic sugar to the language. Sometimes, they're necessary, but often, it's just syntactic sugar.

Note that you've to invoke it polymorphically (of course, if you want runtime-polymorphism), and there're two ways you can do that:

• using pointer of base type, and
• using reference of base type

Example (demo),

struct A
{
   virtual void operator()() { cout << "A" << endl; }
};
struct B : A
{
   virtual void operator()() { cout << "B" << endl; }
};

int main() {
        B b;

        //using pointer
        A *ptr = &b;
        (*ptr)(); //clumsy!  - prints B

        //using reference
        A &ref = b;
        ref();   //better    - prints B

        //also correct
        b();  //prints B
        return 0;
}

And if you've a function template written as:

template<typename Functor>
void call(Functor fun)
{
   (*fun)();
}

Then you can use this function, for functors and regular functions, both:

void regular_function()
{
   cout << "regular_function" << endl;
}

B b;
call(&b);  //prints B
call(regular_function); //prints regular_function

Demo : http://ideone.com/B9w16