Pass by value to virtual methods

Question

I have a question on pass-by-value object construction and virtual methods.

I have a virtual method like this:

typedef boost::function1<void, void*> Task

class ITaskPool
{
    //......

    virtual AddTask(Task task) = 0;
};

And then an implementation like

class TaskPool : public ITaskPool
{
    //......

    AddTask(Task task);
};

If I use it like this;

void MyFunc(void* arg)
{

}

int main()
{
    TaskPool tp;
    tp.AddTask(&MyFunc); 
}

Will a Task object be created twice, once for when it is passed to the virtual method, and another when it is passed to the method of the derived class?

Thanks

Answer 1

Just one copy would be created. When you declare a function virtual, method of that derived class is called through dynamic binding. Its not the case that first method A is called and then method B is called. COmpiler decides at run time which method to call.

Polymorphism and Dynamic Binding

Answer 2

Your code is wrong (missing return types), but no, only one copy is involved. I think you got the wrong idea about polymorphism. It's not like the base class method is called first, and then gets forwarded to the derived class. TaskPool::AddTask is directly called through dynamic dispatch (if it's called polymorphically).