C++ Functors and Zero

Question

Hello all:

First a disclaimer, I am replacing a bunch of code which uses boost::function and boost::bind. However, I am moving to a codebase which does not allow rtti. I would like to keep using boost but don't know if there is a way around this restriction.

So..., I am trying to mimic some of its functionality, but much more simplified. I have a Callback class:

   template <class Class, typename ReturnType = void> class Callback0 {
   typedef ReturnType (Class::*Method)();

   public:
       Callback0(Class* object, Method method)
           : m_object(object)
           , m_method(method)
       {
          ;
       }

       Callback0(const Callback0& callback)
           : m_object(callback.m_object)
           , m_method(callback.m_method)
       {
          ;
       }

       operator bool() {
           return m_object != 0;
       }

       operator bool() const {
           return m_object != 0;
       }

       ReturnType operator()() {
           return (m_object->*m_method)();
       }

       Callback0<Class, ReturnType>& operator=(const Callback0<Class, ReturnType>& callback) {
           if(this != &callback) {
               m_object = callback.m_object;
               m_method = callback.m_method;
           }

           return *this;
       }

   private:
       Class*   m_object;
       Method   m_method;

};

This allows me to do simple callbacks w/zero parameters:

class Meh {
    public:
        Meh() {;}
        ~Meh() {;}

        void f0() {
            footprint6v("%s\n", __FUNCTION__);
        }
};


static void meh() {
    Meh* m = new Meh;

    Callback0<Meh, void> c0(m, &Meh::f0);

    c0();
}

I would like to be able to assign my callback objects to zero as default parameters like so:

class Wtf {
    public:
        Wtf() : m_callback(0) {;}
        ~Wtf() {;}

        void doSomething(const Callback0<Wtf, void>& callback = 0) {
            m_callback = callback;
        }

    private:
        Callback0<Wtf, void> m_callback;
};

This works when using boost::function as you can do:

class Wtf {
    public:
        Wtf() : m_callback(0) {;}
        ~Wtf() {;}

        void doSomething(const boost::function<void()>& callback = 0) {
            m_callback = callback;
        }

    private:
        boost::function<void()> m_callback;
};

I imagine boost is doing some magic here. I know I can just change the parameter to a pointer rather than a reference but as I said, I am replacing a lot of code and would like to minimize the impact of changing from boost.

Thanks for the help.

Answer 1

Boost isn't doing anything magic. 0 is just a NULL function pointer for the function pointer constructor.

I would suggest in your case you just provide a default constructor

Callback0() : m_object(NULL), m_method(NULL) {}

And make doSomething look like

void doSomething(const Callback0<Wtf, void>& callback = Callback0<Wtf, void>()) {