C++ member function pointers in class and subclass

Question

I have one base class which holds a map for function pointers like this

typedef void (BaseClass::*event_t)();
class BaseClass {
    protected:
        std::map<std::string, event_t> events;
    public:
        // Example event
        void onFoo() {
            // can be added easily to the map
        }
};

Handling this works prefect, but now i want to make BaseClass an abstract base class to derive from like this:

 class SpecificClass : public BaseClass {
     public:
         void onBar() {
             // this is gonna be difficult!
         }
 };

Although i can access the map from SpecificClass i am not able to add onBar because the event_t type is only defined for the BaseClass ! Is there any possibility (maybe with templates?) which does not lead to define the event_t for each class i will use...

(It is not neccessary to use templates! Any good/suitable approach would be nice.)

More background information:

This whole thing is for a text based RPG. My base class could be called Location and the specifc one any location eg CivicCenter . Each Location object subscribes to my EventSystem which notifies all neccessary objects when i fire an event. Therefore i want to store in a map some pointers to private functions holding the actions with their "name" like onSetOnFire (xD) as the key.

Answer 1

This can't be done with your current map as it stands. Think about what would happen if you could put a child method into the map. Then you could pull a pointer-to-child-member (masquerading as base) out of the map, call it on a base class instance pointer, and then how would it call a derived class on a base class instance which obviously couldn't work.

Would a polymorphic approach work?

Answer 2

Yes; stop using member pointers.

The more correct way of doing what you want is to have an event type and an object pointer. So an event fires on a specific object. The event type would be a non-member function (or a static member). It would be passed the object pointer. And it would call some actual member function of that object.

Nowadays, the event type could be a std/boost::function . However, since the function parameters have to stay the same type for all events, this doesn't really fix your problem. You can't call SpecificClass::onBar from a BaseClass pointer unless you do a cast to a SpecificClass . And the event calling function would not know to do this. So you still can't put SpecificClass::onBar in the std/boost::function object; you still need some standalone function to do the cast for you.

This all just seems to be a terrible use of polymorphism. Why does SpecificClass need to derive from BaseClass at all? Can't they just be two unrelated classes?

Answer 3

After some thought and a redesign i was able to achieve what i wanted. Although i am stubborn and still using inheritance i have reimplemented the map. This is how it works now:

class Location {
    // ...

    protected:
        std::map<std::string, std::function<void(void)>> m_mEvents;
};

And now i can handle it like this:

class CivicCenter : public Location {
    public:
        CivicCenter() {
            // this is done by a macro which lookes better than this
            this->m_mEvents["onTriggerSomething"] =
                  std::bind(&CivicCenter::onTriggerSomething, this);
        }

        void onTriggerSomething() {
            // ...
        }

    // ...
};

With easy use of std::bind i am able to implement generic function pointers. When using parameters like in std::function<void(int, int)> remeber to use either boost's _1 and _2 or lambda expressions like me:

std::function<void(int,int)> f = [=](int a, int b) {
    this->anotherFunctionWithParams(a, b);
};

But this is just pointed out due to completeness of my solution.

Answer 4

You have to use static_cast :

event_t evt = static_cast<event_t>(&SpecificClass::onBar);

This is because it is slightly dangerous to cast to event_t , you could accidently apply it to a BaseClass instance.

How it works (for the skeptical):

class BaseClass {
public:
    typedef void (BaseClass::*callback_t)(); // callback method
    void doSomething(callback_t callback) {
        // some code
        this->*callback();
        // more code
    }
    void baseCallback(); // an example callback
};

class DerivedClass : public BaseClass {
public:
    void derivedCallback();
    void doWhatever() {
        // some code
        doSomething(&BaseClass::baseCallback);
        // more code
        doSomething(static_cast<callback_t>(&DerivedClass::derivedCallback));
        // et cetera
};

Here is what you should avoid, and why this is potentially dangerous:

void badCodeThatYouShouldNeverWrite()
{
    BaseClass x;
    // DO NOT DO THIS IT IS BAD
    x.doSomething(static_cast<callback_t>(&DerivedClass::derivedCallback));
}

The requirement for a static_cast makes it so you can't "accidentally" pass DerivedClass method pointers in. And if you think this is dangerous, just remember that it's a pointer, and pointers are always dangerous. Of course, there are ways you can do this that involve creating helper classes, but that requires a lot of extra code (possibly making a class for every function you want to pass as a callback). Or you could use closures in C++11, or something from Boost, but I realize that a lot of us do not have that option.