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Kanefa

Not able to implement classes to get Polymorphic Behavior

6 posts in this topic

I was reading Simple Event Handling article, and I tried to re-implement the Event struct as a class with derived classes for each EventType. The base class is simply the EventType and a getter.  I want each derived type to expand on this by adding a member variable of a unique type and a getter.  However, that doesn't make for a common interface, so I am not able to get the polymorphic behavior I want.

 

In the code sample SendEvent just calls the EventHandler for each listener and at this point I cannot access the derived class' getString() through the base class.  Right now I have to static_cast back to its derived type to use it.  Am I just missing something or is this a bad design on my part? 

class Event
{
public:
typedef unsigned long EventType;

explicit Event(EventType type);
virtual  ~Event();
EventType getType() const;

private:
EventType mType;
};

class EventStr : public Event
{
public:
typedef unsigned long EventType;

EventStr(EventType type, std::string str);
std::string getString() const;

private:
std::string mString;
};

void EventDispatcher::sendEvent(const Event& event);

main()
{
const Event& event = EventStr(0x496bf752, "derived data");
sendEvent(event);
} 
Edited by Kanefa
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The base class is missing getString(). It'll need that if you want all Event types to provide some kind of info string. But if you only want getString in EventStr, then casting seems to be fine.

Edited by ultramailman
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The base class is missing getString(). It'll need that if you want all Event types to provide some kind of info string. But if you only want getString in EventStr, then casting seems to be fine.

 

I did intentionally leave it out.  I wasn't sure if it was possible or just a gap in my knowledge.  Thanks!

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Note getString() needs to be declared virtual in the base class, or you won't get the behaviour you are expecting.

 

If it is declared virtual in the base, it is automatically virtual in any derived classes so entirely optional whether you mark it virtual or not. I tend to, so it is clearer in a derived class which are virtual methods I'm re-implementing.

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Hi.

Are you also talking about EventStr(EventType type, std::string str);

the base class will need some type of template class for data to pass on to that function.

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If you want me to really accurate with your OOP, you could declare it (getString()) as a pure virtual (that is, if you don't want Event::getString() to EVER be called. It shouldn't, as the base class doesn't have a string. The only downside to this is you have to implement this function for every derived class, some of which you may not want to have this function):

 

virtual std::string getString() = 0;

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So it seems, Kanefa , that what you are are trying to do with the getString is not polymorphic behavior.

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