Re: virtual functions/generic access

"Alf P. Steinbach" <>
Tue, 24 Jul 2007 03:01:24 +0200
* Mark Foley:

I am implementing some functionality where there is a single
base class 'A' which provides some common functionality

There are then two classes derived from 'A', call them 'B' and 'C'.
Both 'B' and 'C' define functions specific only to their class. Somethng

class A
  void doX();
  void doY();

class B : class A
  void getInfo();
  char * getBInfo();

class C : class A
  void getInfo();
  char * getCInfo();

I then have a list of objects. Some are based on 'B', some are based
on 'C', but I would like to treat them generically.

ob[0]->type.doX(); // always ok
ob[0]->type.getInfo(); // always ok
ob[0]->type.getCInfo(); // only ok if type is Class C

I know if I make getBInfo() virtual in the base class then I can
make the above call regardless. But then I have to provide some
return value for it in class A.

If I make it a pure virtual, then I need to provide an implementation
for it in the derived class.

Is there a way to treat a list of objects like this generically without
having to know their actual type?


First, you should move the common functionality getInfo() up to A, as a
virtual function.

For functions getBInfo() and getCInfo() you can simply invert the usage
pattern: instead of client code calling these functions on an object o,
you let the o object call back to the client code with the necessary
data. I.e., in class A you provide a function

   virtual void callbackOn( CallbackInterface& someObject ) = 0;

and in class B e.g.

   virtual void callbackOn( CallbackInterface& someObject )
       someObject.doStuffWith( getBInfo() );

although you should probably reconsider the use of raw pointers, and
also the naming convention (get rid of "get" and "info"...).

Ditto for class C.

This is known as the visitor pattern (google).

It seems kludgey to have something like:

virtual char * getBInfo() {return NULL};

in my base class, but perhaps that's the way it's done.

It's kludgy and not the way it's done.


- Alf

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