Re: Simple OO design issue

Stuart Redmann <>
Fri, 01 Jun 2007 11:39:48 +0200
<f3osom$n4j$> wrote:


I am relatively new to object oriented programming and design. I am
developing an application in VS 2005. I am having the following design

I have two interfaces X and Y. Y is derived from X as the following:

__interface X
        virtual void func1(int) = 0;

__interface Y: public X
        virtual void func2(int) = 0;

Common scenario.

X interface is implemented in the class XImpl as follow:

class XImpl: public X
        void func1(int i)
    cout << "In XImpl::func 1: i = " << i;


Now I want to have a class YImpl implementing func2() of Y interface.
func1 implementation should remain the same as in XImpl. How should I
define the class YImpl? Deriving it only from Y interface, as

class YImpl: public Y

will not allow me to use the func1() implementation of XImpl. I have
to rewrite the same function in YImpl then.

On the other hand inheriting YImpl multiply from XImpl and Y as


class YImpl: public XImpl, public Y

is not a good idea as X interface is included twice (and the VC++
complier is not allowing it either, rightly complaining that func1 is

How should the class YImpl be defined and implemented? Please note
that I can change the definition or implementation of XImpl but not
the definition of X and Y interfaces as they have come from some other
sources on which I have no control.

You have stumbled over some implementation issues that come together with
interface programming (when using some component architecture). The standard
solution to your problem is to make the implementation class XImpl a template:

template<class t_BaseInterface>
class XImpl : public t_BaseInterface
   void func1(int i)
     // Implementation of func1 of interface X

If you want to use XImpl for implementing, you have to use it like XImpl<X> (or
you can make X the default base interface for XImpl like template <class
t_BaseInterface = X>, than you can say XImpl<> with empty template parameter list).
For YImpl you have to say:
class YImpl : public XImpl<Y>

Now XImpl implements the func1 of interface Y, and YImpl needs only to implement


BTW: If you cross-post to several newsgroups, you should rather specify a
follow-up newsgroup, so that any replies to your post are accumulated in a
single newsgroup.

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