Issues with multiple inheritance

[Alexander Adam]

Hi!

I got a class structure similar to:

class Base_Object {
... some functions ...
}

class Object:
public Base_Object,
public IDispatch
{
.. implementation of some Base_Object methods
.. implementation of some IDispatch methods
}

class A : public virtual Object
{}

class B : public virtual Object
{}

class C : public A, public B
{}


Now the issue is this -- I need to get access to the right pointer of
the base Object class. When doing this on class C, the returned result
is wrong and the vTable of the returned "Object" class does point to
some methods in C which is obviously wrong. I understand the paradigma
of the "this" pointer and multiple inheritance and I know that I could
downcast C to A, then downcast to Object and I'd get the right one.
But I cannot and do not want to do this as my class hierarchy goes
much further / deeper. The this example:

class C : public A, public virtual B
{}

class D : public B, public C
{}

Now class D has several methods returning instances of D and C. Within
D, I do not really know which kind of "C" it is, just that the
returned class is inherited from C. All I want to do then is to get
the correct "Object" class out of the returned "C" class so that I can
access Object's methods correctly without using downcasting a few
times. See, I do not really care which Object instance I get because
the Object's methods are calling one virtual method of Base_Object
class which each of the subclasses implemenents so it doesn't really
matter.

I hope my description got clear so far, I didn't find any kind of idea
or solution to this issue eventhough having seeked the web for quite a
while. The only solution was downcasting but I cannot do this for each
class because I got hundreds of classes.

Regards
Alexander

 

[Alf P. Steinbach]

* Alexander Adam:

Hi!

Hi. See <url:
http://www.parashift.com/c++-faq-lite/how-to-post.html#faq-5.8>.

 

[Adrian Hawryluk]

Hi!

I got a class structure similar to:

class Base_Object {
... some functions ...
}

class Object:
public Base_Object,
public IDispatch
{
.. implementation of some Base_Object methods
.. implementation of some IDispatch methods
}

class A : public virtual Object
{}

class B : public virtual Object
{}

class C : public A, public B
{}


Now the issue is this -- I need to get access to the right pointer of
the base Object class. When doing this on class C, the returned result
is wrong and the vTable of the returned "Object" class does point to
some methods in C which is obviously wrong.

Obviously? No, it inherited those methods (assuming that they are
virtual). It is correct.

> I understand the paradigma
of the "this" pointer and multiple inheritance and I know that I could
downcast C to A, then downcast to Object and I'd get the right one.

_Assuming_ that you can get the 'right' object doing what you stated
(I'm not sure if what you are saying is correct, I've not touched
multiple-inheritance in a long while), then write a function to do it
for you.

i.e.

Base_Object& C::getBaseObject()
{

But I cannot and do not want to do this as my class hierarchy goes
much further / deeper. The this example:

class C : public A, public virtual B
{}

class D : public B, public C
{}

Now class D has several methods returning instances of D and C. Within
D, I do not really know which kind of "C" it is, just that the
returned class is inherited from C.

You would then call getBaseObject().

All I want to do then is to get
the correct "Object" class out of the returned "C" class so that I can
access Object's methods correctly without using downcasting a few
times. See, I do not really care which Object instance I get because
the Object's methods are calling one virtual method of Base_Object
class which each of the subclasses implemenents so it doesn't really
matter.

I hope my description got clear so far, I didn't find any kind of idea
or solution to this issue eventhough having seeked the web for quite a
while. The only solution was downcasting but I cannot do this for each
class because I got hundreds of classes.

Your description was clear as mud. Good luck.


Adrian
--
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