Re: Why multiple inheritance fails

From:
"Doug Harrison [MVP]" <dsh@mvps.org>
Newsgroups:
microsoft.public.vc.mfc
Date:
Mon, 25 Jun 2007 14:04:33 -0500
Message-ID:
<03408396ht8sagaiuqjd3ndigrl2d9kb90@4ax.com>
On Mon, 25 Jun 2007 10:23:27 -0700, "Vijay Visana"
<vijay_visana@myorganization.com> wrote:

I have following multiple inheritance logic

       A
    / > B C
    \ /
      D
here A B and C are pure abstract base classe and D is derived publicly(no
virtual inheritance ) from B and C and implements all pure virtual methods
of parent classes

I am able to call A::Method from

D* pD = new D;
B* b = pD;
C*c = pD;
c->method_of_A();
b->method_of_A();

but NOW !! in following scenerio

          A
        / > / \ \
    / \ \
  B B2 C
    \ / /
     \ / /
       \ /
          \ /
            D

here A, B, B2 AND C are pure virtual classes and D is derived publicly(no
virtual inheritance ) from B, B2 and C

now here I am not able to call A::Method from

D* pD = new D;
B* b = pD;

b->method_of_A();//not compiling ambiguous call

following is example

/////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
/// why multiple inheritance of interfaces (pure ABC) work when inheritance
hiearchy is liniear but fail when it involve loop
///
class Animal

{
   public:
       virtual void walk()=0;
};

class Cat : public Animal
{
public:
   virtual void Meo()=0;
};

class PussyCat : public Animal
{
public:
       virtual void PMeo()=0;
};

class persianCat: public PussyCat,public Cat
{
public:
   virtual void PerMeo()=0;
};

class Dog : public Animal
{
public:
   virtual void bark()=0;
};

class CatDog : public Dog,public persianCat
{
public:
   void walk()
   {
       printf("Animal Walk");
   }

void Meo()
{
   printf("meo");
}

void PMeo()
{
   printf("Pmeo");
}

void PerMeo()
{
   printf("Permeo");
}

void bark()
{
   printf("bhao bhao");
}

static CatDog* CreateInstance();
private :
   CatDog(){};
};

CatDog* CatDog::CreateInstance()
{
   return new CatDog;
}

int _tmain(int argc, _TCHAR* argv[])

{
persianCat* pPerCat = CatDog::CreateInstance();
CatDog* pCatDog = CatDog::CreateInstance();

pCatDog->walk();//Don't get error
pPerCat->walk(); //Get Error here why I don't know

delete pPerCat;
delete pCatDog;

return 0;

}


You think you're using virtual inheritance and have the diamond-shaped
inheritance graph you've shown, but you're not and you don't. You need to
change your class declarations to:

   class Cat : public virtual Animal
   class PussyCat : public virtual Animal
   class Dog : public virtual Animal

(Concerning "CatDog", I don't see anything good coming out of that. Dogs
rule!)

--
Doug Harrison
Visual C++ MVP

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