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From: garnett@coupe.bellcore.com (Michael Garnett)
Newsgroups: comp.lang.c++
Subject: Re: virtual base class & function member ambiguity question
Message-ID: <1990Aug28.163333@coupe.bellcore.com>
Date: 27 Aug 90 20:03:33 GMT
References: <BRUCEC.90Aug27121305@phoebus.phoebus.labs.tek.com>
Sender: news@bellcore.bellcore.com
Reply-To: garnett@thumper.bellcore.com
Organization: Information Networks Research (Bellcore)
Lines: 67

In article <BRUCEC.90Aug27121305@phoebus.phoebus.labs.tek.com>,
brucec@phoebus.phoebus.labs.tek.com (Bruce Cohen;;50-662;LP=A;) writes:
|> Is this a bug in cfront 2.0, or am I not understanding how virtual
base
|> classes work?
|> 
|> Question 1:
|> I have a simple test case with an inheritance DAG that goes like:
|> 
|>                         Root
|>                          |
|>                        Derived
|>                     _____|_____
|>                     |         |
|>                 DerivedVR1  DerivedVR2
|>                     |_________|
|>                          |
|>                         Leaf
|> 
...
|> Here's the file:
|> // test virtual base function inheritance
...
|> 
|> class DerivedVR1 : public Derived
|> {
|>   public:
|>     DerivedVR1(char* s = "DerivedVR1");
|> };
|> DerivedVR1::DerivedVR1(char* s) : Derived (s) { }
|> 
|> class DerivedVR2 : public Derived
|> {
|>   public:
|>     DerivedVR2(char* s = "DerivedVR2");
|> };
|> DerivedVR2::DerivedVR2(char* s) : Derived (s) { }
|> 
|> class Leaf : public virtual DerivedVR2, public virtual DerivedVR1
|> {
|>   public:
|>     Leaf(char* s = "Leaf");
|>     void PrintLeaf();
|> 

I'm answering this in a hurry but...

shouldn't the class DerivedVR1 be 
         class DerivedVR1 : public virtual Derived 
           ...
and the DerivedVR1 class likewise be
         class DerivedVR2 : public virtual Derived
           ...

and the class Leaf be simply
         class Leaf : public DerivedVR1, public DerivedVR2

reason: (possible reason? <smile>)
   It's the 'Derived' class that you want to avoid multiple copies of..
   ...not the DerivedVR2 and DerivedVR1 classes...
   Each of them (DerivedVR[12]) must have a copy of Derived ...
   ...but Leaf needs only one copy....
   ...yet still needs BOTH DerivedVR1 and DerivedVR2



               Michael