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From: "Victor Bazarov" <vAbazarov@dAnai.com>
Newsgroups: comp.std.c++
Subject: Re: Virtual Base-class constructor calls
Date: 30 Aug 2002 19:10:05 GMT
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"Ken Durden" <ken.durden@augusttech.com> wrote...
> Any class which derives from a virtual base-class (through any number
> of levels of inheritance), must call the constructor of the virtual
> base class. The only invocation which is actually used, however, is
> the one for the class actually being constructed.
>
> Thus, in the following example classes B and C must call the A ctor
> even though their invocation is never used since they are never
> allocated.
>
> My only suggestion is that B and C be permitted to not call the A ctor
> since they are both pure virtual and cannot be allocated.

What if I write another class that derives from B but not
from C, and which implements f()?  I will have no way to
initialise A, so I will have to rely on B's constructor for
the initialisation with a number.

>
> The solution I've used below works alright when the arguments are
> simply, but it becomes a little harder when you start taking in
> references to different types, or objects, etc...

What are you referring to?  In your example D is still
abstract (since it doesn't provide a definition of f())
and initialising bases (B and C) in the D's c-tor is
completely unnecessary.

>
> -ken
>
>
> struct A
> {
>   A(int x) {}
>   virtual void f() = 0;
> };
>
> struct B : public virtual A
> {
> B() :
>   A(2) /* not used */

"Not used" by whom?

>   {}
> };
>
> struct C : public virtual A
> {
> C() :
>   A(2) /* not used */
>   {}
> };
>
> struct D : public B, public C
> {
> D() :
>   A(4),
>   B(),
>   C()
>   {}

You can do just

    D() : A(4) {}

But you have to add

    void f();

to be able to instantiate it.

> };
>
> int main()
> {
> D d;

Error: abstract class

> }

Victor
--
Please remove capital A's from my address when replying by mail


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