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From: "Andrei Alexandrescu" <alexandrescua@micromodeling.com>
Subject: Re: Protected Inheritance
Date: 1999/02/05
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Approved: Fergus Henderson <fjh@cs.mu.oz.au>
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Scott Meyers wrote in message ...
[snip]
>I fed this to VC6:
>
>  class Base {
>  public:
>    Base();
>  };
>
>  class Derived: private virtual Base {};
>
>  class MoreDerived: public Derived {};
>
>  MoreDerived m;
>
>It took it without complaint.  Since we all know VC6 is a perfect
>compiler,
>this should settle the matter.  Ahem.
[snip]

I don't want to look like evangelizing VC6, but here it's not its problem.
The "final class" C++ idiom is:

class DontDeriveFromSomething
{
    friend class Something;
    DontDeriveFromSomething() {}
};

class Something : virtual public DontDeriveFromSomething
{
public:
    Something() {}
};

class SomethingElse : public Something
{
};

SomethingElse s;    // compile-time error

Please note that the name of the very base class is important, because it
will appear in the error message, which otherwise is not very informative.

By the way, I tried to templatize the idiom, and I thought it should work.
But VC5 chokes ignores the case when the template parameter is made a friend
of the template class. Who's wrong? Here's the code:

template <class T>
class DontDeriveFromMyTemplateParameter
{
    friend class T;
    DontDeriveFromMyTemplateParameter() {}
};

class Something : virtual public
DontDeriveFromMyTemplateParameter<Something>
{
public:
    Something() {}    // compile-time error here???
};

class SomethingElse : public Something
{
};

SomethingElse s;

As of the protected inheritance issue, I also look forward to seeing a sound
conceptual definition of it.

Andrei

P.S. Nice to see you on clcm again.



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