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From: allan_w@my-dejanews.com (Allan W)
Newsgroups: comp.std.c++
Subject: Re: proposal: "virtual restrict" inheritance
Date: Thu, 13 Feb 2003 19:29:40 +0000 (UTC)
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dlagno@mail.nnov.ru (Denis Lagno) wrote

> [Problem description]
> Imagine that you have typical rhomb shaped inheritance framework
> 
>      B
>    /   \
>    C   D
>    \   /
>      X
> 
> If compiler knows that nothing is grafted badly to this structure in
> other translation units, it can implement one of inheritances
> C:virtual B or D:virtual B not as it usually does -- thru pointer and
> indirect access but directly thru statically known offset.
> 
> But since compiler sees only one translation unit at a time it cannot
> do such optimization.

What does it mean for a structure to be "grafted badly?" As opposed
to "grafted well?" I assume "grafted" means "added to the class
heirarchy" i.e. a derived class. But what makes this good or bad?

> [Proposed solution]
> add "virtual restrict" inheritance.

> "virtual restrict" inheritance logically shall be the same as usual
> "virtual" but with one restriction -- if for any derived class some
> base class occurs more than 1 time as "virtual restrict" then the
> program is ill-formed.

So one class gets "virtual restrict" and the others just get "virtual."
Is that right?

What would the compiler do differently for a "virtual restrict"
derivation?

> So changes to standard required are very small and local and without
> possible interference with other language features -- because
> semantically "virtual restrict" shall everywhere act as usual
> "virtual".  But beforementioned restriction enables compiler to
> optimize virtual inheritance.

How?

> I assumed that keyword "restrict" already included in C99 so it is
> good to reuse it in another sense in another place.
> 
> [example]
> former inheritance framework may be now written as:
> 
> class B { ... };
> 
> class C: public virtual B { ... };
> 
> class D: public virtual restrict B { ... };
> 
> class X: public C, public D { ... };
> 
> I'll be glad to hear comments.

I don't understand what optimization this makes possible. I suppose
you're saying that the compiler could arrange class B and D next to
each other, so that class D doesn't need to use a pointer to find
the members of B. But surely all the other classes still need that
pointer. Also, what happens if something else derives from C and/or
D? What happens if something derives from both X and Y, where Y
also derives from D?

      B
     / \
    C   D
     \ / \
      X   Y
       \ /
        W

class B { ... };
class C: public virtual B { ... };
class D: public virtual restrict B { ... };
class X: public C, public D { ... };
class Y: public D { ... };
class W: public X, public Y { ... };

Without the "restrict" keyword, we have two instances of D, but
only one instance of B. What happens with the restrict keyword?
Why is this good?

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