🚀 go-pugleaf

In article 9ej@fido.asd.sgi.com, cardboard.mti.sgi.com!box.mti.sgi.com!sgi.sgi.
com!ncar.UCAR.EDU!uunet!chatz!chatz (David Chatterton) writes:
>
>There is very little on virtual inheritence in the draft. I cannot find nay
>statement saying that you cannot do this:
>
>class A {};
>class B : public virtual A {};
>
>int main()
>{
>    A* a1 = new A;
>    A* a2 = new B;
>    B* b1 = (B *)A;	// Illegal due to virtual inheritence
>    return 0;
>}

In the current draft standard, section 5.4 "Explicit type conversion (cast
notation)" lists exactly those conversions which may be performed, and
down-casting from a virtual base class is not among them.

In the ARM, section 5.4 "Explicit Type Conversion" says about the same
thing, the difference being that the four new cast keywords were not
in the language at that time.

In other words, the cast noted above has never been allowed in C++, and
still is not.

>Yet g++ and cfront won't let you. However, g++ (2.7.0) will let you do this:
>
>    B* b1 = dynamic_cast<B*>(A);

>Why can't you force the cast to a B (I guess it has to do with the virtual
>tables)? And is g++ correct in allowing dynamic_cast to do it?

Dynamic_cast was created specifically to allow safe down-casting, and in
particular allows you to down-cast pointers or references from a virtual
base class to a derived class. The only requirement is that the base class
have one or more virtual functions. See section 5.2.6 "Dynamic cast" in the
current draft.

---
Steve Clamage, stephen.clamage@eng.sun.com
---
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