Access to private base class?

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2 total messages Started by Igor Boukanov Fri, 15 Mar 1996 00:33

#3678

Author: Igor Boukanov
Date: Fri, 15 Mar 1996 00:33

23 lines
1508 bytes

Here the lines from September DWP, 5.2.6.9:

 [Example:
class A { virtual void f(); };
class B { virtual void g(); };
class D : public virtual A, private B {};
void g()
{
    D   d;
    B*  bp = (B*)&d;  // cast needed to break protection
...

Does "cast needed to break protection" mean that in this case it is possible 
to get access to the private base class by simple cast expression? 

--
Regards, Igor Boukanov (igor.boukanov@fi.uib.no).
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Re: Access to private base class?

#3685

Author: Fergus Henderson
Date: Fri, 15 Mar 1996 02:03

29 lines
1723 bytes

boukanov@sentef1.fi.uib.no (Igor Boukanov) writes:

>Here the lines from September DWP, 5.2.6.9:
>
>class D : public virtual A, private B {};
>void g()
>{
>    D   d;
>    B*  bp = (B*)&d;  // cast needed to break protection
>...
>
>Does "cast needed to break protection" mean that in this case it is possible 
>to get access to the private base class by simple cast expression? 

Yes.

From Stroustrup's book Design and Evolution of C++, page 328:
"The fact that an old-stype cast can be used to gain access to the part of a
representation that is a private base is an unfortunate historical accident."

--
Fergus Henderson             	WWW: http://www.cs.mu.oz.au/~fjh
fjh@cs.mu.oz.au              	PGP: finger fjh@128.250.37.3
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