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From: mwb@ulysses.att.com (Michael W. Balk)
Newsgroups: comp.lang.c++
Subject: Re: protected member functions
Summary: Protected member access is restricted to derived objects, not contained objects
Message-ID: <14474@ulysses.att.com>
Date: 16 Mar 91 22:48:42 GMT
References: <1991Mar14.215717.28199@asc.slb.com>
Sender: netnews@ulysses.att.com
Lines: 49

In article <1991Mar14.215717.28199@asc.slb.com>, wkd@asc.slb.com (Bill Duttweiler) writes:
> 
> Is it illegal to call a protected member function on an instance other
> than 'this'? Consider the following simple example:
> 
> class A
> {
> public:
>     A();
> protected:
>     void SetState();
> };
> 
> class B : public A
> {
> public:
>     B(A* a) { MyA = a; }
>     void Foo() { MyA->SetState(); }
> protected:
>     A *MyA;
> };


The problem here is that MyA is a pointer to an instance of A -- it is *not* an
instance of B.  In otherwords, MyA->SetState() is not permitted regardless
of whether Foo is a member function of class B or not.  If this were not the
case, then one could access the protected and private members of any object
simply by deriving from that object's class.  In order to access A::SetState
from an instance of class B, class B should be modified as follows:

class B : public A
{
public:
	B() {}
	void Foo() { SetState(); }
};



Since B is derived from A it already contains everything that is in A
through the derivation.  In the above definition of class B, Foo executes a
member function of the base part of the same object.  This is legal since
and instance of B is also an A.


Michael W. Balk
AT&T Bell Laboratories
Murray Hill, NJ 07974
mwb@ulysses.att.com