TITLE: pointer type and access specifiers

(Newsgroups: comp.std.c++, 7 Jul 97)

SCHEPLER: Daniel Schepler (daniel@frobnitz.wustl.edu) wrote:

: class base {
:   protected:
:     void foo(int);
: };
:
: class derived : public base {
:   private:
:     base *obj;
:   public:
:     void doFoo(int a) {
:       foo(a);
:       obj->foo(a);   // Access error
:     }
: };
:
: It seems that according to the C++ draft standard, the first access in
: doFoo is legal because base::foo is also accessible as a protected
: member of derived.  However, the second access is illegal, since
: derived::doFoo is not a member function of base.

KUEHL: kuehl@horn.informatik.uni-konstanz.de (Dietmar Kuehl)

Not exactly. The second access is illegal because it is unknown whether
'obj' is indeed a 'derived'. ... and it makes a lot sense to deny
access in this case: If 'obj' is actually pointing to an object of
another class, say 'sibling', also derived from 'base', it would be
possible to access 'sibling's state using a 'derived' member function.
Since 'sibling' and 'derived' are not really related, access to the
protected part of 'sibling', or any other class derived from 'base', by
a method of 'derived' is denied.

SCHEPLER:

: Anyway, I think this type of access should be allowed in this
: context.

KUEHL:

I would definitely be opposed to allow this access, although I once
also wanted to have it: It would break encapsulation.