TITLE: access protection, inheritance, and base class pointers

(Newsgroups: comp.lang.c++.moderated, 4 Dec 97)


LEROY: Jean-Louis Leroy
> 
> consider the following illegal code:
> 
> class B
> {
>         protected:
>                 void f();
> };
> 
> class D : public B
> {
>         protected:
>                 void g(B* p)
>                 {
>                         f(); // ok
>                         p->f(); // error
>                 }
> };
> 

> 
> By the same token, wouldn't it be a better world if the code above did
> compile?
> 

WAN: Chichiang Wan <wanc@cup.hp.com>

FDIS 11.5 says "...the access must be through a pointer to, reference
to, or object  of  the  derived  class itself..." (the full paragraph is
attached below).

However, that paragraph may not be a answer to your question which seems
to be why there is such restriction.

Let me try to give an answer.  When I(someone) design a (non-static)
protect data member (pdm), I am only giving permissions to derived
classes (directly/indirectly, but drived properly) to modify THEIR
copies of pdm, not pdm in instances of my designed class.  Without the
above restriction, there would be be a loophole: you could modify pdm in
instances of my class without going through interface fucntions of my
class by simply designing class derived from mine with a member function
taking reference type of my class.

Given that, it is easy to understand that the above restriction only
applies to nonstatic data member.  The above reasoning seems less
obvious for non data members.  However, access checking is uniformly for
names. 

p.s.  one of your alternative is to let D be friend of A, so permission
is already given when you design A.  That does not contradict to the
following: 

 11.5  Protected member access                        [class.protected]

1 When  a  friend  or  a member function of a derived class references a
  protected nonstatic member of a base class, an access check applies in
  addition to those described earlier in clause _class.access_.4) Except
  when forming a pointer to member (_expr.unary.op_), the access must be
  through a pointer to, reference to, or object  of  the  derived  class
  itself  (or  any  class derived from that class) (_expr.ref_).  If the
  access is to form a pointer to member, the nested-name-specifier shall
  name the derived class (or any class derived from that class).

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