On 4 Sep 1998 16:24:28 GMT, Ryszard Kabatek
<rysio@rumcajs.chemie.uni-halle.de> wrote:

>The sample below does not compile,
>because 'x' is a protected member of 'A'.

>But 'B' does inherit 'x' from 'A'!
>Why is this not allowed?

>class A {
>  public:
>    virtual ~A() {}
>  protected:
>    int x;
>};

>class B : public A {
>  public:
>    int func1(A& a) {return a.x;} // Does not compile!
>    int func2(B& b) {return b.x + b.y;} // OK
>  protected:
>    int y;
>};

If class B inherits a protected member of class A, class B may only
access this member through an object of class B, i.e. 'x' is only
available to B as a member of its _own_ base class A.

Otherwise anybody could make public the protected member x of the
"pure" class A just by writing a function int& B::func1(A&) in a class
B:A. If somebody else would define now a class C:A without defining a
public member function permitting public access to C's 'x' member, you
would be able to access _his_ C::x through

        class B:public A {
        public:
    int& B::func1(A& a) { return a.x; }  // ERROR
        };


 C c;   // somebody else's class
 B b;
 B.func1(c) = 42; //  c.x=42 !!!

just as if x was a public member of C.

Regards
Horst
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>The sample below does not compile,
>because 'x' is a protected member of 'A'.
>
>But 'B' does inherit 'x' from 'A'!
>Why is this not allowed?
>
>class A {
>  protected:
>    int x;
>};
>
>class B : public A {
>  public:
>    int func1(A& a) {return a.x;} // Does not compile!
>};

Because:

  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 this clause.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_).

--
Nate Lewis, MCSD
nlewis@mindspring.com
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Hi!

The sample below does not compile,
because 'x' is a protected member of 'A'.

But 'B' does inherit 'x' from 'A'!
Why is this not allowed?


class A {
  public:
    virtual ~A() {}

  protected:
    int x;
};

class B : public A {
  public:
    int func1(A& a) {return a.x;} // Does not compile!

    int func2(B& b) {return b.x + b.y;} // OK

  protected:
    int y;
};


Ryszard Kabatek
--
Martin-Luther University Halle-Wittenberg
Department of Physical Chemistry
Geusaer Str. 88, 06217 Merseburg, Germany
Tel. +49 3461 46 2487 Fax. +49 3461 46 2129


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On 4 Sep 1998 16:24:28 GMT, Ryszard Kabatek

>The sample below does not compile,
>because 'x' is a protected member of 'A'.
>
>But 'B' does inherit 'x' from 'A'!
>Why is this not allowed?

The A part of B is a completely different class from A.

It's sort of like this:

class A { protected: int x; }
class B { protected: int x; protected: int y; }


--
----------------------------------
Siemel B. Naran (sbnaran@uiuc.edu)
----------------------------------


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