Title
How does protected member check of 11
Status
cd1
Section
11.8.5 [class.protected]
Submitter
Vincent Korstanje

Created on 2002-09-24.00:00:00 last changed 161 months ago

Messages

Date: 2004-10-15.00:00:00

[Voted into WP at October 2004 meeting.]

Date: 2004-09-15.00:00:00

Additional discussion (September, 2004):

Steve Adamczyk: I wonder if this wording is incorrect. Consider:

    class A {
    public:
      int p;
    };
    class B : protected A {
      // p is a protected member of B
    };
    class C : public B {
      friend void fr();
    };
    void fr() {
      B *pb = new B;
      pb->p = 1;  // Access okay?  Naming class is B, p is a protected member of B,
                  // the "C" of the issue 385 wording is C, but access is not via
                  // an object of type C or a derived class thereof.
    }

I think the formulation that the member is a protected member of its naming class is not what we want. I think we intended that the member is protected in the declaration that is found, where the declaration found might be a using-declaration.

Mike Miller: I think the proposed wording makes the access pb->p ill-formed, and I think that's the right thing to do.

First, protected inheritance of A by B means that B intends the public and protected members of A to be part of B's implementation, available to B's descendants only. (That's why there's a restriction on converting from B* to A*, to enforce B's intention on the use of members of A.) Consequently, I see no difference in access policy between your example and

    class B {
    protected:
        int p;
    };

Second, the reason we have this rule is that C's use of inherited protected members might be different from their use in a sibling class, say D. Thus members and friends of C can only use B::p in a manner consistent with C's usage, i.e., in C or derived-from-C objects. If we rewrote your example slightly,

    class D: public B { };

    void fr(B* pb) {
        pb->p = 1;
    }

    void g() {
        fr(new D);
    }

it's clear that the intent of this rule is broken — fr would be accessing B::p assuming C's policies when the object in question actually required D's policies.

(See also issues 471 and 472.)

Date: 2003-04-15.00:00:00

Proposed resolution (April 2003, revised October 2003):

Change 11.8.5 [class.protected] paragraph 1 from

When a friend or a member function of a derived class references a protected nonstatic member function or protected nonstatic data member of a base class, an access check applies in addition to those described earlier in clause 11.8 [class.access]. [Footnote: This additional check does not apply to other members, e.g. static data members or enumerator member constants.] Except when forming a pointer to member (7.6.2.2 [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 (7.6.1.5 [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).

to

An additional access check beyond those described earlier in clause 11.8 [class.access] is applied when a nonstatic data member or nonstatic member function is a protected member of its naming class (11.8.3 [class.access.base]). [Footnote: This additional check does not apply to other members, e.g., static data members or enumerator member constants.] As described earlier, access to a protected member is granted because the reference occurs in a friend or member of some class C. If the access is to form a pointer to member (7.6.2.2 [expr.unary.op]), the nested-name-specifier shall name C or a class derived from C. All other accesses involve a (possibly implicit) object expression (7.6.1.5 [expr.ref]). In this case, the class of the object expression shall be C or a class derived from C.
Date: 2003-04-15.00:00:00

Notes from April 2003 meeting:

We agreed that the example should be allowed.

Date: 2002-09-24.00:00:00

We consider it not unreasonable to do the following

  class A { 
    protected: 
    void g();
  }; 
  class B : public A { 
    public: 
      using A::g; // B::g is a public synonym for A::g 
  }; 

  class C: public A {
    void foo();
  };

  void C::foo() { 
    B b; 
    b.g(); 
  } 

However the EDG front-end does not like and gives the error

  #410-D: protected function "A::g" is not accessible through a "B" pointer or  object 
    b.g();
      ^

Steve Adamczyk: The error in this case is due to 11.8.5 [class.protected] of the standard, which is an additional check on top of the other access checking. When that section says "a protected nonstatic member function ... of a base class" it doesn't indicate whether the fact that there is a using-declaration is relevant. I'd say the current wording taken at face value would suggest that the error is correct -- the function is protected, even if the using-declaration for it makes it accessible as a public function. But I'm quite sure the wording in 11.8.5 [class.protected] was written before using-declarations were invented and has not been reviewed since for consistency with that addition.

History
Date User Action Args
2008-10-05 00:00:00adminsetstatus: wp -> cd1
2005-05-01 00:00:00adminsetstatus: dr -> wp
2004-11-07 00:00:00adminsetmessages: + msg1097
2004-11-07 00:00:00adminsetstatus: ready -> dr
2004-09-10 00:00:00adminsetmessages: + msg1036
2004-04-09 00:00:00adminsetstatus: review -> ready
2003-04-25 00:00:00adminsetmessages: + msg827
2003-04-25 00:00:00adminsetmessages: + msg826
2003-04-25 00:00:00adminsetstatus: open -> review
2002-09-24 00:00:00admincreate