This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 112e. See for the official list.


472. Casting across protected inheritance

Section: 11.8.5  [class.protected]     Status: drafting     Submitter: Mike Miller     Date: 16 Jun 2004

Does the restriction in 11.8.5 [class.protected] apply to upcasts across protected inheritance, too? For instance,

    struct B {
        int i;
    struct I: protected B { };
    struct D: I {
        void f(I* ip) {
            B* bp = ip;    // well-formed?
            bp->i = 5;     // aka "ip->i = 5;"

I think the rationale for the 11.8.5 [class.protected] restriction applies equally well here — you don't know whether ip points to a D object or not, so D::f can't be trusted to treat the protected B subobject consistently with the policies of its actual complete object type.

The current treatment of “accessible base class” in 11.8.3 [class.access.base] paragraph 4 clearly makes the conversion from I* to B* well-formed. I think that's wrong and needs to be fixed. The rationale for the accessibility of a base class is whether “an invented public member” of the base would be accessible at the point of reference, although we obscured that a bit in the reformulation; it seems to me that the invented member ought to be considered a non-static member for this purpose and thus subject to 11.8.5 [class.protected].

(See also issues 385 and 471.).

Notes from October 2004 meeting:

The CWG tentatively agreed that casting across protective inheritance should be subject to the additional restriction in 11.8.5 [class.protected].

Proposed resolution (April, 2011)

Change 11.8.3 [class.access.base] paragraph 4 as follows:

A base class B of N is accessible at R, if


    class B {
      int m;

    class S: private B {
      friend class N;
    class N: private S {
      void f() {
        B* p = this;    // OK because class S satisfies the fourth condition
                        // above: B is a base class of N accessible in f() because
                        // B is an accessible base class of S and S is an accessible
                        // base class of N.

    class N2: protected B { };

    class P2: public N2 {
      void f2(N2* n2p) {
        B* bp = n2p;    // error: invented member would be protected and naming
                        // class N2 not the same as or derived from the referencing
                        // class P2
        n2p->m = 0;     // error (cf 11.8.5 [class.protected]) for the same reason

end example]