This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 117a. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2025-04-13

472. Casting across protected inheritance

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].

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

• an invented public member of B would be a public member of N, or

• R occurs in a member or friend of class N, and an invented public member of B would be a private or protected member of N, or

• R occurs in a member or friend of a class P derived from N, and an invented public member of B would be a private or(but not a protected [Footnote: A protected invented member is disallowed here for the same reason the additional check of 11.8.5 [class.protected] is applied to member access: it would allow casting a pointer to a derived class to a protected base class that might be a subobject of an object of a class that is different from the class context in which the reference occurs. —end footnote]) member of P, or

• there exists a class S such that B is a base class of S accessible at R and S is a base class of N accessible at R.

[Example:

 class B { public: 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]