An additional access check beyond those described earlier in Clause [class.access] is applied when a non-static data member or non-static member function is a protected member of its naming class ([class.access.base])115 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 ([expr.unary.op]), the nested-name-specifier shall denote C or a class derived from C. All other accesses involve a (possibly implicit) object expression ([expr.ref]). In this case, the class of the object expression shall be C or a class derived from C. [ Example:
class B { protected: int i; static int j; }; class D1 : public B { }; class D2 : public B { friend void fr(B*,D1*,D2*); void mem(B*,D1*); }; void fr(B* pb, D1* p1, D2* p2) { pb->i = 1; // ill-formed p1->i = 2; // ill-formed p2->i = 3; // OK (access through a D2) p2->B::i = 4; // OK (access through a D2, even though // naming class is B) int B::* pmi_B = &B::i; // ill-formed int B::* pmi_B2 = &D2::i; // OK (type of &D2::i is int B::*) B::j = 5; // OK (because refers to static member) D2::j = 6; // OK (because refers to static member) } void D2::mem(B* pb, D1* p1) { pb->i = 1; // ill-formed p1->i = 2; // ill-formed i = 3; // OK (access through this) B::i = 4; // OK (access through this, qualification ignored) int B::* pmi_B = &B::i; // ill-formed int B::* pmi_B2 = &D2::i; // OK j = 5; // OK (because j refers to static member) B::j = 6; // OK (because B::j refers to static member) } void g(B* pb, D1* p1, D2* p2) { pb->i = 1; // ill-formed p1->i = 2; // ill-formed p2->i = 3; // ill-formed }
— end example ]
This additional check does not apply to other members, e.g., static data members or enumerator member constants.