11 Classes [class]

11.8 Member access control [class.access]

11.8.5 Protected member access [class.protected]

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An additional access check beyond those described earlier in [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]).104
As described earlier, access to a protected member is granted because the reference occurs in a friend or direct 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 1: 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; // error p1->i = 2; // error 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; // error int B::* pmi_B2 = &D2::i; // OK (type of &D2​::​i is int B​::​*) B::j = 5; // error: not a friend of naming class B D2::j = 6; // OK (because refers to static member) } void D2::mem(B* pb, D1* p1) { pb->i = 1; // error p1->i = 2; // error i = 3; // OK (access through this) B::i = 4; // OK (access through this, qualification ignored) int B::* pmi_B = &B::i; // error 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; // error p1->i = 2; // error p2->i = 3; // error } — end example]
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This additional check does not apply to other members, e.g., static data members or enumerator member constants.