对于受保护的成员变量，子类对其的访问有一个额外的限制： Clause 11.5: 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 11.102) Except when forming a pointer to member (5.3.1), the access must be through a pointer to, reference to, or object of the derived class itself (or any class derived from that class) (5.2.5). 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). 例子： class Base{ protected:      Base(int){}      int i; }; class Derive:public Base{ public:      Derive():Base(100){              //OK           Base b(100);                //ill-formed: Cannot access protected member           int Base::* p = &Base::i;   //ill-formed: Cannot access protected member           int Base::*pd = &Derive::i; //OK: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)      }      void foo( Derive *pd, Base *pb ){           int temp1  = pb->i; //ill-formed: cannot access protected member           int temp2  = pd->i; //OK:   access through pointer to derived class           int temp3  = i;     //OK:   access through 'this'       } };

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