class b1: public bo∥公有派生 i public void display i cout<<Bl: display( "endl class dl: public bl∥公有派生 i public void display i cout<< DI: display"<<endlclass B1: public B0 //公有派生 { public: void display() { cout<<"B1::display()"<<endl; } }; class D1: public B1 //公有派生 { public: void display() { cout<<"D1::display()"<<endl; } };