层次继承 A class A .…; B C class B public A .…; class c public A D E .…; class D: public c . class E public C 13 北大青鸟13 层次继承 A B C D E class A {…}; class B :public A {…}; class C :public A {…}; class D :public C {…}; class E :public C {…};