“三看原则”
1.看调用语句，这句话写在子类的内部还是外部
2.看子类如何从父类继承
3.看父类的访问级别
public：可以在类的内部和外部访问
private：只能在类的内部访问（派生类中访问不了基类的私有变量）
proteced：只能在类的内部访问和类的派生类中访问
class Parent { public: int a; private: int b; protected: int c; }; class Child1:public Parent { public: void SetValue() { a = 10; b = 20;//error c = 30; } }; class Child2:private Parent { public: void SetValue() { a = 10; b = 20;//error c = 30; } }; class Child3:protected Parent { public: void SetValue() { a = 10; b = 20;//error c = 30; } }; //main Child1 c1; c1.a = 100; public c1.b = 200; private //error c1.c = 300; protected //error Child2 c2; c2.a = 100; private //error c2.b = 200; private //error c2.c = 300; private //error Child3 c3; c3.a = 100; protected //error c3.b = 200; private //error c3.c = 300; protected //error
