菜鸟的疑惑，求解答

初学者的疑惑，求解答

C/C++ code

#include <iostream.h>#include <string.h>class Base{ private:     char msg[30];  protected:     int n;  public:     Base(char s[],int m=0):n(m)     {  strcpy(msg,s);     }  void output(void)  {  cout<<n<<endl<<msg<<endl;  }};class Derived1:public Base{private:    int n;public:    Derived1(int m=1):Base("Base",m-1)    {  n=m; }    void output(void){  cout<<n<<endl;       Base::output();    }};class Derived2:public Derived1{private:    int n;public:    Derived2(int m=2):Derived1(m-1)    { n=m; }    void output(void)    {  cout<<n<<endl;       Derived1::output();    }};int main(){    Base B("Base Class",1);    Derived2 D;    B.output();    D.output();}