不明白呀不明白，求讲

#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();
}