派生的类为什么不能访问自己的私有成员啊

#include <iostream.h>
enum br {aa,bb,cc,dd,ee,ff,gg};
class mammal
{
public:
    mammal();
    mammal(int i);
    ~mammal();
    int getage(){return age;}
    void setage(int j){age = j;}
    int getwidth(){return width;}
    void setwidth(int w){width=w;}
protected:
    int age;
    int width;
};
class dog:public mammal
{
    dog();
    dog(int itsage);
    dog(int itsage,int itswidth);
    dog(int itsage,br hh);
    dog(int itsage,int itswidth ,br hh);
    ~dog();
    br getbr(){return mm;}
    void setbr(br mmm){mm=mmm;}
private:
    br mm;
};
mammal::mammal():
age(1),
width(50)
{
    cout << "初始化\n";
}
mammal::mammal(int i):
age(i),
width(5)
{
    cout << "初始化年龄\n";
}
mammal::~mammal()
{
}
dog::dog():
mammal(),
mm(bb)
{
}
dog::dog(int itsage):
mammal(itsage),
mm(bb)
{
}
dog::dog(int itsage,int itswidth):
mammal(itsage),
mm(bb)
{
    width=itswidth;
}
dog::dog(int itsage,int itswidth ,br hh):
mammal(itsage),
mm(hh)
{
    width=itswidth;
}
dog::dog(int itsage,br hh):
mammal(itsage),
mm(hh)
{
}
dog::~dog()
{
}
int main()
{
    mammal p(50);
    cout << p.getage() << "\n" << p.getwidth()<< endl;
    p.setage(03);
    p.setwidth(06);
    cout << p.getage() << "\n" << p.getwidth()<< endl;
    dog p1;
    cout << p1.getbr() << endl;


    return 0;
}