#include <iostream> 

using namespace std; 

class a{
friend class pal;
private:
 int i;
protected:
 int j;
public:
 int k;
};

class b:public a{
protected:
 int l;
};

class c:protected a{};
class d:private a{};
class e:public b{};

class pal{
public:
 void f(a a1){
  cout<<a1.i;//正确，友元能访问私有成员
 }
 void f1(b b1){
  cout<<b1.i;//正确，友元能访问派生类的基类部分
  cout<<b1.l;//出错，pal类不是b的友元
 }
};

class deri_pal:public pal{
public:
 void f2(a a1){
  cout<<a.i;//出错，友元不能继承
 }
};

int main() 
{ 
 a a1;
 b b1;
 c c1;
 d d1;
 e e1;
 a1=b1;//正确，只有公共继承的派生类才能向基类转换
 a1=e1;//正确
 a1=c1;//出错，只有公共继承的派生类才能向基类转换
 a1=d1;//出错，只有公共继承的派生类才能向基类转换
 system("pause");
 return 0; 
} 

#include <iostream> 

using namespace std; 

class a{};
class b:public a{};
class c:protected a{};
class d:private a{};
class bb:public b{};
class cc:public c{};
class dd:public d{};




int main() 
{ 
 a a1;b b1;c c1;d d1;bb bb1;cc cc1;dd dd1;
 a* a1=&b1;//ok
 a* a2=&c1;//error
 a* a3=&d1;//error
 a* a4=&bb1;//ok
 a* a5=&cc1;//error
 a* a6=&dd1;//error
 system("pause");
 return 0; 
}