C++继承与友元、继承与类型转换

#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;
}