实例如下：
#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
}
