1 class B;
 2 class C;
 3 class D;
 4 class A{
 5     public:
 6         virtual ~A(){}
 7         friend void gg(D* d);
 8         void ga(B *b,C* c,D* d); int mem;
 9     };
10
11 class B:protected A{
12     public:
13         void gb(B *b,C* c,D* d);
14 };
15 class C:public B{};
16 class D:public C{};
17
18 void A::ga(B *b,C*c,D*d){
19     A *a1=b;  // error: 'A' is an inaccessible base of 'B' 原因：在A类中B类指针b无法访问base A部分的成员,即这里的b->mem是非法的（B是protected继承A），所以也就不能转换。
20     A *a2=c;  // error: 'A' is an inaccessible base of 'C' 原因同上，这里的c->mem也是非法的，因为mem在C类中是protected的。
21     A *a3=d;  // error: 'A' is an inaccessible base of 'D' 原因同上。
22 }
23 void B::gb(B *b,C*c,D*d){
24     A *a1=b;  // no problem here ，这里没问题是因为在类B中，b->mem是合法的。
25     A *a2=c;  //why do classes derived from B can sucessfully convert to A here? 这里c->mem是非法的，只有在类C或friend of class c中c->mem才合法。
26     A *a3=d;  //why do classes derived from B can sucessfully convert to A here? 原因同上
27 }
28 void gg(D* d){
29     A* a=d;  // 这里也会报错，同上
30 }
31 int main(){
32     B b;
33     C c;
34     D d;
35     A a;
36     gg(&d);  // error: 'A' is an inaccessible base of 'D'
37     a.ga(&b,&c,&d);
38     b.gb(&b,&c,&d);
39     A a1(d); //error here;Does it mean the implicit conversion in the user code is also user code?
40     A a4=d;  //same as above
41     return 0;
42 }

 1 class Base
 2 {
 3 public:
 4     int mem;
 5 };
 6
 7 class Derived : protected Base
 8 {
 9     static void f(Derived *d)
10     {
11         d->mem; // OK, in this context, a Derived IS-A Base
12         Base *b = d;
13     }
14 };
15
16 int main()
17 {
18     Derived d;
19     //d.mem;        // Compilation error : in this context a Derived IS-NOT-A Base
20     //Base *b = &d; // Compilation error too : consistent with the intuitive rule
21     return 0;
22 }
23
24 class Derived_Derived : public Derived
25 {
26     static void f(Derived *d)
27     {
28         //d->mem;    // Compilation error : in this context a Derived IS-NOT-A Base (as expected)
29         Base *b = d; // COMPILATION OK : which seems to violate the rule above
30     }
31 };