虚函数指针和虚函数表

 1 #include <iostream>
 2 
 3 using namespace std;
 4 
 5 class father
 6 {
 7 public:
 8     father(int x):m_idata(x)
 9     {}
10     
11     virtual void show(int idata)
12     {
13         cout << "papa" << endl;
14         cout << idata-20 << endl;
15     }
16     virtual ~father(){}
17     
18 private:
19     int m_idata;
20 };
21 
22 class child:public father
23 {
24 public:
25     child(int x, int y):father(x),m_ichild_data(y)
26     {
27     }
28     
29     virtual void show(int idata)
30     {
31         cout << "wawa" << endl;
32         cout << idata << endl;
33         //cout << m_ichild_data << endl;  通过虚函数表直接调用，无法访问成员变量，没有对应到对象上 
34     }
35     
36     virtual ~child(){}
37 private:
38     int m_ichild_data;
39     
40 };
41 
42 typedef void (* FUNC)(int);
43 
44 int main()
45 {
46     father fa(9);
47     cout << "object address is vptr  address: " << &fa << endl;
48     cout << "111 vtable address: " << *(int *)(&fa) << endl; //suppose pointer is 4 bytes ,value of vptr
49     cout << "func show address: " << *(int *)(*(int *)(&fa)) << endl;
50     int x=66;
51     //firstchild.show(x);
52     //FUNC func = ( void (*)(void) )(*(int *)(*(int *)(&firstchild)) );
53     FUNC func3 = ( FUNC )(*(int *)(*(int *)(&fa) ));
54     func3(x);
55     
56     child firstchild(88,3456);
57     cout << "------------first child-------------" << endl;
58     cout << "object address is vptr  address: " << &firstchild << endl;
59     cout << "vtable address: " << *(int *)(&firstchild) << endl; //suppose pointer is 4 bytes ,value of vptr
60     cout << "func show address: " << *(int *)(*(int *)(&firstchild)) << endl;
61     //int x=66;
62     //firstchild.show(x);
63     //FUNC func = ( void (*)(void) )(*(int *)(*(int *)(&firstchild)) );
64     FUNC func = ( FUNC )(*(int *)(*(int *)(&firstchild)) );
65     func(x);        //没有this指针 无法显示数据成员 
66     
67     cout << "first data member: " << *(int *)((int *)(&firstchild)+1) << endl;
68     
69     cout << endl << "-------------second child--------------" << endl;
70     child secondchild(888,222);
71     cout << "object address is vptr  address: " << &secondchild << endl;
72     cout << "value of vptr: " << *(int *)(&secondchild) << endl; //suppose pointer is 4 bytes
73     cout << "func show address: " << *(int *)(*(int *)(&secondchild)) << endl;
74     
75     FUNC func2 = ( FUNC )(*(int *)(*(int *)(&secondchild)) );
76     func2(x);        //没有this指针 无法显示数据成员 
77     
78     return 0;
79 }

1 每个类对应一个虚函数表，不同对象的第一个slot都是相同值，即该表的地址。

2 从地址看7c和88之间只有12个字节，应该是x和func3被优化掉了。
3 inside c++ object model说虚表里第一个是type_info，是否是指最后一个？