C++ 中的对象布局

C++中的涉及到虚表时，类对象的布局分为：虚表与数据成员，子类包含派生类布局，假设下面一个程序:

 1 #include <iostream>
 2
 3 using namespace std;
 4
 5 class B {
 6 private:
 7     int a;
 8 public:
 9     B() {
10         a = 11;
11     }
12
13     virtual void fun() {
14         cout << "B::fun" << endl;
15     }
16 };
17
18 class A:public B {
19 public:
20     A() {
21         cout << "class A constructor" << endl;
22         m = 3;
23         n = 2;
24     }
25
26     ~A() {
27         cout << "~A()" << endl;
28     }
29     virtual void fun() {
30         cout << "A::fun" << endl;
31     }
32
33 private:
34     int m,n;
35 };
36
37 int main(int argc, char *argv[])
38 {
39     A a;
40     int *p;
41     typedef void (*pf_t)(void);
42
43     pf_t pf;
44     p = static_cast<int *>((int *)&a);
45     pf = (pf_t)p;
46
47     int *vptr = *(int **)&a;
48     int *vtable = (int *)*vptr;
49     pf = (pf_t )vtable[2];
50
51     printf("address A::fun %p\n",pf);
52
53     cout << "p:" << *p << endl;
54     p ++ ;
55     cout << "p:" << *p << endl;
56     p++;
57     cout << "p:" << *p << endl;
58     p++;
59     cout << "p:" << *p << endl;
60
61     p++;
62     cout << "p:" << *p << endl;
63
64
65     return 0;
66 }

通过VS 2010显示对象布局图(cl -d1reportAllClassLayout)，其中meta为对应对象的类型信息:

类B:

 1 class B    size(8):
 2     +---
 3  0    | {vfptr}
 4  4    | a
 5     +---
 6
 7 B::[email protected]:
 8     | &B_meta
 9     |  0
10  0    | &B::fun

类A:

class A    size(16):
    +---
    | +--- (base class B)
 0    | | {vfptr}
 4    | | a
    | +---
 8    | m
12    | n
    +---

A::[email protected]:
    | &A_meta
    |  0
 0    | &A::fun

A::fun this adjustor: 0