vptr的初始化

来源：互联网发布：减脂练马甲线软件编辑：程序博客网时间：2024/05/18 05:39

class base
{
int i;
public:
base(int I):i(I){}
virtual int sum(){return i;}

};

主程序

base b(100);

b.sum();

看base的构造函数

class base
{
int i;
public:
base(int I):i(I){}
00401250 push        ebp
00401251 mov         ebp,esp
00401253 push        ecx
00401254 mov         dword ptr [ebp-4],ecx
00401257 mov         eax,dword ptr [this]
0040125A mov         dword ptr [eax],offset base::`vftable' (402164h)   //对象首地址写入offset vftable
00401260 mov         ecx,dword ptr [this]
00401263 mov         edx,dword ptr [I]                                                          //i 写入edx
00401266 mov         dword ptr [ecx+4],edx                                                //edx写入首地址+4
00401269 mov         eax,dword ptr [this]                                                   //返回this
0040126C mov         esp,ebp
0040126E pop         ebp
0040126F ret         4

我们看到内存 402164放着401280

virtual int sum(){return i;}
00401280 push        ebp
00401281 mov         ebp,esp
00401283 push        ecx
00401284 mov         dword ptr [ebp-4],ecx
00401287 mov         eax,dword ptr [this]
0040128A mov         eax,dword ptr [eax+4]
0040128D mov         esp,ebp
0040128F pop         ebp
00401290 ret

现在有了个小结论:

vptr的初始化在构造函数内，由编译器悄悄生成, 成员初始化之前,问题来了如果在构造函数内调用未被初始化的成员，会发生什么？

什么都没发生,一切正常

class base
{
int i;
public:
base(int I):i(I){add();}
00401250 push        ebp
00401251 mov         ebp,esp
00401253 push        ecx
00401254 mov         dword ptr [ebp-4],ecx
00401257 mov         eax,dword ptr [this]
0040125A mov         dword ptr [eax],offset base::`vftable' (402164h)
00401260 mov         ecx,dword ptr [this]
00401263 mov         edx,dword ptr [I]
00401266 mov         dword ptr [ecx+4],edx
00401269 mov         ecx,dword ptr [this]
0040126C call        base::add (4012A0h)                     //不论是不是虚函数，都在成员初始化之后
00401271 mov         eax,dword ptr [this]
00401274 mov         esp,ebp
00401276 pop         ebp
00401277 ret         4

不清楚各个编译器的是不是不同，这是我在vc2005下的结果

再看派生类的,从汇编可以看出,是用派生类的vptr 代替了base的vptr

base(int I):i(I){printf("base vptr=%x",*(int*)this);}

derived(int I,int J):base(I),j(J) {printf("derived vptr=%x",*(int*)this);}

在构造函数里直接输出了vptr

  base(int I):i(I){printf("base vptr=%x",*(int*)this);}
00401260 push        ebp
00401261 mov         ebp,esp
00401263 push        ecx
00401264 mov         dword ptr [ebp-4],ecx
00401267 mov         eax,dword ptr [this]
0040126A mov         dword ptr [eax],offset base::`vftable' (402174h)
00401270 mov         ecx,dword ptr [this]
00401273 mov         edx,dword ptr [I]
00401276 mov         dword ptr [ecx+4],edx
00401279 mov         eax,dword ptr [this]
0040127C mov         ecx,dword ptr [eax]
0040127E push        ecx
0040127F push        offset string "base vptr=%x" (402160h)
00401284 call        dword ptr [__imp__printf (4020ACh)]
0040128A add         esp,8
0040128D mov         eax,dword ptr [this]
00401290 mov         esp,ebp
00401292 pop         ebp
00401293 ret         4

class derived:public base
{
int j;
public:
derived(int I,int J):base(I),j(J) {printf("derived vptr=%x",*(int*)this);}
004012C0 push        ebp
004012C1 mov         ebp,esp
004012C3 push        ecx
004012C4 mov         dword ptr [ebp-4],ecx
004012C7 mov         eax,dword ptr [I]
004012CA push        eax
004012CB mov         ecx,dword ptr [this]
004012CE call        base::base (401260h)
004012D3 mov         ecx,dword ptr [this]
004012D6 mov         dword ptr [ecx],offset derived::`vftable' (40218Ch)
004012DC mov         edx,dword ptr [this]
004012DF mov         eax,dword ptr [J]
004012E2 mov         dword ptr [edx+8],eax
004012E5 mov         ecx,dword ptr [this]
004012E8 mov         edx,dword ptr [ecx]
004012EA push        edx
004012EB push        offset string "derived vptr=%x" (402178h)
004012F0 call        dword ptr [__imp__printf (4020ACh)]
004012F6 add         esp,8
004012F9 mov         eax,dword ptr [this]
004012FC mov         esp,ebp
004012FE pop         ebp
004012FF ret         8

主程序

base b(100);

derived d(200,400);

结果输出了3个

base vptr=402174 base vptr=402174 derived vptr=40218c

是不是跟你想的一样?