虚函数的一些特性

#include <stdio.h> class IBase { public: virtual void f(void)=0; virtual void f1(void)=0; protected: private: }; class IX:public IBase { public: void f(void) { printf("IX::f /r/n"); } void f1(void) { printf("IX::f1 /r/n"); } protected: private: }; class IY:public IBase { public: void f(void) { printf("IY::f/r/n"); } void f1(void) { printf("IY::f1/r/n"); } protected: private: }; class A :public IX,public IY { public: void f(void) { printf("A::f/r/n"); } void f1(void) { printf("A::f1/r/n"); } protected: private: }; int _tmain(int argc, _TCHAR* argv[]) { A *pa = new A; pa->f1(); IX *pIX = dynamic_cast<IX*>(pa); pIX->f1(); IY *pIY = dynamic_cast<IY*>(pa); pIY->f1(); printf("size of A = %d /r/n",sizeof(A)); return 0; }  

 

上面的输出 就不说了。但是编译器是如何找到对应当函数呢？ 还是从汇编看看吧

 

 

 

pa->f1(); 00413BAD 8B 45 F8 mov eax,dword ptr [ebp-8] 00413BB0 8B 10 mov edx,dword ptr [eax] 00413BB2 8B F4 mov esi,esp 00413BB4 8B 4D F8 mov ecx,dword ptr [ebp-8] 00413BB7 8B 42 04 mov eax,dword ptr [edx+4] 00413BBA FF D0 call eax 00413BBC 3B F4 cmp esi,esp 00413BBE E8 B4 D5 FF FF call 00411177 IX *pIX = dynamic_cast<IX*>(pa); 00413BC3 8B 45 F8 mov eax,dword ptr [ebp-8] 00413BC6 89 45 EC mov dword ptr [ebp-14h],eax pIX->f1(); 00413BC9 8B 45 EC mov eax,dword ptr [ebp-14h] //得到this指针 00413BCC 8B 10 mov edx,dword ptr [eax] 00413BCE 8B F4 mov esi,esp 00413BD0 8B 4D EC mov ecx,dword ptr [ebp-14h] //ecx存放this指针，用于操作具体的类成员 00413BD3 8B 42 04 mov eax,dword ptr [edx+4] //eax存放f1函数的地址 00413BD6 FF D0 call eax 00413BD8 3B F4 cmp esi,esp 00413BDA E8 98 D5 FF FF call 00411177 IY *pIY = dynamic_cast<IY*>(pa); 00413BDF 83 7D F8 00 cmp dword ptr [ebp-8],0 00413BE3 74 0E je 00413BF3 00413BE5 8B 45 F8 mov eax,dword ptr [ebp-8] 00413BE8 83 C0 04 add eax,4 00413BEB 89 85 0C FF FF FF mov dword ptr [ebp+FFFFFF0Ch],eax 00413BF1 EB 0A jmp 00413BFD 00413BF3 C7 85 0C FF FF FF 00 00 00 00 mov dword ptr [ebp+FFFFFF0Ch],0 00413BFD 8B 8D 0C FF FF FF mov ecx,dword ptr [ebp+FFFFFF0Ch] 00413C03 89 4D E0 mov dword ptr [ebp-20h],ecx pIY->f1(); 00413C06 8B 45 E0 mov eax,dword ptr [ebp-20h] 00413C09 8B 10 mov edx,dword ptr [eax] 00413C0B 8B F4 mov esi,esp 00413C0D 8B 4D E0 mov ecx,dword ptr [ebp-20h] 00413C10 8B 42 04 mov eax,dword ptr [edx+4] 00413C13 FF D0 call eax 00413C15 3B F4 cmp esi,esp 00413C17 E8 5B D5 FF FF call 00411177

 

 很明显了，编译器是通过查询vptr来动态得到执行函数的指针，当子类实现了父类的成员函数后，则用父类成员函数指针覆盖掉vptr原来的指针，实现c++所谓的动态特性。这也是COM一种实现方法