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代码有点长,如果你没耐心读完,请不要在此发言,谢谢合作!!
请帮我分析一下,下面作了标记的一句代码到底发生了什么事?
里面内容涉及VTABLE,模拟了通过VTABLE来调用指定的成员函数.
- //---------------------CODE From HERE--------------------
- #include <iostream>
- using namespace std;
- class C1{
- public:
- C1(){};
-
- void SetValue(int var1){
- mVal=var1;
- };
- virtual int PrintValue(){
- cout<<"In Class C1,Value="<<mVal<<endl;return 0;
- };
- virtual int PrintHello(){
- cout<<"Hello World!!(In C1!)"<<endl;return 0;
- };
- protected:
- int mVal;
- };
- class C2: public C1{
- public:
- C2():C1(){};
- int PrintValue(){
- cout<<"In Class C2,Value="<<mVal<<endl;return 0;
- };
- int PrintHello(){
- cout<<"Hello World!!(In C2!)"<<endl;return 0;
- };
- };
- int main() {
- C1 obj1;
- C2 obj2;
- int (*pfn)(C1 *);//function pointer point to the member function
- int **p; //a pointer point to the vtable
-
- //设置初值
- obj1.SetValue(999);
- obj2.SetValue(888);
-
- cout<<"------------------Obj1----------------"<<endl;
- p=(int **) &obj1;
- int var1=(int)*(p+1);//如果不要这一句,一切正常,没问题,如果执行了一这句就有问题了.
- pfn=(int (*) (C1 *)) **p;
- pfn(&obj1);//invoke member function printvalue;
- pfn=(int (*) (C1 *)) *(*p+1);
- pfn(&obj1);//invoke member function printhello;
- cout<<"------------------Obj2----------------"<<endl;
- p=(int **) &obj2;
-
- pfn=(int (*) (C1 *)) **p;
- pfn(&obj2);
- pfn=(int (*) (C1 *)) *(*p+1);
- pfn(&obj2);
- return 0;
- }
- //----------------------------End OF THE CODE-----------------------------
有那一句的执行结果(不正确的结果):
------------------Obj1----------------
In Class C1,Value=4198685
Hello World!!(In C1!)
------------------Obj2----------------
In Class C2,Value=4198880
Hello World!!(In C2!)
没有那一句的结果(正确的结果):
------------------Obj1----------------
In Class C1,Value=999
Hello World!!(In C1!)
------------------Obj2----------------
In Class C2,Value=888
Hello World!!(In C2!)
注意,两个类的实例中的数据都发生了变化.经过调试发现实际内存中的数据并未发生变化(是正确的),应该是这个变量的地址出了问题. |
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发表于 2006-01-10 12:53
