ATL布幔下的秘密之虚函数背后的东西

程序32.

#include ＜iostream＞ using namespace std; class Base { 　public: 　Base() { 　　cout ＜＜ "In Base" ＜＜ endl; 　　cout ＜＜ "Virtual Pointer = " ＜＜ (int*)this ＜＜ endl; 　　cout ＜＜ "Address of Vtable = " ＜＜ (int*)*(int*)this ＜＜ endl; 　　cout ＜＜ "Value at Vtable 1st entry = " ＜＜ (int*)*((int*)*(int*)this+0) ＜＜ endl; 　　cout ＜＜ "Value at Vtable 2nd entry = " ＜＜ (int*)*((int*)*(int*)this+1) ＜＜ endl; 　　cout ＜＜ endl; 　} 　virtual void f1() = 0; 　virtual void f2() = 0; }; class __declspec(novtable) Drive : public Base { 　public: 　Drive() { 　　cout ＜＜ "In Drive" ＜＜ endl; 　　cout ＜＜ "Virtual Pointer = " ＜＜ (int*)this ＜＜ endl; 　　cout ＜＜ "Address of Vtable = " ＜＜ (int*)*(int*)this ＜＜ endl; 　　cout ＜＜ "Value at Vtable 1st entry = " ＜＜ (int*)*((int*)*(int*)this+0) ＜＜ endl; 　　cout ＜＜ "Value at Vtable 2nd entry = " ＜＜ (int*)*((int*)*(int*)this+1) ＜＜ endl; 　　cout ＜＜ endl; 　} }; class __declspec(novtable) MostDrive : public Drive { 　public: 　MostDrive() { 　　cout ＜＜ "In MostDrive" ＜＜ endl; 　　cout ＜＜ "Virtual Pointer = " ＜＜ (int*)this ＜＜ endl; 　　cout ＜＜ "Address of Vtable = " ＜＜ (int*)*(int*)this ＜＜ endl; 　　cout ＜＜ "Value at Vtable 1st entry = " ＜＜ (int*)*((int*)*(int*)this+0) ＜＜ endl; 　　cout ＜＜ "Value at Vtable 2nd entry = " ＜＜ (int*)*((int*)*(int*)this+1) ＜＜ endl; 　　cout ＜＜ endl; 　} 　virtual void f1() { cout ＜＜ "MostDrive::f1" ＜＜ endl; } 　virtual void f2() { cout ＜＜ "MostDrive::f2" ＜＜ endl; } }; int main() { 　MostDrive d; 　return 0; }

　　现在，程序的输出为：

In Base Virtual Pointer = 0012FF7C Address of Vtable = 0046C0C0 Value at Vtable 1st entry = 00420E50 Value at Vtable 2nd entry = 00420E50 In Drive Virtual Pointer = 0012FF7C Address of Vtable = 0046C0C0 Value at Vtable 1st entry = 00420E50 Value at Vtable 2nd entry = 00420E50 In MostDrive Virtual Pointer = 0012FF7C Address of Vtable = 0046C0C0 Value at Vtable 1st entry = 00420E50 Value at Vtable 2nd entry = 00420E50

　　在MSDN中，对__declspec(novtable)的解释是：它应该使用在纯虚类中。那么，让我们再做一个实验来更好地弄懂这一含义吧。

　　程序33.

#include ＜iostream＞ using namespace std; class Base { 　public: 　Base() { 　　cout ＜＜ "In Base" ＜＜ endl; 　　cout ＜＜ "Virtual Pointer = " ＜＜ (int*)this ＜＜ endl; 　　cout ＜＜ "Address of Vtable = " ＜＜ (int*)*(int*)this ＜＜ endl; 　　cout ＜＜ "Value at Vtable 1st entry = " ＜＜ (int*)*((int*)*(int*)this+0) ＜＜ endl; 　　cout ＜＜ "Value at Vtable 2nd entry = " ＜＜ (int*)*((int*)*(int*)this+1) ＜＜ endl; 　　cout ＜＜ endl; 　} 　virtual void f1() = 0; 　virtual void f2() = 0; }; class __declspec(novtable) Drive : public Base { 　public: 　Drive() { 　　cout ＜＜ "In Drive" ＜＜ endl; 　　cout ＜＜ "Virtual Pointer = " ＜＜ (int*)this ＜＜ endl; 　　cout ＜＜ "Address of Vtable = " ＜＜ (int*)*(int*)this ＜＜ endl; 　　cout ＜＜ "Value at Vtable 1st entry = " ＜＜ (int*)*((int*)*(int*)this+0) ＜＜ endl; 　　cout ＜＜ "Value at Vtable 2nd entry = " ＜＜ (int*)*((int*)*(int*)this+1) ＜＜ endl; 　　cout ＜＜ endl; 　} }; class __declspec(novtable) MostDrive : public Drive { 　public: 　MostDrive() { 　　cout ＜＜ "In MostDrive" ＜＜ endl; 　　cout ＜＜ "Virtual Pointer = " ＜＜ (int*)this ＜＜ endl; 　　cout ＜＜ "Address of Vtable = " ＜＜ (int*)*(int*)this ＜＜ endl; 　　cout ＜＜ "Value at Vtable 1st entry = " ＜＜ (int*)*((int*)*(int*)this+0) ＜＜ endl; 　　cout ＜＜ "Value at Vtable 2nd entry = " ＜＜ (int*)*((int*)*(int*)this+1) ＜＜ endl; 　　cout ＜＜ endl; 　　 　　// 尝试调用第一个虚函数 　　typedef void (*Fun)(); 　　Fun pFun = (Fun)*((int*)*(int*)this+0); 　　pFun(); 　} 　virtual void f1() { cout ＜＜ "MostDrive::f1" ＜＜ endl; } 　virtual void f2() { cout ＜＜ "MostDrive::f2" ＜＜ endl; } }; int main() { 　MostDrive d; 　return 0; }

　　我们在程序中添加的新东西是：

// 尝试调用第一个虚函数 typedef void (*Fun)(); Fun pFun = (Fun)*((int*)*(int*)this+0); pFun();

　　并且，当我们运行这个应用程序的时候，我们会面对与前一个程序相同的问题——也就是尝试调用虚函数发生的错误。这就意味着虚函数表并未初始化。MostDrive类并不是一个抽象类，所以我们应该从类中移除__declspec(novtable)。

　　程序34.

#include ＜iostream＞ using namespace std; class Base { 　public: 　　virtual void f1() = 0; 　　virtual void f2() = 0; }; class __declspec(novtable) Drive : public Base { }; class MostDrive : public Drive { 　public: 　MostDrive() { 　　// 尝试调用第一个虚函数 　　typedef void (*Fun)(); 　　Fun pFun = (Fun)*((int*)*(int*)this+0); 　　pFun(); 　} 　virtual void f1() { cout ＜＜ "MostDrive::f1" ＜＜ endl; } 　virtual void f2() { cout ＜＜ "MostDrive::f2" ＜＜ endl; } }; int main() { 　MostDrive d; 　return 0; }

　　现在，程序就可以正常工作了。它的输出为： MostDrive::f1

　　这个属性并不一定只用在ATL的类中，它可以对任何不能创建对象的类使用。同样，它也并不一定非要用在ATL类中，也就是说它可以从ATL类中省略，不过这就意味着这样会产生更多的代码。

　　我希望能在下篇文章中探究更多ATL的秘密。

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