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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