ATL布幔下的秘密之虚函数背后的东西

来源：VCKBASE 作者：李马编译出处：巧巧读书 2005-12-02 进入讨论组

关键词：
microsoft
c++
mfc

谢谢收藏 http://www.qqread.com/vc/n782121200.html

　　现在，你会发现在基类中含有多个虚函数的情况下，派生类并不能完全重写它们。

　　程序26.

#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 ＜＜ "Value at Vtable 3rd entry = " ＜＜ (int*)*((int*)*(int*)this+2) ＜＜ endl;
　　cout ＜＜ endl;
　}
　virtual void f1() { cout ＜＜ "Base::f1" ＜＜ endl; }
　virtual void f2() { cout ＜＜ "Base::f2" ＜＜ endl; }
};

class 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 ＜＜ "Value at Vtable 3rd entry = " ＜＜ (int*)*((int*)*(int*)this+2) ＜＜ endl;
　　cout ＜＜ endl;
　}
　virtual void f1() { cout ＜＜ "Drive::f1" ＜＜ endl; }
};

int main() {
　Drive d;
　return 0;
}

　　程序的输出为：

In Base
Virtual Pointer = 0012FF7C
Address of Vtable = 0046C0E0
Value at Vtable 1st entry = 004010F0
Value at Vtable 2nd entry = 00401145
Value at Vtable 3rd entry = 00000000

In Drive
Virtual Pointer = 0012FF7C
Address of Vtable = 0046C0C8
Value at Vtable 1st entry = 0040121C
Value at Vtable 2nd entry = 00401145
Value at Vtable 3rd entry = 00000000

　　这个程序的输出表明基类的虚函数在派生类中并未被重写，然后，派生类的构造函数没有对虚函数的入口做任何的事情。
　　
　　那么现在，让我邀请纯虚函数来加入这一游戏，再来看看它的行为吧。请看以下的程序：

　　程序27.

#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 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;
　}
　virtual void f1() { cout ＜＜ "Drive::f1" ＜＜ endl; }
　virtual void f2() { cout ＜＜ "Drive::f2" ＜＜ endl; }
};

int main() {
　Drive d;
　return 0;
}

　　在debug和release模式下，程序的输出有所不同。下面是debug模式的输出：

In Base
Virtual Pointer = 0012FF7C
Address of Vtable = 0046C0BC
Value at Vtable 1st entry = 00420CB0
Value at Vtable 2nd entry = 00420CB0

In Drive
Virtual Pointer = 0012FF7C
Address of Vtable = 0046C0A4
Value at Vtable 1st entry = 00401212
Value at Vtable 2nd entry = 0040128F

　　以下则是release模式的输出：

In Base
Virtual Pointer = 0012FF80
Address of Vtable = 0042115C
Value at Vtable 1st entry = 0041245D
Value at Vtable 2nd entry = 0041245D

In Drive
Virtual Pointer = 0012FF80
Address of Vtable = 00421154
Value at Vtable 1st entry = 00401310
Value at Vtable 2nd entry = 00401380

　　为了更好地弄懂这一原理，我们需要对程序作少许的改动，并尝试使用函数指针来调用虚函数。

　　程序28.

#include ＜iostream＞
using namespace std;

typedef void(*Fun)();

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;

　　// 尝试执行第一个虚函数
　　Fun pFun = (Fun)*((int*)*(int*)this+0);
　　pFun();

　　cout ＜＜ endl;
　}
　virtual void f1() = 0;
　virtual void f2() = 0;
};

class 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;
　}
　virtual void f1() { cout ＜＜ "Drive::f1" ＜＜ endl; }
　virtual void f2() { cout ＜＜ "Drive::f2" ＜＜ endl; }
};

int main() {
　Drive d;
　return 0;
}

　　现在程序的行为在debug和release模式下仍然不同。在debug模式下，它会显示一个运行时错误的对话框：

　　并且，当你按下“忽略”按钮之后，它会显示下面的对话框：

　　而在release模式下运行的话，它只会在控制台窗口中输出错误信息：

In Base
Virtual Pointer = 0012FF80
Address of Vtable = 0042115C
Value at Vtable 1st entry = 0041245D
Value at Vtable 2nd entry = 0041245D

runtime error R6025
- pure virtual function call

进入讨论组讨论。