c++对象虚表

虚表(virtual table)

The virtual table is actually quite simple, though it’s a little complex to describe in words. First, every class that uses virtual functions (or is derived from a class that uses virtual functions) is given it’s own virtual table. This table is simply a static array that the compiler sets up at compile time. A virtual table contains one entry for each virtual function that can be called by objects of the class. Each entry in this table is simply a function pointer that points to the most-derived function accessible by that class.

带有虚函数的类都有自己的虚表，虚表是编译器在编译时就确定的一个静态数组。虚表中包含每个虚函数的一个入口(函数指针)，且这个函数指针是对象能访问到最近的一个函数指针。

Second, the compiler also adds a hidden pointer to the base class, which we will call *__vptr. *__vptr is set (automatically) when a class instance is created so that it points to the virtual table for that class. Unlike the *this pointer, which is actually a function parameter used by the compiler to resolve self-references, *__vptr is a real pointer. Consequently, it makes each class object allocated bigger by the size of one pointer. It also means that *__vptr is inherited by derived classes, which is important.

编译器会为基类设置一个隐藏的指针(*__vptr), 该指针是在类实例化时自动设置指向类的虚表的。

1、在进入构造函数时，虚表已经构建完成。

2、虚构时，虚表可以回滚。

3、虚表与虚表指针的区别：每个类只有一个虚表，每个对象实例都有个虚表指针。

如下：

 1 #include "stdafx.h"
 2 #include <iostream>
 3 
 4 class CBase
 5 {
 6 public:
 7     CBase()
 8     {
 9         std::cout<<"CBase::CBase()"<<std::endl;
10         print();
11         //print1();
12     }
13     virtual ~CBase()
14     {
15         std::cout<<"CBase::~CBase()"<<std::endl;
16         print();
17     }
18     //virtual void print1() = 0;
19     virtual void print()
20     {
21         std::cout<<"CBase::print()"<<std::endl;
22         //print1();
23     }
24 };
25 
26 class CDerived : public CBase
27 {
28 public:
29     CDerived()
30     {
31         std::cout<<"CDerived::CDerived()"<<std::endl;
32         print();
33     }
34     
35     virtual ~CDerived()
36     {
37         std::cout<<"CDerived::~CDerived()"<<std::endl;
38         print();
39     }
40 
41     virtual void print()
42     {
43         std::cout<<"CDerived::print()"<<std::endl;
44     }
45 
46     virtual void print1()
47     {
48         std::cout<<"CDerived::print1()"<<std::endl;
49     }
50 };
51 
52 void DoPrint(CBase* pBase)
53 {
54     pBase->print();
55 }
56 int _tmain(int argc, _TCHAR* argv[])
57 {
58     CBase* pBase = new CBase;
59     CDerived* pDerived = new CDerived;
60     CBase* pBase1 = pDerived;
61 
62     std::cout<<"---------"<<std::endl;
63     DoPrint(pBase);
64     DoPrint(pDerived);
65     DoPrint(pBase1);
66     std::cout<<"---------"<<std::endl;
67 
68     delete pBase;
69     delete pDerived;
70 
71     return 0;
72 }