VPTR与构造函数和继承

た入场券 2022-09-20 05:49 283阅读 0赞

C++中类的成员函数默认情况下是non-virtual，即被调用时为静态绑定。  
  
 至少 包含一个virtual成员函数的类，都有一个VTABLE——虚函数映射表，表中的每项对应类中一个virtual成员函数的函数体地址。相应的该类的每个对象在为其分配存储空间时，编译器会额外的为每个对象附加一个指针VPTR，该指针指向该对象所属类的VTABLE。  
  
一定要明确概念，VTABLE是在类这个层次上的概念，而VPTR则是在对象这个层次上的概念。  
  
将VPTR正确设置、指向合适的VTABLE，这是由谁负责完成的？类的构造函数。编译器会自动的在构造函数中插入设置VPTR的代码。  
  
常见的实现中，编译器会将VPTR放在对象所占空间的头部。  
  
对于单重继承关系中的子类对象，其VPTR的设置则经历如下两个阶段  
  
A).首先，在基类构造函数中，VPTR被设置为指向基类的VTABLE  
B).之后，在子类构造函数中，VPTR被设置为指向子类的VTABLE  
  
注意，这里只存在一个VPTR，在子对象的构建过程中被重写。  
  
而对于多重继承，有类似的过程，不过子类对象中存在不止一个VPTR。  
  
下面是一个简单的验证代码，可以观察VPTR是如何被重写，以及指向何处

![None.gif][] \#include  < iostream >   
![None.gif][]  
![None.gif][] using  namespace  std;  
![None.gif][]  
![None.gif][] class  Base  
![ExpandedBlockStart.gif][]![ContractedBlock.gif][] ... \{  
![InBlock.gif][]public:  
![InBlock.gif][]int x;  
![InBlock.gif][] Base():x(0)  
![ExpandedSubBlockStart.gif][]![ContractedSubBlock.gif][]...\{  
![InBlock.gif][]void\* pv=this;  
![InBlock.gif][]int\* pi=static\_cast<int\*>(pv);  
![InBlock.gif][] printf("vptr in base ctor point to : %x ",\*pi);  
![InBlock.gif][]  
![ExpandedSubBlockEnd.gif][] \}  
![ExpandedSubBlockStart.gif][]![ContractedSubBlock.gif][]virtual~Base()...\{\}  
![ExpandedBlockEnd.gif][]\} ;  
![None.gif][]  
![None.gif][] class  Derived: public  Base  
![ExpandedBlockStart.gif][]![ContractedBlock.gif][] ... \{  
![InBlock.gif][]public:  
![InBlock.gif][] Derived()  
![ExpandedSubBlockStart.gif][]![ContractedSubBlock.gif][]...\{  
![InBlock.gif][]  
![InBlock.gif][]void\* pv=this;  
![InBlock.gif][]int\* pi=static\_cast<int\*>(pv);  
![InBlock.gif][] printf("vptr in derived point to : %x ",\*pi);  
![ExpandedSubBlockEnd.gif][] \}  
![InBlock.gif][]  
![ExpandedBlockEnd.gif][]\} ;  
![None.gif][]  
![None.gif][]  
![None.gif][] int  main( int  argc,  char \*  argv\[\])  
![ExpandedBlockStart.gif][]![ContractedBlock.gif][] ... \{  
![InBlock.gif][]  
![InBlock.gif][] Base Ba;  
![InBlock.gif][] Derived Da;  
![InBlock.gif][]  
![InBlock.gif][]void\* pv=&Ba;  
![InBlock.gif][]int\* pi=static\_cast<int\*>(pv);  
![InBlock.gif][] printf("address of Base's vTable : %x ",(\*pi));  
![InBlock.gif][]  
![InBlock.gif][]  
![InBlock.gif][] pv=&Da;  
![InBlock.gif][] pi=static\_cast<int\*>(pv);  
![InBlock.gif][] printf("address of Derived's vTable : %x ",(\*pi));  
![InBlock.gif][]  
![InBlock.gif][]return1;  
![ExpandedBlockEnd.gif][]\}

运行结果

![None.gif][] vptr  in  base  ctor point to : 8048af0  
![None.gif][]  
![None.gif][]vptr  in  base  ctor point to : 8048af0  // initialized by base ctor   
![None.gif][] vptr  in  derived point to : 8048b10  // overwritten by derived ctor    
![None.gif][]   
![None.gif][]address of Base ' s vTable : 8048af0   
![None.gif][] address of Derived ' s vTable : 8048b10   
![None.gif][]

[None.gif]: /images/20220828/473659ee851e4e0c84c15214b2fefb92.png
[ExpandedBlockStart.gif]: http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockStart.gif
[ContractedBlock.gif]: /images/20220828/160e5ada1b5f4cfeadf9348b305ffa4a.png
[InBlock.gif]: /images/20220828/3e5e3ec973f64926857298de0a3d7e98.png
[ExpandedSubBlockStart.gif]: http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedSubBlockStart.gif
[ContractedSubBlock.gif]: /images/20220828/81d0cf03bf7f421c8427c84c3f7f11e1.png
[ExpandedSubBlockEnd.gif]: /images/20220828/4f0e882ad0d54e9086a7fcef7208832b.png
[ExpandedBlockEnd.gif]: /images/20220828/5d471c7d8046448da8a70cedeba4d78e.png