析构函数的virtual在子类

Posted 2022-03-12 windmissing

运行结果是什么？

father *pf = new father();
delete pf;
father *ps = new son();
delete ps;
son *ps1 = new son();
delete ps1;
son *pss = new sonson();
delete pss;
father *pss1 = new sonson();
delete pss1;

运行结果

delete father()
delete father()
delete son()
delete father()
delete sonson()
delete son()
delete father()
delete father()

解释与结论

virtual关键字在父类和virtual关键字在子类已经说明virtual关键字在父类的情况能够正常地进行多态。因此这里仅把关注点放在virtual关键字在子类的情况。

由于析构函数与普通函数略有不同，把virtual关键字在子类的情况对析构函数再测一遍。结论与virtual关键字在子类类似。

id	指针名	指针的类型	实际的类型	运行结果	解释
1	pf	father	father	delete father()
2	ps	father	son	delete father()	实际调用的是指针的类型(father)的析构函数，说明没有多态
3	ps1	son	son	delete son() delete father()	调用了son()，son()又调用了~father() ，由编译器完成。
4	pss	son	sonson	delete sonson() delete son() delete father()	实际调用的是对象的类型(sonson)的函数，说明有多态的效果。sonson()又调用了son()以及~father()，由编译器完成。
5	pss1	father	sonson	delete father()	实际调用的是指针的类型(father)的析构函数，说明没有多态

结论：

1. 如果virtual只写在派生类的析构函数中，而没有写在基类中的析构函数中，则不会有多态的效果。有可能只析构基类部分而不析构派生类部分，造成内存泄漏。

2. 如果在某一层的派生类中加了virtual标签，那么从这一层开始以后的每一层，这个函数都会有多态的效果。

其它测试

virtual关键字在父类

virtual关键字在子类

间接调用虚函数

完整代码

#include <iostream>
using namespace std;

class father

public:
  ~father()
  
    cout<<"delete father()"<<endl;
  
;

class son : public father

public:
  virtual ~son()
  
    cout<<"delete son()"<<endl;
  
;

class sonson : public son

public:
  ~sonson()
  
    cout<<"delete sonson()"<<endl;
  
;

int main()

  father *pf = new father();
  delete pf;
  father *ps = new son();
  delete ps;
  son *ps1 = new son();
  delete ps1;
  son *pss = new sonson();
  delete pss;
  father *pss1 = new sonson();
  delete pss1;
  return 0;