1. 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;
2. 运行结果
delete father()
delete father()
delete son()
delete father()
delete sonson()
delete son()
delete father()
delete father()
3. 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：
结论与virtual关键字在子类类似，可以对照着看。
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。
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。
4. 其它测试
5. 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;
}