c++虚析构函数

1.为什么基类的析构函数是虚函数？

　　在实现多态时，当用基类操作派生类，在析构时防止只析构基类而不析构派生类的状况发生。

　　下面转自网络：源地址 http://blog.sina.com.cn/s/blog_7c773cc50100y9hz.html

　　a.第一段代码

#include<iostream>
using namespace std;
class ClxBase{
public:ClxBase() {};~ClxBase() {cout << "Output from the destructor of class ClxBase!" << endl;};void DoSomething() { cout << "Do something in class ClxBase!" << endl; };
};class ClxDerived : public ClxBase{
public:ClxDerived() {};~ClxDerived() { cout << "Output from the destructor of class ClxDerived!" << endl; };void DoSomething() { cout << "Do something in class ClxDerived!" << endl; };
};int   main(){  ClxDerived *p =  new ClxDerived;p->DoSomething();delete p;return 0;}

运行结果：

　　Do something in class ClxDerived!

　　Output from the destructor of class ClxDerived!

　　Output from the destructor of class ClxBase!

　　这段代码中基类的析构函数不是虚函数,在main函数中用继承类的指针去操作继承类的成员,释放指针P的过程是:先释放继承类的资源,再释放基类资源.

　　b.第二段代码

#include<iostream>
using namespace std;
class ClxBase{
public:ClxBase() {};~ClxBase() {cout << "Output from the destructor of class ClxBase!" << endl;};void DoSomething() { cout << "Do something in class ClxBase!" << endl; };
};class ClxDerived : public ClxBase{
public:ClxDerived() {};~ClxDerived() { cout << "Output from the destructor of class ClxDerived!" << endl; };void DoSomething() { cout << "Do something in class ClxDerived!" << endl; }
};int   main(){  ClxBase *p =  new ClxDerived;p->DoSomething();delete p;return 0;}

输出结果：

　　Do something in class ClxBase!
　　Output from the destructor of class ClxBase!

这段代码中基类的析构函数同样不是虚函数,不同的是在main函数中用基类的指针去操作继承类的成员,释放指针P的过程是:只是释放了基类的资源,而没有调用继承类的析构函数.调用　　dosomething()函数执行的也是基类定义的函数.

一般情况下,这样的删除只能够删除基类对象,而不能删除子类对象,形成了删除一半形象,造成内存泄漏.

在公有继承中,基类对派生类及其对象的操作,只能影响到那些从基类继承下来的成员.如果想要用基类对非继承成员进行操作,则要把基类的这个函数定义为虚函数.

析构函数自然也应该如此:如果它想析构子类中的重新定义或新的成员及对象,当然也应该声明为虚的.

　　c.第三段代码：

#include<iostream>
using namespace std;
class ClxBase{
public:ClxBase() {};virtual ~ClxBase() {cout << "Output from the destructor of class ClxBase!" << endl;};virtual void DoSomething() { cout << "Do something in class ClxBase!" << endl; };
};class ClxDerived : public ClxBase{
public:ClxDerived() {};~ClxDerived() { cout << "Output from the destructor of class ClxDerived!" << endl; };void DoSomething() { cout << "Do something in class ClxDerived!" << endl; };
};int   main(){  ClxBase *p =  new ClxDerived;p->DoSomething();delete p;return 0;}