C++中虚析构函数的作用

C++中用来做基类的类的析构函数一般都是虚函数。把析构函数声明为vitual有什么作用呢？请看下面的代码：

#include
using namespace std;

class Base
{
public:
Base() {};       //Base的构造函数
~Base()        //Base的析构函数
{
cout << "Output from the destructor of class Base!" << endl;
};
virtual void DoSomething()
{
cout << "Do something in class Base!" << endl;
};
};

class Derived : public Base
{
public:
Derived() {};     //Derived的构造函数
~Derived()      //Derived的析构函数
{
cout << "Output from the destructor of class Derived!" << endl;
};
void DoSomething()
{
cout << "Do something in class Derived!" << endl;
};
};

int main()
{
Derived *pTest1 = new Derived();   //Derived类的指针
pTest1->DoSomething();
delete pTest1;

cout << endl;

Base *pTest2 = new Derived();      //Base类的指针
pTest2->DoSomething();
delete pTest2;

return 0;
}

先看程序输出结果：

1 Do something in class Derived!

2 Output from the destructor of class Derived!

3 Output from the destructor of class Base!

4

5 Do something in class Derived!

6 Output from the destructor of class Base!

代码第36行可以正常释放pTest1的资源，而代码第42行没有正常释放pTest2的资源，因为从结果看Derived类的析构函数并没有被调用。通常情况下类的析构函数里面都是释放内存资源，而析构函数不被调用的话就会造成内存泄漏。原因是指针pTest2是Base类型的指针，释放pTest2时只进行Base类的析构函数。在代码第8行前面加上virtual关键字后的运行结果如下：

1 Do something in class Derived!

2 Output from the destructor of class Derived!

3 Output from the destructor of class Base!

4

5 Do something in class Derived!

6 Output from the destructor of class Derived!

7 Output from the destructor of class Base!

此时释放指针pTest2时，由于Base的析构函数是virtual的，就会先找到并执行Derived类的析构函数，然后再执行Base类的析构函数，资源正常释放，避免了内存泄漏。

因此，只有当一个类被用来作为基类的时候，才会把析构函数写成虚函数。这样当用一个基类的指针删除一个派生类的对象时，派生类的析构函数会被调用。