析构函数前加virtual关键字的作用?

来源：互联网发布：多益网络上市了吗编辑：程序博客网时间：2024/06/06 10:07

大家知道，析构函数是为了在对象不被使用之后释放它的资源，虚函数是为了实现多态。那么把析构函数声明为vitual有什么作用呢？请看下面的代码：

#include<iostream>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。

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