析构函数什么情况下要定义为虚函数?

深藏阁楼爱情的钟 2022-01-07 13:17 250阅读 0赞

原文出处：[点击我][Link 1]：）

1.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.

2.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.

3.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;  
\}

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被定义为虚函数,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:只是释放了继承类的资源,再调用基类的析构函数.调用dosomething()函数执行的也是继承类定义的函数.

如果不需要基类对派生类及对象进行操作,则不能定义虚函数,因为这样会增加内存开销.当类里面有定义虚函数的时候,编译器会给类添加一个虚函数表,里面来存放虚函数指针,这样就会增加类的存储空间.所以,只有当一个类被用来作为基类的时候,才把析构函数写成虚函数.

转载于:https://www.cnblogs.com/yangtze736-2013-3-6/p/4438739.html

[Link 1]: http://blog.sina.com.cn/s/blog_7c773cc50100y9hz.html