C++中的析构函数为什么通常为虚函数

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#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;
}

运行结果：

Do something in classClxDerived!

Output from the destructor of classClxDerived!

Output from the destructor of classClxBase!

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。

#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;
}

运行结果：

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。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。

#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;
}

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的过程是先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。