c++ 虚析构函数的作用分析

1.为什么基类的析构函数是虚函数？　　在实现多态时，当用基类指针操作派生类对象时，在析构时防止只析构基类而不析构派生类的状况发生。       什么是多态：根据面向对象的继承规则，派生类跟基类是IS-A的关系。也就是说派生类的对象也是一个基类对象。所以基类的指针可以指向派生类的对象以便实现多态。（让基类实现多态）      亦即：这样做是为了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；     下面看代码----------------------------------------------------------------------------------------------------------　　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;
}  

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.    当然，如果基类ClxBase中的dosomething()不是虚成员，那么指针p->dosomething()调用的还是基类对象的成员。    另外，基类对象如果调用了一个派生类对象专有的成员，那么编译不通过：

#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; }<span style="color:#ff6666;">   void test() {cout << "I am the test! << endl";}</span>};int   main(){ClxBase *p =  new ClxDerived;p->DoSomething();

  p->Test();delete p;return 0;}

编译结果如下：

xigou.cpp: In function 'int main()':xigou.cpp:21: error: 'class ClxBase' has no member named 'Test'

    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,而且基类指针用来指向派生类对象时（通常这样做 是为了实现基类对象的多态）才把析构函数写成虚函数.