类的继承之构造函数和析构函数的顺序

#include <iostream>

/* run this program using the console pauser or add your own getch, system("pause") or input loop */
class someThing
{
    public:
        someThing(){std::cout << "someThing's construction function" << std::endl;}
        ~someThing(){std::cout << "someThing's destructor function" << std::endl;}
};
class parent
{
    public:
        parent(){std::cout << "parent's construction function" << std::endl;}
        ~parent(){std::cout << "parent's destructor function" << std::endl;}    
};

class child : public parent
{
    public:
        child(){std::cout << "child's construction function" << std::endl;}
        ~child(){std::cout << "child's destructor function" << std::endl;}
    private :
    someThing nameChild;    
};

int main(int argc, char** argv) 
{
    child myChild;
        
    return 0;
}

结果是：

parent's construction function
someThing's construction function
child's construction function
child's destructor function
someThing's destructor function
parent's destructor function

可见构造函数是从父类开始的，然后才是子类的构造函数。

析构函数刚好相反

这里存在一个问题就是：当我们声明一个指向parent的指针访问chlid的对象并且删除这个指针的时候就会发现，出问题啦！

parent* myChlid = new child;
delete myChlid;

结果是：

parent's construction function
someThing's construction function
child's construction function

parent's destructor function

因为当我们delete一个指针时，他是仅仅delete指向的那个类，这使得整个析构函数调用链断了。怎么解决这个问题呢？

我们只需要使得每个析构函数为virtual即可

virtual ~someThing(){std::cout << "someThing's destructor function" << std::endl;}

virtual ~parent(){std::cout << "parent's destructor function" << std::endl;}

virtual ~child(){std::cout << "child's destructor function" << std::endl;}

这样的话就可以得到我们期望的结果：

parent's construction function
someThing's construction function
child's construction function
child's destructor function
someThing's destructor function
parent's destructor function

最够贴上完整的代码：

#include <iostream>

/* run this program using the console pauser or add your own getch, system("pause") or input loop */
class someThing
{
    public:
        someThing(){std::cout << "someThing's construction function" << std::endl;}
        virtual ~someThing(){std::cout << "someThing's destructor function" << std::endl;}
};
class parent
{
    public:
        parent(){std::cout << "parent's construction function" << std::endl;}
        virtual ~parent(){std::cout << "parent's destructor function" << std::endl;}    
};

class child : public parent
{
    public:
        child(){std::cout << "child's construction function" << std::endl;}
        ~child(){std::cout << "child's destructor function" << std::endl;}
    private :
    someThing nameChild;    
};

int main(int argc, char** argv) 
{
    parent* myChlid = new child;
    delete myChlid;
        
    return 0;
}

结果是：

parent's construction function
someThing's construction function
child's construction function
child's destructor function
someThing's destructor function
parent's destructor function