C++知识点10：构造函数和析构函数

构造函数：

生成对象时调用构造函数
生成派生类对象时先调用基类构造函数在调用派生类构造函数

#include <iostream>
using namespace std;

class base
{
public:
    base(){cout<<"base"<<endl;}
    virtual ~base(){}
};

class derived:public base
{
public:
    derived(){cout<<"derived"<<endl;}
    ~derived(){}
};
int main()
{
    base *point=new derived();
    return 0;
}

输出结果为：
base
derived

析构函数：

撤销对象时调用析构函数
撤销派生类对象时，先调用派生类的析构函数再调用基类的析构函数。

#include <iostream>
using namespace std;

class base
{
public:
    base(){}
    //virtual ~base(){}
    ~base(){cout<<"base"<<endl;}
};

class derived:public base
{
public:
    derived(){}
    ~derived(){cout<<"derived"<<endl;}
};
int main()
{
    derived *p=new derived;
    delete p;
    return 0;
}

输出结果为：
derived
base

虚拟的析构函数：

基类指针指向派生类对象时，撤销指针
当基类析构函数不叫virtual关键字，则下列代码输出结果为base（只调用基类析构函数）

#include <iostream>
using namespace std;

class base
{
public:
    base(){}
    ~base(){cout<<"base"<<endl;}//正常的析构函数
};

class derived:public base
{
public:
    derived(){}
    ~derived(){cout<<"derived"<<endl;}
};
int main()
{
    base *point=new derived();
    delete point;
    return 0;
}

基类指针指向派生类对象时，
当基类析构函数加virtual关键字时，撤销指针先调用派生类析构函数再调用基类析构函数
输出结果：
derived
base

#include <iostream>
using namespace std;

class base
{
public:
    base(){}
    virtual ~base(){cout<<"base"<<endl;}//虚拟的析构函数
};

class derived:public base
{
public:
    derived(){}
    ~derived(){cout<<"derived"<<endl;}
};
int main()
{
    base *point=new derived();
    delete point;
    return 0;
}