C++自学笔记32（虚析构函数）

在以往的笔记中我们讲到过析构函数和虚函数。析构函数是释放被初始化的变量，虚函数是告诉编译器有重名的函数被复写去派生类找对应函数。

虚析构函数就是在基类析构函数前加入virtual表示派生类引用析构函数需要找派生类。看以下栗子。

#include <iostream>

class Base
{
publi：
    Base(){std::cout<<Base Constructor<<std::endl;}
    ~Base(){std::cout<<Base Destructor<<std::endl;}
}；
class Derived：public Base
{
public：
    Derived(){std::cout<<Derived Constructor<<std::endl;}
    ~Derived(){std::cout<<Derived Destructor<<std::endl;}
}；
int mian()
{
    Base* base = new Base();
    delete base;
    std::cout<<"_____________________________________"<<std::endl;
    Derived* derived= new Derived();
    delete derived;
    std::cout<<"_____________________________________"<<std::endl;
    Base* ptr = new Derived();
    delete ptr;
    std::cout<<"_____________________________________"<<std::endl;
    std::cin.get();
}

基类和派生类分别有自己的构造函数和析构函数，打印出的为

Base Constructor

Base Destructor

_____________________________________

Base Constructor

Derived Constructor

Derived Destructor

Base Destructor

_____________________________________

Base Constructor

Derived Constructor

Base Destructor

_____________________________________

我们来逐行解析main中代码，第一块创建Base基本类并删除base，打印对应构造与析构函数没有问题。

第二块创建Derived派生类，因为其基于Base所以需要将Base类初始化和析构一同执行，从打印结果也可以看出先执行基类构造、派生构造、派生析构、基类析构。

第三块代码什么问题，我们创建了Base类指针但实际上指向的是Derived类型指针（new是实际创建的，前面只是命名的）怎么解决呢？

#include <iostream>

class Base
{
publi：
    Base(){std::cout<<Base Constructor<<std::endl;}
    virtual ~Base(){std::cout<<Base Destructor<<std::endl;}
}；
class Derived：public Base
{
public：
    Derived(){std::cout<<Derived Constructor<<std::endl;}
    ~Derived(){std::cout<<Derived Destructor<<std::endl;}
}；
int mian()
{
    Base* base = new Base();
    delete base;
    std::cout<<"_____________________________________"<<std::endl;
    Derived* derived= new Derived();
    delete derived;
    std::cout<<"_____________________________________"<<std::endl;
    Base* ptr = new Derived();
    delete ptr;
    std::cout<<"_____________________________________"<<std::endl;
    std::cin.get();
}

我们看到没有执行派生类的析构函数，所以我们在基类的析构函数前加上virtual使其变为虚函数。virtual意味着其存在的类存在派生类且析构函数可能会被调用，所以建立一个表供调用时查找。

Base是Derived的基类所以Derived就是Base，注意下类的范围。

Base Constructor

Base Destructor

_____________________________________

Base Constructor

Derived Constructor

Derived Destructor

Base Destructor

_____________________________________

Base Constructor

Derived Constructor

Derived Destructor

Base Destructor

_____________________________________