C++虚函数(5) - 虚析构函数

使用一个指向基类(定义了一个non-virtual析构函数)的指针，来删除一个子类时，会导致未知行为。这种情况下，基类必须定义虚析构函数。

Source: https://www.securecoding.cert.org/confluence/display/cplusplus/OOP34-CPP.+Ensure+the+proper+destructor+is+called+for+polymorphic+objects
例如，下面程序会导致未知行为。

//此程序中基类没有定义虚析构函数，会导致异常行为
#include<iostream>
using namespace std;

class base {
public:
base()
{ cout<<"Constructing base \n"; }
~base()
{ cout<<"Destructing base \n"; }
};

class derived: public base {
public:
derived()
{ cout<<"Constructing derived \n"; }
~derived()
{ cout<<"Destructing derived \n"; }
};

int main(void)
{
derived *d = new derived();
base *b = d;
delete b;
return 0;
}

不同的编译器可能会打印不同结果。下面是visual studio 2015的结果：

Constructing base

Constructing derived

Destructing base

将基类的析构函数定义为virtual后, 就可以确保子类能被正确的析构了。

参考下面例子：

// 使用虚析构函数的例子程序
#include<iostream>
using namespace std;
class base {
public:
base()
{ cout<<"Constructing base \n"; }
virtual ~base()
{ cout<<"Destructing base \n"; }
};

class derived: public base {
public:
derived()
{ cout<<"Constructing derived \n"; }
~derived()
{ cout<<"Destructing derived \n"; }
};

int main(void)
{
derived *d = new derived();
base *b = d;
delete b;
return 0;
}

输出：
Constructing base

Constructing derived

Destructing derived

Destructing base

做为一个原则, 任何时候只有一个类中拥有虚函数，则应该立即给这个类增加一个虚析构函数(尽管它也许不会做什么)。