为什么一般要定义析构函数为虚析构函数

没有使用虚析构函数可能会出现的问题：

      #include <iostream>

#include <string>

using namespace std;



class A {

public:

	A() { cout << "
      
        A constructor
      
      " << endl; }

	~A() { cout << "
      
        A destructor
      
      " << endl; }

};



class B: public A {

	
      
        char
      
       *buf;

public:

	B() { buf = new 
      
        char
      
      [10]; cout << "
      
        B constructor
      
      " << endl; }

	~B() { cout << "
      
        B destructor
      
      " << endl; }

};




      
        int
      
       main()

{

	A *p = new B;

	delete p;  // p是基类类型指针，仅调用基类析构函数，造成B中申请的10字节内存没被释放。

	
      
        return
      
       0;

}




    

      #include <iostream>

#include <string>

using namespace std;



class A {

public:

	A() { cout << "
      
        A constructor
      
      " << endl; }

	virtual ~A() { cout << "
      
        A destructor
      
      " << endl; }

};



class B: public A {

	
      
        char
      
       *buf;

public:

	B() { buf = new 
      
        char
      
      [10]; cout << "
      
        B constructor
      
      " << endl; }

	~B() { cout << "
      
        B destructor
      
      " << endl; }

};




      
        int
      
       main()

{

	A *p = new B;

	delete p; //A中的析构函数说明为虚析构函数后，B中析构函数自动成为虚析构函数，由于P
    

                        //指向派生类对象，因此会调用派生类B的析构函数。

	
      
        return
      
       0;

}