virtual 析构函数

 有时候我们需要用基类的指针删除其子类的对象，这时候要非常注意，倘若基类的析构函数是 non-virtual析构函数，那么事实上，只有基类的析构函数被调用，派生类的析构函数并没有被调用，这可能会导致资源泄漏。为避免这中情况发生，我们可以将基类的析构函数声明为virtual，这样的话，子类对象才能被完全销毁.

#include <iostream>
#include <stdlib.h>
#include <crtdbg.h>

#ifdef _DEBUG
#define new new(_NORMAL_BLOCK, __FILE__, __LINE__)
#endif

using namespace std;

class BaseClass
{
public:
    virtual ~BaseClass() = 0;
    //~BaseClass();
};

BaseClass::~BaseClass()
{
    cout << "the ~BaseClass is called" << endl;
}

class DerivedClass : public BaseClass
{
public:
    DerivedClass(int x, int y)
        :X(x)
        ,Y(y)
    {
        a = new int[y];
    }

    ~DerivedClass()
    {
        cout << "the ~Derivedclass is called" << endl;
        if(a)
        {
            delete a;
            a = NULL;
        }
    }


private:
    int X;
    int Y;
    int *a;
};

void main()
{
    //memory check
    _CrtSetDbgFlag(_CrtSetDbgFlag(_CRTDBG_REPORT_FLAG) | _CRTDBG_LEAK_CHECK_DF);

    BaseClass *bs= new DerivedClass(2, 3);

    delete bs;

    system("pause");

}

程序运行结果为

the ~Derivedclass is called
the ~BaseClass is called

基类和派生类的析构函数都被调用

倘若基类的析构函数为~BaseClass();即 non-virtual 析构函数，

程序运行结果为

the ~BaseClass is called,

只有基类的析构函数被调用，子类中指针 a 指向的内存并没有被释放

Dumping objects ->
e:\work\test\test\test.cpp(287) : {128} normal block at 0x00395E38, 12 bytes long.
 Data: <            > CD CD CD CD CD CD CD CD CD CD CD CD
Object dump complete.

12 字节的内存泄漏了