cpp虚函数，基类命名指向子类实例必备佳选

Posted 2021-11-11 飞凡可期

简单虚函数

//============================================================================
// Name        : p1022VirtualDeconstructor.cpp
// Author      : perfey
// Version     :
// Copyright   : Your copyright notice
// Description : Hello World in C++, Ansi-style
//============================================================================

#include <iostream>
using namespace std;

class Base
{
public:
	Base(){cout << "Conctructing Base!"<<endl;}

	virtual ~Base(){cout << "Deconstructing Base!"<<endl;}//多一个空格，virtual和~之间
};

class Derive: public Base
{
public:
	  Derive(){cout << "Constructing Derive!"<<endl;}
      ~ Derive(){cout << "Deconstructing Derive!"<<endl;}
     /*
      * 若是不加virtual，只输出构建base, Derive, 解构Base 三步，没有解构 derive？ 这个有问题了
      *
      * */
};


int main() {
	cout << "!!!Hello World!!!" << endl; // prints !!!Hello World!!!

	Base *bptr = new Derive(); //父类指针指向之类实例；
	delete bptr;

	return 0;
}

结果

Conctructing Base!
Constructing Derive!
Deconstructing Derive!
Deconstructing Base!

（如果没有 tilde~前面那个virtual修饰，那么不会有Deconstructing Derive！这一句）

继续传递，继承下去

//============================================================================
// Name        : p1022VirtualDeconstructor.cpp
// Author      : perfey
// Version     :
// Copyright   : Your copyright notice
// Description : Hello World in C++, Ansi-style
//============================================================================

#include <iostream>
using namespace std;

class Base
{
public:
	Base(){cout << "Conctructing Base!"<<endl;}

	virtual ~Base(){cout << "Deconstructing Base!"<<endl;}//多一个空格，virtual和~之间
};

class Derive: public Base
{
public:
	  Derive(){cout << "Constructing Derive!"<<endl;}
      ~ Derive(){cout << "Deconstructing Derive!"<<endl;}
     /*
      * 若是不加virtual，只输出构建base, Derive, 解构Base 三步，没有解构 derive？ 这个有问题了
      *
      * */
};

class Derive2: public Derive
{
public:
	  Derive2(){cout << "Constructing DeriveSon!"<<endl;}
      ~ Derive2(){cout << "Deconstructing DeriveSon!"<<endl;}
     /*
      * 若是不加virtual，只输出构建base, Derive, 解构Base 三步，没有解构 derive？ 这个有问题了
      *
      * */
};


int main() {
	cout << "!!!Hello World!!!" << endl; // prints !!!Hello World!!!

	Base *bptr = new Derive2(); //父类指针指向之类实例；
	delete bptr;

	return 0;
}

结果

!!!Hello World!!!
Conctructing Base!
Constructing Derive!
Constructing DeriveSon!
Deconstructing DeriveSon!
Deconstructing Derive!
Deconstructing Base!

可以看到每一个子类都继承虚函数，都调用了Deconstructor这是预期的效果。

继续花样

//============================================================================
// Name        : p1022VirtualDeconstructor.cpp
// Author      : perfey
// Version     :
// Copyright   : Your copyright notice
// Description : Hello World in C++, Ansi-style
//============================================================================

#include <iostream>
using namespace std;

class Base
{
public:
	Base(){cout << "Conctructing Base!"<<endl;}

	virtual ~Base(){cout << "Deconstructing Base!"<<endl;}//多一个空格，virtual和~之间

    virtual void Eat() = 0; //纯需函数，无需在基类中实现，奇怪吧，类似interface中概念
};

class Derive: public Base
{
public:
	  Derive(){cout << "Constructing Derive!"<<endl;}

       void Eat()override
    		  {
    	  cout <<" 我吃生肉，茹毛饮血！！"<<endl;
              }

      ~ Derive(){cout << "Deconstructing Derive!"<<endl;}
};

/*
 * 若是不加virtual，只输出构建base, Derive, 解构Base 三步，没有解构 derive？ 这个有问题了
 *
 * */

class Derive2: public Derive
{
public:
	  Derive2(){cout << "Constructing DeriveSon!"<<endl;}
      ~ Derive2(){cout << "Deconstructing DeriveSon!"<<endl;}
     void Eat() {
    	  cout <<" 我今晚大口吃肉！"<<endl;
      }
};


int main() {
	cout << "!!!Hello World!!!" << endl; // prints !!!Hello World!!!

	Base *bptr = new Derive(); //父类指针指向之类实例；
	bptr->Eat();
	bptr = new Derive2();
	bptr->Eat();

	delete bptr;

	return 0;
}

纯虚函数来结果

!!!Hello World!!!
Conctructing Base!
Constructing Derive!
我吃生肉，茹毛饮血！！
Conctructing Base!
Constructing Derive!
Constructing DeriveSon!
我今晚大口吃肉！
Deconstructing DeriveSon!
Deconstructing Derive!
Deconstructing Base!