虚函数的使用 以及虚函数与重载的关系, 空虚函数的作用，纯虚函数->抽象类，基类虚析构函数使释放对象更彻底

为了访问公有派生类的特定成员，可以通过讲基类指针显示转换为派生类指针。

也可以将基类的非静态成员函数定义为虚函数（在函数前加上virtual）

 1 #include<iostream>

 2 using namespace std;

 3 

 4 class base{

 5 public:

 6     /*virtual*/ void who(){    //define this function to virtual will be normal

 7         cout << "this is the class of base !" << endl;

 8     }

 9 };

10 

11 class derive1:public base{

12 public:

13     void who(){

14         cout << "this is the class of derive1 !" << endl;

15     }

16 };

17 class derive2 :public base{

18 public:

19     void who(){

20         cout << "this is the class of derive2 !" << endl;

21     }

22 };

23 int main(){

24     base beseObject, *p;

25     derive1 obj1;

26     derive2 obj2;

27     p = &beseObject;

28     p->who();

29     cout << "------------------------------" << endl;

30     p = &obj1;

31     p->who();

32     ((derive1*)p)->who();

33     cout << "------------------------------" << endl;

34     p = &obj2;

35     p->who();

36     ((derive2*)p)->who();

37     cout << "------------------------------" << endl;

38     obj1.who();

39     obj2.who();

40     int i;

41     cin >> i;

42     return 0;

43 }

View Code

两次结果比较： 没加virtual:；加virtual后：

 

与重载的关系：

 1 #include<iostream>

 2 using namespace std;

 3 

 4 class base{

 5 public:

 6     virtual void f1(){    //virtual function

 7         cout << "f1 function of base " << endl;

 8     }

 9     virtual void f2(){    //virtual function

10         cout << "f2 function of base " << endl;

11     }

12     virtual void f3(){    //virtual function

13         cout << "f3 function of base " << endl;

14     }

15     void f4(){    

16         cout << "f4 function of base " << endl;

17     }

18 };

19 

20 class derive:public base{

21 public:

22     void f1(){    //virtual function

23         cout << "f1 function of derive " << endl;

24     }

25     virtual void f2(int x){    //lose virtual characteristic

26         cout << "f2 function of derive " << endl;

27     }

28     //f3(){    //wrong, not the same return type

29     //    cout << "f3 function of base " << endl;

30     //}

31     void f4(){    //normal overload

32         cout << "f4 function of derive " << endl;

33     }

34 };

35 int main(){

36     base obj1, *ptr;

37     derive obj2;

38     ptr = &obj1;

39     ptr->f1();

40     ptr->f2();

41     ptr->f3();

42     ptr = &obj2;

43     ptr->f1();

44     ptr->f2();

45     ptr->f4();

46     int i;

47     cin >> i;

48     return 0;

49 }

View Code

结果：

 

 

空虚函数在中间类里必须声明，以保证其后的派生类能使用该虚函数。建立一条从虚函数到派生类的虚函数路径。

 1 #include<iostream>

 2 using namespace std;

 3 

 4 class base{

 5 public:

 6     virtual void print(){

 7         cout << "calss base!" << endl;

 8     }

 9 };

10 class son:public base{

11 public:

12     virtual void print(){ //empty virtual class

13     }

14 };

15 class grandson :public son{

16 public:

17     void print(){

18         cout << "calss grandson!" << endl;

19     }

20 };

21 void show(base* b){

22     b->print();

23 }

24 int main(){

25     base *pbase = new base;

26     son *pson = new son;

27     grandson *pgrandson = new grandson;

28     show(pbase);

29     show(pson);

30     show(pgrandson);

31     int i;

32     cin >> i;

33     return 0;

34 }

View Code

结果：

 

存虚函数与抽象类

 1 #include<iostream>

 2 using namespace std;

 3 

 4 class shape{  //抽象类里必须有一个纯虚函数

 5 public:

 6     virtual float area() = 0;

 7 };

 8 class triangle :public shape{

 9 protected:

10     float h, w;

11 public:

12     triangle(float hh, float ww){

13         h = hh; w = ww;

14     }

15     float area(){

16         return h*w*0.5;

17     }

18 };

19 class rectangle :public shape{

20 protected:

21     float h, w;

22 public:

23     rectangle(float hh, float ww){

24         h = hh; w = ww;

25     }

26     float area(){

27         return h*w;

28     }

29 };

30 float total(shape* s[], int n){

31     float sum = 0;

32     for (int i = 0; i < n - 1; i++)

33         sum += s[i]->area();

34     return sum;

35 }

36 int main(){

37     shape* s[2];

38     s[0] = new triangle(3, 4);

39     s[1] = new rectangle(3, 4);

40     float sum = total(s, 2);

41     cout <<"total area is: "<< sum << endl;;

42     int i;

43     cin >> i;

44     return 0;

45 }

View Code

结果：算出总面积 为 6