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 }
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 }
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 }
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 }
结果：算出总面积 为 6
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
时间： 2024-08-02 17:19:57