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）
#include<iostream>
using namespace std; class base{
public:
/*virtual*/ void who(){ //define this function to virtual will be normal
cout << "this is the class of base !" << endl;
}
}; class derive1:public base{
public:
void who(){
cout << "this is the class of derive1 !" << endl;
}
};
class derive2 :public base{
public:
void who(){
cout << "this is the class of derive2 !" << endl;
}
};
int main(){
base beseObject, *p;
derive1 obj1;
derive2 obj2;
p = &beseObject;
p->who();
cout << "------------------------------" << endl;
p = &obj1;
p->who();
((derive1*)p)->who();
cout << "------------------------------" << endl;
p = &obj2;
p->who();
((derive2*)p)->who();
cout << "------------------------------" << endl;
obj1.who();
obj2.who();
int i;
cin >> i;
return ;
}
两次结果比较： 没加virtual:；加virtual后：
与重载的关系：
#include<iostream>
using namespace std; class base{
public:
virtual void f1(){ //virtual function
cout << "f1 function of base " << endl;
}
virtual void f2(){ //virtual function
cout << "f2 function of base " << endl;
}
virtual void f3(){ //virtual function
cout << "f3 function of base " << endl;
}
void f4(){
cout << "f4 function of base " << endl;
}
}; class derive:public base{
public:
void f1(){ //virtual function
cout << "f1 function of derive " << endl;
}
virtual void f2(int x){ //lose virtual characteristic
cout << "f2 function of derive " << endl;
}
//f3(){ //wrong, not the same return type
// cout << "f3 function of base " << endl;
//}
void f4(){ //normal overload
cout << "f4 function of derive " << endl;
}
};
int main(){
base obj1, *ptr;
derive obj2;
ptr = &obj1;
ptr->f1();
ptr->f2();
ptr->f3();
ptr = &obj2;
ptr->f1();
ptr->f2();
ptr->f4();
int i;
cin >> i;
return ;
}
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。
#include<iostream>
using namespace std; class base{
public:
virtual void print(){
cout << "calss base!" << endl;
}
};
class son:public base{
public:
virtual void print(){ //empty virtual class
}
};
class grandson :public son{
public:
void print(){
cout << "calss grandson!" << endl;
}
};
void show(base* b){
b->print();
}
int main(){
base *pbase = new base;
son *pson = new son;
grandson *pgrandson = new grandson;
show(pbase);
show(pson);
show(pgrandson);
int i;
cin >> i;
return ;
}
结果：
存虚函数与抽象类
#include<iostream>
using namespace std; class shape{ //抽象类里必须有一个纯虚函数
public:
virtual float area() = ;
};
class triangle :public shape{
protected:
float h, w;
public:
triangle(float hh, float ww){
h = hh; w = ww;
}
float area(){
return h*w*0.5;
}
};
class rectangle :public shape{
protected:
float h, w;
public:
rectangle(float hh, float ww){
h = hh; w = ww;
}
float area(){
return h*w;
}
};
float total(shape* s[], int n){
float sum = ;
for (int i = ; i < n - ; i++)
sum += s[i]->area();
return sum;
}
int main(){
shape* s[];
s[] = new triangle(, );
s[] = new rectangle(, );
float sum = total(s, );
cout <<"total area is: "<< sum << endl;;
int i;
cin >> i;
return ;
}
结果：算出总面积 为 6