多态性与虚拟函数一个典型的例子第一步

**#include <iostream> #include <strings.h> #include <cstring> using namespace std; class Point { public: Point(float x=0,float y=0); //输入带有默认参数的构造函数 void setPoint(float,float); //设置坐标值 float getX()const{return x;} //读入x float getY()const{return y;} //定义读入y值的常函数 friend ostream & operator<<(ostream &,const Point &); //重载运算符“<<” protected: float x,y; }; //下面定义Point类的成员函数 Point::Point(float a, float b) // 对x,y进行初始化 { x=a; y=b; } void Point::setPoint(float a, float b) { //对x,y赋新值 x=a; y=b; } ostream & operator <<(ostream &output,const Point &p) { output <<"["<<p.x<<","<<p.y<<"]"<<endl; return output; } int main() { Point p(3.5,6.4); cout<<"x="<<p.getX()<<",y="<<p.getY()<<endl; p.setPoint(8.5,6.8); cout<<"p(new):"<<p<<endl; }** **/home/andrew/文档/Clion/untitled5/cmake-build-debug/untitled5 x=3.5,y=6.4 p(new):[8.5,6.8] Process finished with exit code 0 **

函数说明，建立一个典型的例子； 一步一步的增加内容以免调试程序出现错误无法纠正

**#include <iostream> #include <strings.h> #include <cstring> using namespace std; class Point { public: Point(float x=0,float y=0); //输入带有默认参数的构造函数 void setPoint(float,float); //设置坐标值 float getX()const{return x;} //读入x float getY()const{return y;} //定义读入y值的常函数 friend ostream & operator<<(ostream &,const Point &); //重载运算符“<<” protected: float x,y; }; //下面定义Point类的成员函数 Point::Point(float a, float b) // 对x,y进行初始化 { x=a; y=b; } void Point::setPoint(float a, float b) { //对x,y赋新值 x=a; y=b; } class Circle:public Point { public: Circle(float x=0,float y=0,float r=0); void setRadius(float); //设置半径值 float getRadius()const; //定义常成员函数 float area()const; friend ostream &operator<<(ostream &,const Circle &); private: float radius; }; Circle::Circle(float a, float b, float r):Point(a,b),radius(r) {} //设置半径 void Circle::setRadius(float r) { radius=r; } float Circle::getRadius() const { return radius; } float Circle::area() const { return 3.1415926*radius*radius; } ostream & operator <<(ostream &output,const Circle &c) { output <<"Center=["<<c.x<<","<<c.y<<"],r="<<c.radius<<",area="<<c.area()<<endl; return output; } int main() { Circle c(3.5,6.4,5.2); cout<<"original circle :\nx="<<c.getX()<<",y="<<c.getY()<<",r="<<c.area()<<endl; return 0; }** **/home/andrew/文档/Clion/untitled5/cmake-build-debug/untitled5 original circle : x=3.5,y=6.4,r=84.9487 Process finished with exit code 0 **