#include"iostream" using namespace std; class Shape { public: virtual float area()const { return 0.0; } virtual float volume()const { return 0.0; } virtual void shapeName()const = 0; }; class Point :public Shape { protected: float x, y; public: Point(float = 0, float = 0); void setPoint(float, float); float getX()const { return x;} float getY()const { return y; } virtual void shapeName()const { cout << "Point: ";} friend ostream & operator<<(ostream&, const Point &); }; Point::Point(float a, float b) { x = a; y = b; } void Point::setPoint(float a, float b) { x = a; y = b; } ostream & operator<<(ostream &output, const Point &p) { return output; } class Circle:public Point { public: Circle(float x = 0,float y = 0,float r = 0); void setRadius(float); float getRadius() const; virtual float area() const; virtual void shapeName() const { cout << "Circle:"; } friend ostream &operator<<(ostream &, const Circle &); protected: float radius; }; Circle::Circle(float a, float b, float r) : Point(a, b),radius(r) {} void Circle::setRadius(float r) {radius = r;} float Circle::getRadius() const { return radius; } float Circle::area() const { return 3.14159 * radius * radius; } ostream &operator<<(ostream &output,const Circle &c) { output << "[" << c.x << "," << c.y << "],r=" << c.radius; return output; } class Cylinder : public Circle { public: Cylinder(float x = 0, float y = 0, float r = 0, float h = 0); void setHeight(float); virtual float area() const; virtual float volume() const; virtual void shapeName() const { cout<<"Cylinder:"; } friend ostream &operator<<(ostream &, const Cylinder&); protected: float height; }; Cylinder::Cylinder(float a, float b, float r, float h): Circle(a, b, r), height(h) {} void Cylinder::setHeight(float h) { height = h; } float Cylinder :: area() const{return 2 * Circle::area() + 2 * 3.14159 * radius * height;} float Cylinder :: volume() const { return Circle::area() * height; } ostream &operator << (ostream &output, const Cylinder& cy) { output << "[" << cy.x << "," << cy.y << "],r=" << cy.radius << ",h=" << cy.height; return output; } int main() { Point point(3.2, 4.5); Circle circle(2.4, 1.2, 5.6); Cylinder cylinder(3.5, 6.4, 5.2, 10.5); point.shapeName(); cout << point<<endl; circle.shapeName(); cout<< circle << endl; cylinder.shapeName(); cout << cylinder << endl << endl; Shape * pt; pt = &point; pt->shapeName(); cout << "x=" << point.getX() << ",y=" << point.getY() << "\narea=" << pt->area() << "\nvolume=" << pt->volume() << "\n\n"; pt = &circle; pt->shapeName(); cout << "x=" << circle.getX() << ",y=" << circle.getY() << "\narea=" << pt->area() << "\nvolume=" << pt->volume() << "\n\n"; pt=&cylinder; pt->shapeName(); cout << "x=" << cylinder.getX() << ",y=" << cylinder.getY() << "\narea=" << pt->area() << "\nvolume=" << pt->volume() << "\n\n"; return 0; }为什么第一个Point不能输出数据

在你的代码中，第一个Point对象的输出运算符重载函数operator<<没有实际的输出语句，所以调用该函数时没有任何输出。你需要在该函数中添加输出语句，例如：

ostream & operator<<(ostream &output, const Point &p) {
    output << "[" << p.x << "," << p.y << "]";
    return output;
}

这样就能正确输出第一个Point对象的数据了。