float getRadius（）const中const怎么理解

在这个函数声明中，`const`关键字被用于限定该成员函数不会修改该类的任何数据成员，即该函数是一个只读函数，它不会对对象的状态进行修改。这样做的目的是为了保证函数的安全性和可靠性，避免在函数调用过程中意外地修改了对象的状态，从而导致代码出现难以调试的错误。

因此，`const`关键字可以被理解为一种修饰符，用于声明该函数是一个只读函数，不会修改任何数据成员。在函数的实现中，如果尝试修改数据成员或调用其他非const成员函数，编译器会报错。

相关问题

#include<iostream> #include<cmath> using namespace std; class Point{ protected: float x,y; public: Point(float a,float b); void setPoint(float,float); float getX(){return x;} float getY(){return y;} 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){ output<<"["<<p.x<<","<<p.y<<"]"<<endl; return output; } //======================================== class Circle:public Point{ protected: float radius; public: Circle(float x=0,float y=0,float r=0);//构造函数 void setRadius(float); float getRadius() const; float Area() const; friend ostream &operator<<(ostream &,const Circle &); }; Circle::Circle(float x,float y,float r):Point(x,y),radius(r){} void Circle::setRadius(float r){radius=r;} float Circle::getRadius() const {return radius;} float Circle::Area() const {return 3.14*radius*radius;} ostream &operator<<(ostream &output,const Circle &c){ output<<"Center=["<<c.x<<","<<c.y<<"],r="<<c.radius<<",area="<<c.Area()<<endl; return output; } class Cylinder:public Circle{ protected: float height; public: Cylinder(float x=0,float y=0,float r=0,float h=0); void setHeight(float); float getHeight() const; float Area() const; float volume() const; friend ostream& operator<<(ostream & ,const Cylinder &); }; 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::getHeight() const {return height;} float Cylinder::Area() const {return 2*Circle::Area()+2*3.14*radius*height;} float Cylinder::volume() const {return Circle::Area()*height;} ostream& operator<<(ostream &output ,const Cylinder &cy) { output<<"Center=["<<cy.x<<","<<cy.y<<"],r="<<cy.radius<<",h="<<cy.height<<"area"<<cy.Area()<<",volume"<<cy.volume()<<endl; return output; } int main(){ Cylinder cy1(3.5,6.4,5.2,10); cout<<"x="<<cy1.getX()<<",y="<<cy1.getY()<<",r="<<cy1.getRadius()<<",h="<<cy1.getHeight<<",area="<<cy1.Area()<<",volume="<<cy1.volume()<<endl; cy1.setHeight(15); cy1.setRadius(7.5); cy1.setPoint(5,5); cout<<"new"<<cy1;

你提供的代码存在一些问题。在调用 `cy1.getHeight` 和 `cy1.getHeight` 函数时，缺少了函数调用的括号。此外，在使用 `operator<<` 输出 `cy1` 时，缺少了分号。以下是修改后的代码：

#include<iostream>
#include<cmath>
using namespace std;

class Point{
protected:
    float x, y;
public:
    Point(float a, float b);
    void setPoint(float, float);
    float getX() { return x; }
    float getY() { return y; }
    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){
    output << "[" << p.x << "," << p.y << "]" << endl;
    return output;
}

class Circle: public Point{
protected:
    float radius;
public:
    Circle(float x = 0, float y = 0, float r = 0);
    void setRadius(float);
    float getRadius() const;
    float Area() const;
    friend ostream& operator<<(ostream&, const Circle&);
};

Circle::Circle(float x, float y, float r): Point(x, y){
    radius = r;
}

void Circle::setRadius(float r){
    radius = r;
}

float Circle::getRadius() const {
    return radius;
}

float Circle::Area() const {
    return 3.14 * radius * radius;
}

ostream& operator<<(ostream& output, const Circle& c){
    output << "Center=[" << c.x << "," << c.y << "], r=" << c.radius << ", area=" << c.Area() << endl;
    return output;
}

class Cylinder: public Circle{
protected:
    float height;
public:
    Cylinder(float x = 0, float y = 0, float r = 0, float h = 0);
    void setHeight(float);
    float getHeight() const;
    float Area() const;
    float volume() const;
    friend ostream& operator<<(ostream&, const Cylinder&);
};

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::getHeight() const {
    return height;
}

float Cylinder::Area() const {
    return 2 * Circle::Area() + 2 * 3.14 * radius * height;
}

float Cylinder::volume() const {
    return Circle::Area() * height;
}

ostream& operator<<(ostream& output, const Cylinder& cy){
    output << "Center=[" << cy.x << "," << cy.y << "], r=" << cy.radius << ", h=" << cy.height << ", area=" << cy.Area() << ", volume=" << cy.volume() << endl;
    return output;
}

int main(){
    Cylinder cy1(3.5, 6.4, 5.2, 10);
    cout << "x=" << cy1.getX() << ", y=" << cy1.getY() << ", r=" << cy1.getRadius() << ", h=" << cy1.getHeight() << ", area=" << cy1.Area() << ", volume=" << cy1.volume() << endl;
    
    cy1.setHeight(15);
    cy1.setRadius(7.5);
    cy1.setPoint(5, 5);
    
    cout << "new " << cy1;
    
    return 0;
}

在这个代码中，我们定义了三个类：Point、Circle 和 Cylinder。Point类表示一个点，Circle类表示一个圆，Cylinder类表示一个圆柱体。每个类都有相应的成员函数和友元函数用于操作和输出对象。在main函数中，创建了一个 Cylinder 对象 cy1，并输出其属性。然后修改 cy1 的属性，并再次输出。

#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。