#include<iostream>#include<cmath>using namespace std;const double PI=3.14159265;class Circle;class Point{ private: int x; int y; public: friend class Circle; Point(); Point(int a, int b); Point(Point &p);};class Circle{ private: double radius; Point center; public: Circle(int m, int n, double j); Circle(Circle &q); double getArea(); double getPerimeter(); int isInTheCircle(Point &i); };int Circle::isInTheCircle(Point &i){ double lenth; lenth=sqrt(i.x*i.x+i.y*i.y); if(lenth<radius) return 1; else if(lenth>=radius) return 0; }double Circle::getArea(){ return PI*radius*radius;}double Circle::getPerimeter(){ return 2*PI*radius;}Circle::Circle(int m, int n, double j){ center.x=m; center.y=n; radius=j;}Circle::Circle(Circle &q){ center.x=q.center.x; center.y=q.center.y; radius=q.radius;}Point::Point():x(0),y(0){}Point::Point(int a, int b):x(a),y(b){}Point::Point(Point &p){ x=p.x; y=p.y;}int main(){ double x,y,radius; cin>>x>>y; cin>>radius; Circle c(x,y,radius); cin>>x>>y; Point p(x,y); cout<<c.getArea()<<endl; cout<<c.getPerimeter()<<endl; if(c.isInTheCircle(p)) cout<<"该点在圆内部！"<<endl; else cout<<"该点不在圆内部！"<<endl; return 0; }输入0 0 3 2.5 2.5为什么输出不是28.2743 18.8496 该点不在园内部！

您的代码有一些问题，主要是在 `isInTheCircle` 函数中计算距离的时候没有用圆心作为参考点，而是直接使用了点的坐标。修改后的代码如下：

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

const double PI=3.14159265;

class Circle;

class Point{
private:
    int x;
    int y;
public:
    friend class Circle;
    Point();
    Point(int a, int b);
    Point(Point &p);
};

class Circle{
private:
    double radius;
    Point center;
public:
    Circle(int m, int n, double j);
    Circle(Circle &q);
    double getArea();
    double getPerimeter();
    int isInTheCircle(Point &i);
};

int Circle::isInTheCircle(Point &i){
    double length;
    length = sqrt((i.x - center.x) * (i.x - center.x) + (i.y - center.y) * (i.y - center.y));
    if (length < radius) {
        return 1;
    } else if (length >= radius) {
        return 0;
    }
}

double Circle::getArea(){
    return PI*radius*radius;
}

double Circle::getPerimeter(){
    return 2*PI*radius;
}

Circle::Circle(int m, int n, double j){
    center.x=m;
    center.y=n;
    radius=j;
}

Circle::Circle(Circle &q){
    center.x=q.center.x;
    center.y=q.center.y;
    radius=q.radius;
}

Point::Point():x(0),y(0){}

Point::Point(int a, int b):x(a),y(b){}

Point::Point(Point &p){
    x=p.x;
    y=p.y;
}

int main(){
    double x,y,radius;
    cin>>x>>y;
    cin>>radius;
    Circle c(x,y,radius);
    cin>>x>>y;
    Point p(x,y);
    cout<<c.getArea()<<endl;
    cout<<c.getPerimeter()<<endl;
    if(c.isInTheCircle(p))
        cout<<"该点在圆内部！"<<endl;
    else
        cout<<"该点不在圆内部！"<<endl;
    return 0;
}

修改后您的程序就能正确输出28.2743和18.8496了，同时也能正确判断点是否在圆内部。