#include<iostream.h>
#define p 3.14

class Circle
{
public:
    Circle(float r);
    float Circumference();
    float Area();
private:
    float radius;
};
Circle::Circle(float r)
{
    radius=r;
}
float Circle::Circumference()
{
    return 2*p*radius;
}
float Circle::Area()
{
    return p*radius*radius;
}
int main()
{
    float radius;
    float Circumference,Area;
    cin>>radius;
    cout<<Circumference()<<Area()<<endl;
}
哪里错了，该怎么改啊

#include<iostream.h>
#define p 3.14

class Circle
{
public:
    Circle(float r);
    float Circumference();
    float Area();
private:
    float radius;
};
Circle::Circle(float r)
{
    radius=r;
}
float Circle::Circumference()
{
    return 2*p*radius;
}
float Circle::Area()
{
    return p*radius*radius;
}
int main()
{
    float radius;
    float Circumference,Area;
    cin>>radius;
    Circle A(radius);//创建一个Circle对象
    cout<<A.Circumference()<<A.Area()<<endl;
}

必须再主函数内定义一个类的对象，然后用"对象.类的变量或者类的函数"进行操作。就像2楼所写的程序。

#include<iostream>
#define p 3.14
using namespace std;

class Circle
{
public:
    Circle(float r);
    float Circumference();
    float Area();
private:
    float radius;
};
Circle::Circle(float r)
{
    radius=r;
}
float Circle::Circumference()
{
     return float(2*p*radius);
}
float Circle::Area()
{
   
   
    return float(p*radius*radius);
}
int main()
{
    float radius;
    float Circumference(),Area();
    cin>>radius;
    Circle A(radius);
    cout<<A.Circumference()<<A.Area()<<endl;
}