#include<iostream>
using namespace std;
class Round
{
    float radius;
public:
    Round(float radiusn)
    {
        radius=radiusn;
    }
    ~Round(){}
    friend float perimeter();
    friend float area();
};
float perimeter()
{
    return 2*3.14*radius;
}
float area()
{
    return 3.14*radius*radius;
}
void main()
{
    Round R1(5);
    cout<<"周长为："<<perimeter()<<endl;
    cout<<"面积为："<<area()<<endl;
}

float perimeter(Round &r)
{
    return 2*3.14*r.radius;
}

void main()
 {
     Round R1(5);
     cout<<"周长为："<<perimeter()<<endl;
     cout<<"面积为："<<area()<<endl;
 }
明显 需要赋予函数对象名啊float perimeter(Round &r)
 

只有方法木有对象