求教：友元函数无法访问类的私有成员。帮我改一下

#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;
}