通过引用实现，，谢谢

#include<iostream>
using namespace std;
double area(double &r)
{
   double s=r*r*3.141;
   return s;
}
double circle(double &r)
{
    double len=2*3.14*r;
     return len;
}
int main()
{
    double r;
    cin>>r;
    cout<<area(r)<<endl<<circle(r)<<endl;
    return 0;
}

谢谢了