#include <iostream.h>
const double p=3.14;
class circle {
   
    double length,area,r;
    double length (double x);
    double area (double x);
public:
    length(double x)
    {  double l;
    l=2*p*r;
    reture l;
    }
    area (double x)
    {  double s;
    s=P*r*r;
    reture s;


}
  void main ()
  {
      double r;
      cin<<r;
      length=length (r);
      area=area(r);
      cout>>"length=">>length>>endl;
      cout >>"area=">>area>>endl;
  }

我个人觉得，你的这段代码有许多错误，比如说C++的封装性可能你都没有理解，还有输入输出，总之错误很多，建议回去好好看看书吧，
此话皆为心里话，如果认为我说的不对，还请见谅。

#include <iostream.h>
#define pi 3.14
 class circle
 {
     
 private: double r;
     
 public:
     circle(double c){r=c;}
     double length()
     {
         double l;
         l=2*pi*r;
        return l;
     }
     double area ()
     {
         double s;
        s=pi*r*r;
        return s;
     }
 

};
double main ()
 {
      
       circle tom(1);
       cout<<tom.area()<<endl;
       cout<<tom.length()<<endl;
       return 0;
 }

#include <iostream.h>
#define pi 3.14
 class circle
 {
     
 private: double r;
     
 public:
     circle(double c){r=c;}
     double length()
     {
         double l;
         l=2*pi*r;
        return l;
     }
     double area ()
     {
         double s;
        s=pi*r*r;
        return s;
     }
 

};
double main ()
 {
       double i;
       cin>>i;
       circle tom(i);
       cout<<tom.area()<<endl;
       cout<<tom.length()<<endl;
       return 0;
 }