类中构造函数问题

#include <iostream.h>
const float pi=3.14159;
const float fenceprice=35;
const float concreteprice=20;
class circle
{float radius;
 public:
    circle(float r);
    float circumference();
    float area();
};
circle::circle(float r)   //没有这句无法运行,请问下这里为什么要这样写?
{radius=r;}                 //radius=r但程序从没给r赋值啊?
float circle::circumference()
{return 2*pi*radius;}
float circle::area()
{return pi*radius*radius;}
void main()
{float radius,fencecost,concretecost;
 cin>>radius;
 circle pool(radius);
 circle poolrim(radius+3);
 fencecost=poolrim.circumference()*fenceprice;
 cout<<"langanqian="<<fencecost;
 concretecost=(poolrim.area()-pool.area())*concreteprice;
 cout<<"ludaoqian="<<concretecost<<endl;
}
问题在程序里