怎么给对象输入值

如何给对象输入值
#include <iostream>
using   namespace   std;
const   float   PI=3.14;
class   circle
{      
public:
circle(float   r=0);
float   display();
int   fun();
private:
float   r;
};
float   circle::display()
{
    int   cc;
    cc=2*PI*r;
    return   cc;
}
int   circle::fun()
{       int   r;
cout < < "please   enter   radious: ";
        cin> > r;
return   0;
}
int   main()
{
circle   c1;
        c1.fun();
cout < <c1.display();
return   0;

}


------解决方案--------------------
改正的程序:

#include <iostream>
using namespace std;
const float PI=3.14;

class circle
{
public:
circle(float _r=0);

float display();
int fun();
private:
float r;
};

circle::circle(float _r)
:r(_r)
{
}

float circle::display()
{
float cc;
cc=2.0*PI*r;
return cc;
}

int circle::fun()
{
cout < < "please enter radious: ";
cin> > r;
return 0;
}


int main()
{
circle c1;
c1.fun();
cout < <c1.display() < <endl;

system( "pause ");
return 0;

}