求指教 ，看不出这个关于类跟对象的代码的错误

#include <iostream.h>
class A
{
      public:
             A()
             {
                     m=0;
             }
             A(double i)
             {
                      m=i;
             }
             void print()
             {
                  cout<<m<<endl;
             }
             private;
             double m;
             };
void main ()
{
                  A a(5);
                  a.print();
                  a=10;
                  a.print();
}