Catching An Exception

 int  main()  {
     int  num;
     while  (cin  >>  num)  {
         try  {
             cout  <<  square_root(num)  <<  '\n';
         }  catch(Domain_Error  e)  {
             cerr  <<  e.number 
                           <<  "  is  not  valid\n";
         }
     }
 }