Convert Double to String

[ccubed]

Okay, so I have this code to convert Doubles to Strings.

string stringify(double x){
             
         ostringstream o;
         o << x;
         return o.str();
             
      }

My question is two fold. One, assume the double was originally 1.2E+003. I convert it to a string and i'll get something like XXXX.XX, but convert it back to a double via istringstream and I get 1.2E+003. Why?

Two, is there any way to get all the decimal numbers from the double and not round when it converts to string? I'm trying to create a nice round function so that I can convert a number that could have 20+ decimals to 2 decimals. The process would be really simple if i could get it into a string form and then just loop through each number behind the decimal and see if it's higher than 5.