hello. i've got a monkey of a problem with a program that converts some text into a single string. Exerpt follows:

// include files
#include <iostream>
#include <sstream>
#include <string>
#include "Room.h"
#include "String.h"
using namespace std;

and the problem area of code in the program:

string Room::display_room()
{

ostringstream format_message;

format_message << "Room details" << endl
<< "===========" << endl
<< endl
<< "Room Name : " << room_name << endl
<< "Length : " << length << endl
<< "Width : " << width << endl
<< "Height : " << height << endl;

string a_string = format_message.str();
cout << a_string << endl;
return a_string;
}

there's also a header file but i've made the same alterations (String display_room, and pretty much the same files included).

i get errors like 'string not defined'. how can I get this routine to return string without error?