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Connecting Streams to External Files

The stream must first be declared. The names for streams follow the same rules of formation as identifiers. Thus the following declaration declares the stream ins for input and the stream outs for output:

ifstream ins;  // input stream
ofstream outs; // output stream

To connect these streams to the appropriate external files the open function is used. The open function is a member function of the stream class and to connect the stream ins to a file indata.dat the following statement is used:

ins.open("indata.dat");

The general form for a stream with identifier streamname and a file with name filename is:

streamname.open(filename);

If the file indata.dat did not exist or could not be found then this open function will fail. Failure of the open function is tested by using the the function streamname.fail() which returns true if the open function failed. When using the open function failure should always be tested for as in the following example:

ifstream ins;
ins.open("indata.dat");
if (ins.fail())
  {
    cout << "Error opening file indata.dat"
         << endl;
    return 1;
  }

In this example it is assumed that the failure occurs within the main() program. This means that the return statement exits the main program and hence the program is terminated. The value 1 that is returned indicates to the operating system that the program has terminated with an error condition. As the main program now returns a value it must be declared as having the type int rather than void.

Having connected the external files to the streams any input/output from/to these streams will then be from/to the connected files.

In the following example two streams are declared, one for input called ins and another for output called outs. These streams are then connected to files with names indata.dat and results.dat respectively by:

int main()
{
 ifstream ins;
 ofstream outs;
 ins.open("indata.dat");
 if (ins.fail())
   {
     cout << "Error opening indata.dat"
          << endl;
     return 1;
   }
 outs.open("results.dat");
 if (outs.fail())
   {
     cout << "Error opening results.dat"
          << endl;
     return 1;
   }
      .
      .

All access to these files now uses the declared stream names ins and outs and the input/output operators << and >>. Input/output manipulators can also be used on the output stream.

Thus a data item is entered from the stream ins by:

     ins >> x;

and results are output to the stream outs by:

outs << "Result is " << setw(6) << count << endl;