State of an I/O Stream

We know any of the following can cause an input stream to enter the fail state:
  1. Invalid input data
  2. An attempt to read beyond the end of a file
  3. An attempt to open a nonexistent file to input
C++ provides a way to test the state of a stream: The stream name used in the expression returns true value if the state is ok and false value if the state is in the fail state. Here is an example program that reads four floating point data values from a file and write to another file in the reverse order. 

// Program IODemo demonstrates how to use files

#include <iostream>
#include <fstream>
using namespace std;

int main()
{
    cout  << fixed  << showpoint;

    float val1, val2, val3, val4;	// declares 4 variables
    ifstream inData;			// declares input stream
    ofstream outData;			// declares output stream

    // binds program variable inData to file "inputfile.txt"
    inData.open("inputfile.txt");

    //Testing the state of the stream
    //true means the last I/O operation on that stream succeeded
    //false means the last I/O operation on that stream failed
    if (!inData)
    {
       cout << "Can't open the input file successfuly." << endl;
       return 1;
    }

    // binds program variable outData to file "outputfile.txt"
    outData.open("outputfile.txt");

    //Testing the state of the stream
    if (!outData)
    {
       cout << "Can't open the output file successfuly." << endl;
       return 2;
    }


    inData  >> val1 >> val2 >> val3 >> val4;	// inputs 4 values

    

    outData  << val4  << endl;
    outData  << val3  << endl;
    outData  << val2  << endl;
    outData  << val1  << endl;	// outputs 4 values
    return 0;
}
Note that inData and outData are two varibales in the program; "inputfile.txt" and "outputfile.txt" are character strings. inputfile.txt is the name of the input data file that we have created; outputfile.txt is the name of the output data file where the answers are stored.

If the input file inputfile.txt cannot be found, 1 is returned to the operating system. If the output file outputfile.txt cannot be opened or created, 2 is returned to the operating system. If there is no input and output error, 0 is returned to the operating system. Notice that the main is exited as soon as a value is returned. Therefore, Returning 0 value means normal completion of a program; returning any other value signals an error. When you write your own program, you may choose the value to return to indicate different error conditions.

You can use pico or vi text editor to create the input data file according to the requirement of the data type and format in your program. Input data file must exist and contain correct data. Otherwise, you will get input failure.

For example, in the preceding IODemo program, the input file should look like this: 

5.5
6.6
7.7
8.8
You may run the program and and test the state of the I/O stream.


Copyright: Department of Computer Science, University of Regina.