Input File State

State of an I/O Stream

We know any of the following can cause an input stream to enter the fail state:

Invalid input data
An attempt to read beyond the end of a file
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 if the state is OK and false if the file stream is in a fail state. Here is an example program that reads four floating point data values from a file and writes them to another file in reverse order.

// Program IODemo demonstrates how to use files

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

int main()
{
    cout  << fixed;

ifstream inData;       // declare input stream
    ofstream outData;      // declare output stream

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

// test the state of the input 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;
    }

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

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

float val1, val2, val3, val4;           // declare 4 variables
    inData >> val1 >> val2 >> val3 >> val4; // input 4 values

// see if the read succeeded
    if (!inData)
    {
       cout << "Could not read 4 floats from the input file." << endl;
       return 3;
    }

outData  << val4  << endl;  // output 4 values
    outData  << val3  << endl;
    outData  << val2  << endl;
    outData  << val1  << endl;

inData.close();   // close input file
    outData.close();  // close output file

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 the data format read from inData is incorrect or there is not enough data, 3 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 a plain text editor or your Desktop IDE to create the input data file according to the requirements of the data types and format in your program. The input data file must exist and contain correct data before you run the program. 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.

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