C++ Primer Plus

Now let’s look at the second prototype. Values of the streampos type locate a position in a file. It can be a class, but, if so, the class includes a constructor with a streamoff argument and a constructor with an integer argument, providing a path to convert both types to streampos values. A streampos value represents an absolute location in a file, measured from the beginning of the file. You can treat a streampos position as if it measures a file location in bytes from the beginning of a file, with the first byte being byte 0. So the following statement locates the file pointer at byte 112, which would be the 113th byte in the file:

fin.seekg(112);

If you want to check the current position of a file pointer, you can use the tellg() method for input streams and the tellp() methods for output streams. Each returns a streampos value representing the current position, in bytes, measured from the beginning of the file. When you create an fstream object, the input and output pointers move in tandem, so tellg() and tellp() return the same value. But if you use an istream object to manage the input stream and an ostream object to manage the output stream to the same file, the input and output pointers move independently of one another, and tellg() and tellp() can return different values.

You can then use seekg() to go to the file beginning. Here is a section of code that opens a file, goes to the beginning, and displays the file contents:

fstream finout;     // read and write streams

finout.open(file,ios::in | ios::out |ios::binary);

//NOTE: Some Unix systems require omitting | ios::binary

int ct = 0;

if (finout.is_open())

{

    finout.seekg(0);    // go to beginning

    cout << "Here are the current contents of the "

          << file << " file:\n";

    while (finout.read((char *) &pl, sizeof pl))

    {

        cout << ct++ << ": " << setw(LIM) << pl.name << ": "

        << setprecision(0) << setw(12) << pl.population

        << setprecision(2) << setw(6) << pl.g << endl;

    }

    if (finout.eof())

        finout.clear(); // clear eof flag

    else

    {

        cerr << "Error in reading " << file << ".\n";

        exit(EXIT_FAILURE);

    }

}

else

{

    cerr << file << " could not be opened -- bye.\n";

    exit(EXIT_FAILURE);

}

This is similar to the start of Listing 17.19, but there are some changes and additions. First, as just described, the program uses an fstream object with a read/write mode, and it uses seekg() to position the file pointer at the start of the file. (This isn’t really needed for this example, but it shows how seekg() is used.) Next, the program makes the minor change of numbering the records as they are displayed. Then it makes the following important addition:

if (finout.eof())

    finout.clear(); // clear eof flag

else

{

    cerr << "Error in reading " << file << ".\n";

    exit(EXIT_FAILURE);

}

The problem this code addresses is that when the program finishes reading and displaying the entire file, it sets the eofbit element. This convinces the program that it’s finished with the file and disables any further reading of or writing to the file. Using the clear() method resets the stream state, turning off eofbit. Now the program can once again access the file. The else part handles the possibility that the program quits reading the file for some reason other than reaching the end-of-file, such as a hardware failure.

The next step is to identify the record to be changed and then change it. To do this, the program asks the user to enter a record number. Multiplying the number by the number of bytes in a record yields the byte number for the beginning of the record. If record is the record number, the desired byte number is record * sizeof pl:

cout << "Enter the record number you wish to change: ";

long rec;

cin >> rec;

eatline();              // get rid of newline

if (rec < 0 || rec >= ct)

{

    cerr << "Invalid record number -- bye\n";

    exit(EXIT_FAILURE);

}

streampos place = rec * sizeof pl;  // convert to streampos type

finout.seekg(place);    // random access

The variable ct represents the number of records; the program exits if you try to go beyond the limits of the file.

Next, the program displays the current record:

finout.read((char *) &pl, sizeof pl);

cout << "Your selection:\n";

cout << rec << ": " << setw(LIM) << pl.name << ": "

<< setprecision(0) << setw(12) << pl.population

<< setprecision(2) << setw(6) << pl.g << endl;

if (finout.eof())

    finout.clear();     // clear eof flag

After displaying the record, the program lets you change the record:

cout << "Enter planet name: ";

cin.get(pl.name, LIM);

eatline();

cout << "Enter planetary population: ";

cin >> pl.population;

cout << "Enter planet's acceleration of gravity: ";

cin >> pl.g;

finout.seekp(place);    // go back

finout.write((char *) &pl, sizeof pl) << flush;

if (finout.fail())

{

    cerr << "Error on attempted write\n";

    exit(EXIT_FAILURE);

}

The program flushes the output to guarantee that the file is updated before proceeding to the next stage.