C++ Primer Plus полностью

    cout << fixed << right;

// show initial contents

    ifstream fin;

    fin.open(file, ios_base::in |ios_base::binary);  // binary file

    //NOTE: some systems don't accept the ios_base::binary mode

    if (fin.is_open())

    {

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

        << file << " file:\n";

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

    {

        cout << setw(20) << pl.name << ": "

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

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

    }

    fin.close();

    }

// add new data

    ofstream fout(file,

             ios_base::out | ios_base::app | ios_base::binary);

    //NOTE: some systems don't accept the ios::binary mode

    if (!fout.is_open())

    {

        cerr << "Can't open " << file << " file for output:\n";

        exit(EXIT_FAILURE);

    }

    cout << "Enter planet name (enter a blank line to quit):\n";

    cin.get(pl.name, 20);

    while (pl.name[0] != '\0')

    {

        eatline();

        cout << "Enter planetary population: ";

        cin >> pl.population;

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

        cin >> pl.g;

        eatline();

        fout.write((char *) &pl, sizeof pl);

        cout << "Enter planet name (enter a blank line "

                "to quit):\n";

        cin.get(pl.name, 20);

    }

    fout.close();

// show revised file

    fin.clear();    // not required for some implementations, but won't hurt

    fin.open(file, ios_base::in | ios_base::binary);

    if (fin.is_open())

    {

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

             << file << " file:\n";

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

        {

            cout << setw(20) << pl.name << ": "

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

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

        }

        fin.close();

    }

    cout << "Done.\n";

    return 0;

}

Here is a sample initial run of the program in Listing 17.19:

Enter planet name (enter a blank line to quit):

Earth

Enter planetary population: 6928198253

Enter planet's acceleration of gravity: 9.81

Enter planet name (enter a blank line to quit):

Here are the new contents of the planets.dat file:

               Earth:   6928198253  9.81

Done.

And here is a sample follow-up run:

Here are the current contents of the planets.dat file:

               Earth:   6928198253  9.81

Enter planet name (enter a blank line to quit):

Jenny's World

Enter planetary population: 32155648

Enter planet's acceleration of gravity: 8.93

Enter planet name (enter a blank line to quit):

Here are the new contents of the planets.dat file:

               Earth:   6928198253  9.81

       Jenny's World:     32155648  8.93

Done.

You’ve already seen the major features of the program, but let’s re-examine an old point. The program uses this code (in the form of the inline eatline() function) after reading the planet’s g value:

while (std::cin.get() != '\n') continue;

This reads and discards input up through the newline character. Consider the next input statement in the loop:

cin.get(pl.name, 20);

If the newline were left in place, this statement would read the newline as an empty line, terminating the loop.

You might wonder if this program could use a string object instead of a character array for the name member of the planet structure. The answer is no—at least not without major changes in design. The problem is that a string object doesn’t actually contain the string within itself; instead, it contains a pointer to the memory location where the string is stored. So if you copy the structure to a file, you don’t copy the string data, you just copy the address of where the string was stored. When you run the program again, that address is meaningless.

Random Access

For our last file example, let’s look at random access. Random access means moving directly to any location in the file instead of moving through it sequentially. The random access approach is often used with database files. A program will maintain a separate index file, giving the location of data in the main data file. Then it can jump directly to that location, read the data there, and perhaps modify it. This approach is done most simply if the file consists of a collection of equal-sized records. Each record represents a related collection of data. For example, in the example in Listing 17.19, each file record would represent all the data about a particular planet. A file record corresponds rather naturally to a program structure or class.

This example is based on the binary file program in Listing 17.19, to take advantage of the fact that the planet structure provides a pattern for a file record. To add to the creative tension of programming, the example opens the file in a read-and-write mode so that it can both read and modify a record. You can do this by creating an fstream object. The fstream class derives from the iostream class, which, in turn, is based on both the istream and ostream classes, so it inherits the methods of both. It also inherits two buffers, one for input and one for output, and synchronizes the handling of the two buffers. That is, as the program reads the file or writes to it, it moves both an input pointer in the input buffer and an output pointer in the output buffer in tandem.

The example does the following:

1. Displays the current contents of the planets.dat file.