C++ Primer Plus

Opening a file for output this way creates a new file if there is no file of that name. If a file by that name exists prior to opening it for output, the act of opening it truncates it so that output starts with a clean file. Later in this chapter you’ll see how to open an existing file and retain its contents.

Caution

Opening a file for output in the default mode automatically truncates the file to zero size, in effect disposing of the prior contents.

The requirements for reading a file are much like those for writing to a file:

1. Create an ifstream object to manage the input stream.

2. Associate that object with a particular file.

3. Use the object the same way you would use cin.

The steps for reading a file are similar to those for writing to a file. First, of course, you include the fstream header file. Then you declare an ifstream object and associate it with the filename. You can do so in two statements or one:

// two statements

ifstream fin;              // create ifstream object called fin

fin.open("jellyjar.txt");  // open jellyjar.txt for reading

// one statement

ifstream fis("jamjar.txt"); // create fis and associate with jamjar.txt

You can then use fin or fis much as you would use cin. For example, you can use the following:

char ch;

fin >> ch;               // read a character from the jellyjar.txt file

char buf[80];

fin >> buf;              // read a word from the file

fin.getline(buf, 80);    // read a line from the file

string line;

getline(fin, line);      // read from a file to a string object

Input, like output, is buffered, so creating an ifstream object such as fin creates an input buffer, which the fin object manages. As with output, buffering moves data much faster than byte-by-byte transfer.

The connections with a file are closed automatically when the input and output stream objects expire—for example, when the program terminates. Also you can close a connection with a file explicitly by using the close() method:

fout.close();    // close output connection to file

fin.close();     // close input connection to file

Closing such a connection does not eliminate the stream; it just disconnects it from the file. However, the stream management apparatus remains in place. For example, the fin object still exists, along with the input buffer it manages. As you’ll see later, you can reconnect the stream to the same file or to another file.

Let’s look at a short example. The program in Listing 17.16 asks for a filename. It creates a file that has that name, writes some information to it, and closes the file. Closing the file flushes the buffer, guaranteeing that the file is updated. Then the program opens the same file for reading and displays its contents. Note that the program uses fin and fout in the same manner as you’d use cin and cout. Also, the program reads the filename into a string object and then uses the c_str() method to provide C-style string arguments for the ofstream and ifstream constructors.

Listing 17.16. fileio.cpp

// fileio.cpp -- saving to a file

#include // not needed for many systems

#include

#include

int main()

{

    using namespace std;

    string filename;

    cout << "Enter name for new file: ";

    cin >> filename;

// create output stream object for new file and call it fout

    ofstream fout(filename.c_str());

    fout << "For your eyes only!\n";        // write to file

    cout << "Enter your secret number: ";   // write to screen

    float secret;

    cin >> secret;

    fout << "Your secret number is " << secret << endl;

    fout.close();           // close file

// create input stream object for new file and call it fin

    ifstream fin(filename.c_str());

    cout << "Here are the contents of " << filename << ":\n";

    char ch;

    while (fin.get(ch))     // read character from file and

        cout << ch;         // write it to screen

    cout << "Done\n";

    fin.close();

    return 0;

}

Here is a sample run of the program in Listing 17.16:

Enter name for new file: pythag

Enter your secret number: 3.14159

Here are the contents of pythag:

For your eyes only!

Your secret number is 3.14159

Done

If you check the directory that contains the program, you should find a file named pythag, and any text editor should show the same contents that the program output displays. (So much for secrecy.)

Stream Checking and is_open()