Persistent Data: File Input and Output - Reading from a File

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You input or read information from a file into your program using the stream extraction operator (>>) just as you use that operator to input information from the keyboard. The only difference is that you use an ifstream (or fstream) object instead of the cin object.

You input or read information from a file into your program using the operator just as you use that operator to input information from the keyboard. The only difference is that you use an (or ) object instead of the object.

The following program builds on the previous one. After writing information inputted by the user to a file named students.dat, the program reads information from the file and outputs it onto the screen:

#include <fstream>
#include <iostream>
using namespace std;
int main ()
{
char data[80];
ofstream outfile;
outfile.open("students.dat");
cout << "Writing to the file" << endl;
cout << "===================" << endl;
cout << "Enter class name: ";
cin.getline(data, 80);
outfile << data << endl;
cout << "Enter number of students: ";
cin >> data;
cin.ignore();
outfile << data << endl;
outfile.close();
ifstream infile;
cout << "Reading from the file" << endl;
cout << "=====================" << endl;
infile.open("students.dat");
infile >> data;
cout << data << endl;
infile >> data;
cout << data << endl;
infile.close();
return 0;
}

Sample input and output:

Writing to the file
===================
Enter class name: Programming
Enter number of students: 32
Reading from the file
=====================
Programming
32

Reading a Line of a File

With the same program, try entering a class name with an embedded space. The following is some sample input and output:

Writing to the file
===================
Enter class name: Programming Demystified
Enter number of students: 32
Reading from the file
=====================
Programming
Demystified

The following are the contents of the file after the inputted data was written to it:

Programming Demystified
32

The first read of the file did not read the first line of the file, “Programming Demystified.” Instead, the first read of the file only read the word “Programming” and then stopped. Consequently the second line of the program read the remainder of the first line of the file, “Demystified,” instead of the number of students.

The ifstream object together with the stream extraction operator reads the file sequentially, starting with the first byte of the file. The first attempt to read the file starts at the beginning of the file and goes to the first whitespace character (a space, tab, or new line) or the end of the file, whichever comes first. The second attempt starts at the first printable character after that whitespace, and continues to the next whitespace character or the end of the file, whichever comes first.

The first read attempt only read “Programming,” not “Programming Demystified,” because the read stopped at the whitespace between “Programming” and “Demystified.” The second attempt read “Demystified.” There were no further read attempts, so the number of students, 32, was never read.

This should seem like déjà vu. We encountered a similar issue in Chapter 10 using the cin object with the stream extraction operator (>>). As in Chapter 10 with the cin object, the solution is to use getline.

If you are working with C-strings, then you should use the getline member function. The only difference between using the getline member function here and in Chapter 10 is that here the getline member function is called by an ifstream or fstream object instead of a cin object. Accordingly, we need to replace the two calls to infile >> data with the following:

infile.getline(data, 80);

You also can use getline with the C++ string class. The only difference between using the getline member function here and in Chapter 10 is that here the first argument of the getline member function is an ifstream or fstream object instead of a cin object. Accordingly, we need to replace the two calls to infile >> data with the following:

getline(infile, data);

The following modification of the previous program uses the getline function with the C++ string class:

#include <fstream>
#include <iostream>
#include <string>
using namespace std;
int main ()
{
string data;
ofstream outfile;
outfile.open("students.dat");
cout << "Writing to the file" << endl;
cout << "===================" << endl;
cout << "Enter class name: ";
getline(cin, data);
outfile << data<< endl;
cout << "Enter number of students: ";
cin >> data;
cin.ignore();
outfile << data<< endl;
outfile.close();
ifstream infile;
cout << "Reading from the file" << endl;
cout << "=====================" << endl;
infile.open("students.dat");
getline(infile, data);
cout << data << endl;
getline(infile, data);
cout << data << endl;
infile.close();
return 0;
}

As the following sample input and output reflects, the first read now reads the entire first line of the file even when that line contains embedded spaces:

Writing to the file
===================
Enter class name: Programming Demystified
Enter number of students: 32
Reading from the file
=====================
Programming Demystified
32

Next: Looping Through the File >>
 

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This article is excerpted from chapter 13 of the book C++ DeMYSTifieD, written by Jeff Kent (McGraw-Hill, 2004; ISBN: 0072253703). Check it out at your favorite bookstore. Buy this book now.