File Streams: input and output involving disk files

Key points

With these commands, you can either input from a file or output to a file, but not both at once.
You must always use the include statement #include <fstream.h>
The first step is to open the file using the command ifstream (for input) or ofstream (for output).
When you open a file, you give it a name within the program, called its file handle. This isn’t necessarily the same name as the name of the file on the disk.
Then, you can input or output using methods like those for the console (cin >> and cout <<), but using the file handle in place of cin or cout.
If you want to input all the lines in a file, use eof to test whether you’re at the end of the file.
When you’re done working with a file, you should close it.

Example 1: outputting a message to a file


#include <iostream.h>
#include <fstream.h>
int main()
{
	ofstream outfile ("hello.dat");	// open stmt prepares for output to hello.dat
					 	 	// outfile is the file handle
	outfile << "Hello there." << endl;
	outfile << "How are you?" << endl;
	outfile.close();                 // close file when done
	return 0;
}

Example 2: inputting a single line from a file, then outputting it to the screen


#include <iostream.h>
#include <fstream.h>
#include "apstring.h"
int main()
{
	apstring text;
	ifstream infile ("oneline.dat");	// open stmt prepares for input from oneline.dat
							// infile is the file handle
	getline(infile,text);      // used getline rather than >>, to get a whole line
	cout << text << endl;      // now output the line to the screen
	infile.close();                  // close file when done
	return 0;
}

Example 3: inputting many words from a file, and output each to the screen on its own line


#include <iostream.h>
#include <fstream.h>
#include "apstring.h"
int main()
{
	apstring text;
	ifstream filein ("manylines.dat");	// open stmt for input from manylines.dat
													// filein is the file handle
	while (filein.eof() == false) //  keep going as long as you're not at end of file
	{
		filein >> text;         		// input a word from the file
		cout << text << endl;   // now output it to the screen on its own line
	}
	filein.close();                    // close file when done
	return 0;
}