Lesson 10: C++ File I/O
This is a slightly more advanced topic
than what I have covered so far, but I think that it is useful. File I/O is reading from
and writing to files. This lesson will only cover text files, that is, files that are
composed only of ASCII text.
C++ has two basic classes to handle files, ifstream and ofstream. To
use them, include the header file fstream.h. Ifstream handles file input (reading from
files), and ofstream handles file output (writing to files). The way to declare an
instance of the ifstream or ofstream class is:
ifstream a_file;
//or
ifstream a_file("filename");
The constructor for both classes will actually
open the file if you pass the name as an argument. As well, both classes have an open
command (a_file.open()) and a close command (a_file.close()). It is generally a good idea
to clean up after yourself and close files once you are finished.
The beauty of the C++ method of handling files rests in the simplicity
of the actual functions used in basic input and output operations. Because C++ supports
overloading operators, it is possible to use << and >> in front of the
instance of the class as if it were cout or cin.
For example:
#include <fstream.h>
#include <iostream.h>
int main()
{
char str[10];
//Used later
ofstream a_file("example.txt");
//Creates an instance
of ofstream, and opens example.txt
a_file<<"This text will now be inside of example.txt";
//Outputs to
example.txt through a_file
a_file.close();
//Closes up the file
ifstream b_file("example.txt");
//Opens for reading the
file
b_file>>str;
//Reads one string from the file
cout<<str;
//Should output 'this'
b_file.close();
//Do not forget this!
}
The default mode for opening a file with ofstream's constructor is
to create it if it does not exist, or delete everything in it if something does exist in
it. If necessary, you can give a second argument that specifies how the file should be
handled. They are listed below:
ios::app -- Opens the file, and allows additions at the end
ios::ate -- Opens the file, but allows additions anywhere
ios::trunc -- Deletes everything in the file
ios::nocreate -- Does not open if the file must be created
ios::noreplace -- Does not open if the file already exists
For example:
ifstream a_file("test.txt", ios::nocreate);
The above code will only open the file test.txt if that file does
not already exist.