In C++, a file is opened by linking it to a stream. There are three types of
streams: input, output and input/output. To open an input stream you must declare
the stream to be of class ifstream. To open an output stream, it must be declared
as class ofstream. A stream that will be be performing both input and output
operations must be declared as class fstream. For example, this fragment creates
one input stream, one output stream and one stream that is capable of both input
and output.
ifstream in;
ofstream out;
fstream both;
Once you have created s stream, one way to associate it with a file is by using
open(). This function is a member of each of the three stream classes. Its
prototype is shown here
void open(const char *filename, int mode, int access);
Here, filename is the name of the file. The value of the mode determines how the
file is opened. It must be one or more of the following values which are defined
in the fstream.h
ios::app // causes all output to the file to be appended to the end
ios::ate // causes a seek to the end of the file occur when file is opened
ios::binary// causes a file to be opened in binary mode
ios::out // specifies that the file is capable of output
ios::in// specifies that the file is capable of input
ios::nocreate// causes open() to fail if the file doe not already exist.
ios::noreplace// causes open() to fail if the file already exist
ios::trunc// causes the contents of a preexisting file by the same name to be
destroyed and truncate the file to zero length
The value of access determines how the file can be accessed. In DOS/Windows environments, this value generally corresponds to the DOS/Windows file attribute code listed here:
| Attribute | Meaning |
| Norma file; open access | |
| 1 | read-only file |
| 2 | hidden file |
| 4 | system file |
| 8 | archive bit set |
The following fragment opens a normal output file
ofstream out;
out.open("test",ios::out,o);
The default value for mode is ios::out for ofstream and ios::in for ifstream and the default value for access is 0. Hence the previous statement can be written as
out.open("test");
To open a stream for input and output, you must specify both ios::in and ios::out mode values as shown here,
fstream xyst;
xyst.open("test",ios::in | ios::out);
To close a file, use the member function close(). For example to close the file linked to a stream called xyst you would use this statement
xyst.close();
The close function takes no parameters and returns no values.