different forms of the fstream constructors

    ifstream fa(filename);
    ifstream fb(filename, flags);
    ofstream fc(filename);
    ofstream fd(filename, flags);
    fstream fe(filename);
    fstream ff(filename, flags);

filename must be an old-fashioned C string (or a "string literal").

flags is any combination (using the bitwise or | operator) of
    ios::in     - open for reading
    ios::out    - open for writing
    ios::binary - when it is not a text file 
    ios::trunc  - erase content of file if it already exists
    ios::app    - all output is appended to the end of the file

If you do not provide any flags along with the filename, the defaults are
    for ifstream - ios::in
    for ofstream - ios::out | ios::trunc
    for fstream  - ios::in | ios::out

If you do provide flags, of course the default is ignored. If you use
ofstream without specifying ios::trunc, the original contents of the
file (if there are any) are not erased.

Alternatively

    ifstream fg;
    ofstream fh;
    fstream fi;
followed later by
    fg.open(filename, flags);


To read non-text data:

    fa.read(pointer, number);

pointer is a pointer to any variable, array, or struct. The bytes read from
the file will be written directly where that pointer points. Remember to
typecast to (char *).

number is the number of bytes to read.


After using read, it is a good idea to use gcount:

    int n = fa.gcount();

n is the number of bytes that were successfully read by the last read() done. 
It will be less than the number requested if the end of the file is reached.


To write non-text data:

    fc.write(pointer, number);


Don't forget to close files when finished with them.


Current position in the file:

    int p = fa.tellg();       // for input streams
    int q = fc.tellp();       // for output streams

these return the current position in the file, the value is the number of
bytes from the beginning. If you have only just opened the file, the position
will be zero.

    fa.seekg(p, relative);     // input
    fc.seekp(p, relative);     // output

these are used to change the position in the file, so that earlier data can
be re-read (or overwritten) or so that you can skip over data that you do
not need to read.

"relative" must be one of the three constants ios::beg, ios::cur, ios::end.

ios::beg (beginning) - reposition to exactly p bytes from the beginning of the file
ios::cur (current) - reposition p bytes from the current position, so a positive
                     p means p bytes will be skipped, p can be negative.
ios::end - reposition to p bytes beyond the end of the file, p=0 is the most
           common: go directly to the end of the file, but p may be negative.