Open a file.
  Opens a file whose name is s, associating its content with the stream buffer to perform operations allowed by parameter mode.

Parameters.

s

String containg the name of the file to be opened.

mode

Flags describing the requested i/o mode for the file. This is an object of type ios_base::openmode, it generally consist of a combination of the following flags (member constants):

bit	effect
app	(append) Seek to the end of the stream before each output operation.
ate	(at end) Seek to the end of the stream when opening.
binary	Consider stream as binary rather than text.
in	Allow input operations on a stream.
out	Allow output operations on a stream.
trunc	(truncate) Truncate file to zero when opening.

Return Value.
  The function returns this if successful.
  In case of error, function returns a null pointer.

Example.

// filebuf::open ()
#include <iostream>
#include <fstream>
using namespace std;

int main () {

  ifstream is;
  filebuf * fb;

  fb = is.rdbuf();
  fb->open ("test.txt",ios::in);

  // >> file buffer operations here <<

  fb->close();

  return 0;
}

Basic template member declaration ( basic_filebuf<charT,traits> ):

basic_filebuf* open ( const char* s, ios_base::openmode mode );

See also.
  close, is_open
  filebuf class