Open a file.
  Opens a file. The stream's file buffer is associated with the specified file to perform the i/o operations.

Parameters.

filename

String containing 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.
  none

Example.

// ofstream::open
#include <fstream>
using namespace std;

int main () {

  ofstream outfile;

  outfile.open ("test.txt", ofstream::out | ofstream::app);
  outfile << "This sentence is appended to the file content\n";
  outfile.close();

  return 0;
}

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

void open ( const char * filename, openmode mode = out | trunc );

See also.
  close
  ofstream class