#include <fstream>
    void fstream::open(const char *filename, openmode mode = default_mode);

The function open() is used with file streams. It opens filename and associates it with the current stream. The mode parameter defaults to ios::in for ifstream, ios::out for ofstream, and ios::in|ios::out for fstream. If open() fails, the stream is put into a non-good state which may be checked. For example:

   ifstream inputStream;"file.txt");
   if( !inputStream ) {
     cerr << "Error opening input stream" << endl;
   // or
   if ( inputStream ) {
     // use inputStream

Related Topics: I/O Constructors, close, C++ I/O Mode Flags