cppreference.com > C++ I/O > open open Syntax:
      #include <fstream>
      void open( const char *filename );
      void 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 optional io stream mode flag mode defaults to ios::in for ifstream, ios::out for ofstream, and ios::in|ios::out for fstream. If open() fails, the resulting stream will evaluate to false when used in a Boolean expression. For example:
     ifstream inputStream;
     inputStream.open("file.txt");
     if( !inputStream ) {
       cerr << "Error opening input stream" << endl;
       return;
     }
Related topics: I/O Constructors close