When we want to read or write a file, we define a file stream object and associate that object with the file. Each file stream class defines a member function named
open that does whatever system-specific operations are required to locate the given file and open it for reading or writing as appropriate.
When we create a file stream, we can (optionally) provide a file name. When we supply a file name,
open is called automatically:
ifstream in(ifile); // construct an ifstream and open the given fileofstream out; // output file stream that is not associated with any file
This code defines
in as an input stream ...