C++ Primer, Fifth Edition

8.2.1. Using File Stream Objects

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 ...