C++ How to Program, 10/e

... Data can be written anywhere in the file.
ios::in	Open a file for input.
ios::out	Open a file for output.
ios::trunc	Discard the file’s contents (this also is the default action for ios::out).
ios::binary	Open a file for binary, i.e., nontext, input or output.

14.3.2 Opening a File via the open Member Function

You can create an ofstream object without opening a specific file—in this case, a file can be attached to the object later. For example, the statement

ofstream outClientFile;

creates an ofstream object that’s not yet associated with a file. The ofstream member function open opens a file and attaches it to an existing ofstream object as follows:

outClientFile.open("clients.txt", ios::out);

Once again, ios::out is the default ...