Type for opening mode flags.
  Bitmask type to store opening mode flags. The flag values can be any combination of the following constant member values:

bit	effect
ios_base::app	(append) Seek to the end of the stream before each output operation.
ios_base::ate	(at end) Seek to the end of the stream when opening.
ios_base::binary	Consider stream as binary rather than text.
ios_base::in	Allow input operations on a stream.
ios_base::out	Allow output operations on a stream.
ios_base::trunc	(truncate) Truncate file to zero when opening.

See also.
  ios_base class