Question

diff between ios:out and ios:app ??

Answer 1

The ios::out is the default mode of ofstream. With the mode of the file does not exist, it gets created but if the file exists then its existing contents get deleted.
The ios::app is output mode of ofstream. With the mode of the file does not exist, it gets created but if the file exists then its existing contents are retained and new information is appended to it.