Quite simple really,just open word.txt with ios::app (append) added to the options when opening the file.That will automaticaly put the file pointer to the end of the file and you can just write to it normally.

Another way is to open the file and use filehandle.seekp(0,ios::end); to move the pointer to the end of the file and then write the output you want to the file and it will still be at the end.

Checks the tut for the list of options,I covered them there.

Btw,then close the file when you exit or are finished with using it.There you go a new word was added to the file.