How to append to a file with fstream fstream::app flag seems not to work

Question

i simply want to write (append) to a logfile. I looked it up here:
http://www.cplusplus.com/reference/iostream/fstream/open/

so this is what i did

#include <fstream>

fstream outfile;

//outfile.open("/tmp/debug.txt" );  // works, simply for writing
outfile.open("/tmp/debug.txt", fstream::app );  // does nothing

outfile << "START" << endl;

outfile.close();

Answer 1

fstream::app|fstream::out instead of fstream::app. app doesn't make sense without specifying out (one could think it should have implied out, but it doesn't).