It took me quite a while to figure out that my .open() call wasn't opening a file because I had both the trunc and app mode options set. I only figured this out after catching a little note written on the C++ docs.
This seems like a weird gotcha. Why is this the case? Can you not truncate the file and then append only? Or is this considered superfluous specification?
The iostream open modes correspond roughly to the fopen mode in the C library and fopen has a w mode that truncates and an a mode that appends, but no combination of the two.
The allowable combinations of flags are specified in [filebuf.members] in the standard. Table 132 gives the possibilities:
So, since the combination of trunc and app isn't in the table, the open is required to fail.
来源：https://stackoverflow.com/questions/26900392/why-does-fstream-open-fail-if-the-mode-has-both-trunc-and-app-set