Why does fstream.open() fail “If the mode has both trunc and app set”?

Question

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?

Answer 1

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.

Answer 2

iostream打开模式大致对应于C库中的fopen模式，而fopen具有截断的w模式和附加a模式，但不能将两者组合。