According to the standard, fstream doesn't have an explicit destructor, but the standard says "It uses a basic_filebuf<charT, traits> object to control the associated sequences." ~basic_filebuf(), in turn, is defined to call close() (which I think flushes to disk?) and swallow exceptions.

However, I can't seem to find anything that explicitly ties the lifetime of the fstream to the corresponding basic_filebuf. fstream doesn't have an explicitly declared destructor and the standard doesn't require that the basic_filebuf is a member of fstream, so the obvious ways the file would be closed don't seem to be explicitly required. In addition, all of fstream's parents' destructors are specified to perform no operations on the underlying rdbuf(). Which leaves... I don't know?

cppreference says the underlying file is closed, though, which should flush it. And that's what I would expect for an RAII class! But I just can't seem to find the requirement...