Does it makes sense to call sync() on output-only fstream?

Question

I found some legacy company code that was using an std::fstream and calling fs.flush() after each write, then fs.sync() each time a certain amount of data were written to the stream. This code only needs to do output operations, not input ones.

std::fstream fs(filename, ios::out | ios::binary);
fs.write(buff, size);
fs.flush();
if (/* needs to sync */) {
  fs.sync();
}

This is surprising me as I usually use std::ofstream for output-only IO operations and because this class actually does not have the sync() method.

Since std::ofstream does not have the sync() method, does it makes sense to use std::fstream and sync() for output-only IO operations?

In my specific case, should I keep using std::ofstream and call flush without thinking about sync?

Answer 1

If you look at http://www.cplusplus.com/reference/fstream/fstream/ you will find sync under the heading "Public member functions inherited from istream" and its description is:

Synchronize input buffer

Synchronizes the associated stream buffer with its controlled input sequence.

So no, this is not needed for output-only code and will do nothing useful there.

If from the logic of the code it thinks it is doing a fsync equivalent, you would need to do that in a different way. See How to do fsync on an ofstream?