do I need to close a std::fstream? [duplicate]

Solution 1:

I think the previous answers are misleading.

fstream is a proper RAII object, it does close automatically at the end of the scope, and there is absolutely no need whatsoever to call close manually when closing at the end of the scope is sufficient.

In particular, it’s not a “best practice” and it’s not necessary to flush the output.

And while Drakosha is right that calling close gives you the possibility to check the fail bit of the stream, nobody does that, anyway.

In an ideal world, one would simply call stream.exceptions(ios::failbit) beforehand and handle the exception that is thrown in an fstream’s destructor. But unfortunately exceptions in destructors are a broken concept in C++ so that’s not a good idea.

So if you want to check the success of closing a file, do it manually (but only then).

Solution 2:

To append to Amy Lee's answer, it's better to do it manually because this way you can check for errors too.

BTW, according to "close" manpage:

Not checking the return value of close() is a common but nevertheless serious programming error. It is quite possible that errors on a previous write(2) operation are first reported at the final close(). Not checking the return value when closing the file may lead to silent loss of data. This can especially be observed with NFS and with disk quota.

A successful close does not guarantee that the data has been successfully saved to disk, as the kernel defers writes. It is not common for a filesystem to flush the buffers when the stream is closed. If you need to be sure that the data is physically stored use fsync(2). (It will depend on the disk hardware at this point.)