What does std::ofstream::close() actually do?

Question

This question: How to protect log from application crash? has lead me to another - what does std::ofstream::close() actually do? I know it calls flush() and that's one thing. But what else? What closing the file actually is?

Edit: Let me rephrase my question - is anything physically done to the actual file during the call to close() or is it just std::ofstream internal cleanup stuff?

Answer 1

Here is the trace of calls from documentation:

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void std::basic_ofstream::close();

Effectively calls rdbuf()->close(). If an error occurs during operation, setstate(failbit) is called.

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std::basic_streambuf<CharT,Traits>* std::basic_ofstream::rdbuf() const;

Returns the associated stream buffer. If there is no associated stream buffer, returns NULL.

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std::basic_streambuf actually inherits std::basic_filebuf, therefore:

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std::basic_filebuf<CharT, Traits>* std::basic_filebuf::close();

If a put area exist (e.g. file was opened for writing), first calls overflow(Traits::eof()) to write all pending output to the file, including any unshift sequences.

If the most recently called function, out of underflow(), overflow(), seekpos(), and seekoff(), was overflow(), then calls std::codecvt::unshift(), perhaps multiple times, to determine the unshift sequence according to the imbued locale, and writes that sequence to file with overflow(Traits::eof()).

Then, closes the file as if by calling std::fclose, regardless of whether any of the preceding calls succeeded or failed.

NOTE: close() is typically called through the destructor of std::basic_filebuf (which, in turn, is typically called by the destructor of std::basic_fstream.

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First of all, we can see that it doesn't actually call flush() directly as you expected. Nevertheless, the flushing effect indeed occurs in the std::basic_filebuf::close() method. In addition, we can see that it still does a bit of tampering with a file, i.e. writing the unshift sequence. Nothing else special happens then, the file simply closes.

Pay attention to the NOTE above: in most cases you don't even need to call std::basic_ofstream::close() explicitly.