cjcurrie cjcurrie - 3 months ago 40
C++ Question

What is the difference between flush() and sync() in regard to fstream buffers?

I was reading the cplusplus.com tutorial on I/O. At the end, it says fstream buffers are synchronized with the file on disc


Explicitly, with manipulators: When certain manipulators are used on
streams, an explicit synchronization takes place. These manipulators
are: flush and endl.


and


Explicitly, with member function sync(): Calling
stream's member function sync(), which takes no parameters, causes an
immediate synchronization. This function returns an int value equal to
-1 if the stream has no associated buffer or in case of failure. Otherwise (if the stream buffer was successfully synchronized) it
returns 0.


in addition to a few other implicit cases ( such as destruction and stream.close() )

What is the difference between calling fstream::flush() and fstream::sync()? endl?



In my code, I've always used flush().

Documentation on std::flush():


Flush stream buffer

Synchronizes the buffer associated with the stream
to its controlled output sequence. This effectively means that all
unwritten characters in the buffer are written to its controlled
output sequence as soon as possible ("flushed").


Documentation on std::streambuf::sync():


Synchronize input buffer with source of characters

It is called to synchronize the stream buffer with the controlled sequence (like the file in the case of file streams). The public member function pubsync calls this protected member function to perform this action.


Forgive me if this is a newbie question; I am a noob.

Answer

basic_ostream::flush This is a non-virtual function which writes uncommited changes to the underlying buffer. In case of error, it sets an error flag in the used stream object. This is because the return value is a reference to the stream itself, to allow chaining.

basic_filebuf::sync This is a virtual function which writes all pending changes to the underlying file and returns an error code to signal success or failure.

endl This, when applied to an ostream, writes an '\n' to the stream and then calls flush on that stream.

So, essentially: flush is a more general function for any stream, whereas sync is explicitly bound to a file. flush is non-virtual, whereas sync is virtual. This changes how they can be used via pointers (to base class) in the case of inheritance. Furthermore, they differ in how they report errors.

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