Why does ofstream require a flush?

Question

If I run the following code, no file is created at all:

std::ofstream outputFile(strOutputLocation.c_str(), std::ios::binary);
outputFile.write((const char*)lpResLock, dwSizeRes);
outputFile.close();

However, if I add a flush() before the close, it works:

std::ofstream outputFile(strOutputLocation.c_str(), std::ios::binary);
outputFile.write((const char*)lpResLock, dwSizeRes);
outputFile.flush();
outputFile.close();

Does the standard library actually require this, or is it a bug in the Visual C++ CRT?

Answer 1

It's a bug. Reading §27.8.1.10/4, abridged:

void close();
Effects: Calls rdbuf()->close()...

What does rdbuf()->close() do? According to §27.8.1.3/6, abridged, emphasis mine:

basic_filebuf<charT,traits>* close();
If is_open() == false, returns a null pointer. If a put area exists, calls overflow(EOF) to flush characters. ...

That is, it's suppose to flush. (Indeed, the call to flush() ultimately does the same thing.)

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Note the call to close() itself isn't needed, as the destructor of basic_ofstream will call close().

Answer 2

Are you checking the file before you exit from the program? OS will buffer all IO, so may not see any data(unless you flush) in the file before you exit.