c++ - How to disable buffering on a stream?

Question

In C, I can easily set a stream to unbuffered I/O:

FILE * f = fopen( "test", "r" );
setvbuf( f, (char *)NULL, _IONBF, 0 );

How would I achieve similarly unbuffered I/O using C++ IOStreams?

Answer 1

For file streams, you can use pubsetbuf for that :

std::ifstream f;
f.rdbuf()->pubsetbuf(0, 0);
f.open("test");

Explanation

The C++ standard says the following about the effect of setbuf (and thus pubsetbuf) for file streams :

If setbuf(0,0) is called on a stream before any I/O has occurred on that stream, the stream becomes unbuffered. Otherwise the results are implementation-defined. “Unbuffered” means that pbase() and pptr() always return null and output to the file should appear as soon as possible.

The first sentence guarantees that the above code makes the stream unbuffered. Note that some compilers (eg. gcc) see opening a file as an I/O operation on the stream, so pubsetbuf should be called before opening the file (as above).

The last sentence, however, seems to imply that that would only be for output, and not for input. I'm not sure if that was an oversight, or whether that was intended. Consulting your compiler documentation might be useful. For gcc eg., both input and output are made unbuffered (ref. GNU C++ Library Manual - Stream Buffers).