First, are you sure that you understand buffering? Particularly + the fact that C++ may not, in fact, have anything to do with it? +
The rules for buffering can be a little odd, but they aren't any + different from those of C. (Maybe that's why they can be a bit + odd.) Many people think that writing a newline to an output + stream automatically flushes the output buffer. This is true only + when the output stream is, in fact, a terminal and not a file + or some other device -- and that may not even be true + since C++ says nothing about files nor terminals. All of that is + system-dependent. (The "newline-buffer-flushing only occurring + on terminals" thing is mostly true on Unix systems, though.) +
Some people also believe that sending endl down an
+ output stream only writes a newline. This is incorrect; after a
+ newline is written, the buffer is also flushed. Perhaps this
+ is the effect you want when writing to a screen -- get the text
+ out as soon as possible, etc -- but the buffering is largely
+ wasted when doing this to a file:
+
+ output << "a line of text" << endl; + output << some_data_variable << endl; + output << "another line of text" << endl;
The proper thing to do in this case to just write the data out + and let the libraries and the system worry about the buffering. + If you need a newline, just write a newline: +
+ output << "a line of text\n" + << some_data_variable << '\n' + << "another line of text\n";
I have also joined the output statements into a single statement. + You could make the code prettier by moving the single newline to + the start of the quoted text on the last line, for example. +
If you do need to flush the buffer above, you can send an
+ endl if you also need a newline, or just flush the buffer
+ yourself:
+
+ output << ...... << flush; // can use std::flush manipulator + output.flush(); // or call a member fn
On the other hand, there are times when writing to a file should + be like writing to standard error; no buffering should be done + because the data needs to appear quickly (a prime example is a + log file for security-related information). The way to do this is + just to turn off the buffering before any I/O operations at + all have been done (note that opening counts as an I/O operation): +
+ std::ofstream os;
+ std::ifstream is;
+ int i;
+
+ os.rdbuf()->pubsetbuf(0,0);
+ is.rdbuf()->pubsetbuf(0,0);
+
+ os.open("/foo/bar/baz");
+ is.open("/qux/quux/quuux");
+ ...
+ os << "this data is written immediately\n";
+ is >> i; // and this will probably cause a disk read Since all aspects of buffering are handled by a streambuf-derived
+ member, it is necessary to get at that member with rdbuf().
+ Then the public version of setbuf can be called. The
+ arguments are the same as those for the Standard C I/O Library
+ function (a buffer area followed by its size).
+
A great deal of this is implementation-dependent. For example,
+ streambuf does not specify any actions for its own
+ setbuf()-ish functions; the classes derived from
+ streambuf each define behavior that "makes
+ sense" for that class: an argument of (0,0) turns off buffering
+ for filebuf but does nothing at all for its siblings
+ stringbuf and strstreambuf, and specifying
+ anything other than (0,0) has varying effects.
+ User-defined classes derived from streambuf can
+ do whatever they want. (For filebuf and arguments for
+ (p,s) other than zeros, libstdc++ does what you'd expect:
+ the first s bytes of p are used as a buffer,
+ which you must allocate and deallocate.)
+
A last reminder: there are usually more buffers involved than + just those at the language/library level. Kernel buffers, disk + buffers, and the like will also have an effect. Inspecting and + changing those are system-dependent. +