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<html xmlns="http://www.w3.org/1999/xhtml"><head><title>Stream Buffers</title><meta name="generator" content="DocBook XSL-NS Stylesheets V1.76.1"/><meta name="keywords" content=" ISO C++ , library "/><link rel="home" href="../spine.html" title="The GNU C++ Library"/><link rel="up" href="io.html" title="Chapter 13. Input and Output"/><link rel="prev" href="io.html" title="Chapter 13. Input and Output"/><link rel="next" href="stringstreams.html" title="Memory Based Streams"/></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Stream Buffers</th></tr><tr><td align="left"><a accesskey="p" href="io.html">Prev</a> </td><th width="60%" align="center">Chapter 13.
Input and Output
</th><td align="right"> <a accesskey="n" href="stringstreams.html">Next</a></td></tr></table><hr/></div><div class="section" title="Stream Buffers"><div class="titlepage"><div><div><h2 class="title"><a id="std.io.streambufs"/>Stream Buffers</h2></div></div></div><div class="section" title="Derived streambuf Classes"><div class="titlepage"><div><div><h3 class="title"><a id="io.streambuf.derived"/>Derived streambuf Classes</h3></div></div></div><p>
</p><p>Creating your own stream buffers for I/O can be remarkably easy.
If you are interested in doing so, we highly recommend two very
excellent books:
<a class="link" href="http://www.angelikalanger.com/iostreams.html">Standard C++
IOStreams and Locales</a> by Langer and Kreft, ISBN 0-201-18395-1, and
<a class="link" href="http://www.josuttis.com/libbook/">The C++ Standard Library</a>
by Nicolai Josuttis, ISBN 0-201-37926-0. Both are published by
Addison-Wesley, who isn't paying us a cent for saying that, honest.
</p><p>Here is a simple example, io/outbuf1, from the Josuttis text. It
transforms everything sent through it to uppercase. This version
assumes many things about the nature of the character type being
used (for more information, read the books or the newsgroups):
</p><pre class="programlisting">
#include <iostream>
#include <streambuf>
#include <locale>
#include <cstdio>
class outbuf : public std::streambuf
{
protected:
/* central output function
* - print characters in uppercase mode
*/
virtual int_type overflow (int_type c) {
if (c != EOF) {
// convert lowercase to uppercase
c = std::toupper(static_cast<char>(c),getloc());
// and write the character to the standard output
if (putchar(c) == EOF) {
return EOF;
}
}
return c;
}
};
int main()
{
// create special output buffer
outbuf ob;
// initialize output stream with that output buffer
std::ostream out(&ob);
out << "31 hexadecimal: "
<< std::hex << 31 << std::endl;
return 0;
}
</pre><p>Try it yourself! More examples can be found in 3.1.x code, in
<code class="code">include/ext/*_filebuf.h</code>, and in this article by James Kanze:
<a class="link" href="http://kanze.james.neuf.fr/articles/fltrsbf1.html">Filtering
Streambufs</a>.
</p></div><div class="section" title="Buffering"><div class="titlepage"><div><div><h3 class="title"><a id="io.streambuf.buffering"/>Buffering</h3></div></div></div><p>First, are you sure that you understand buffering? Particularly
the fact that C++ may not, in fact, have anything to do with it?
</p><p>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 <span class="emphasis"><em>that</em></span> 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.)
</p><p>Some people also believe that sending <code class="code">endl</code> 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:
</p><pre class="programlisting">
output << "a line of text" << endl;
output << some_data_variable << endl;
output << "another line of text" << endl; </pre><p>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:
</p><pre class="programlisting">
output << "a line of text\n"
<< some_data_variable << '\n'
<< "another line of text\n"; </pre><p>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.
</p><p>If you do need to flush the buffer above, you can send an
<code class="code">endl</code> if you also need a newline, or just flush the buffer
yourself:
</p><pre class="programlisting">
output << ...... << flush; // can use std::flush manipulator
output.flush(); // or call a member fn </pre><p>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 <span class="emphasis"><em>before any I/O operations at
all</em></span> have been done (note that opening counts as an I/O operation):
</p><pre class="programlisting">
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 </pre><p>Since all aspects of buffering are handled by a streambuf-derived
member, it is necessary to get at that member with <code class="code">rdbuf()</code>.
Then the public version of <code class="code">setbuf</code> 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).
</p><p>A great deal of this is implementation-dependent. For example,
<code class="code">streambuf</code> does not specify any actions for its own
<code class="code">setbuf()</code>-ish functions; the classes derived from
<code class="code">streambuf</code> each define behavior that "makes
sense" for that class: an argument of (0,0) turns off buffering
for <code class="code">filebuf</code> but does nothing at all for its siblings
<code class="code">stringbuf</code> and <code class="code">strstreambuf</code>, and specifying
anything other than (0,0) has varying effects.
User-defined classes derived from <code class="code">streambuf</code> can
do whatever they want. (For <code class="code">filebuf</code> and arguments for
<code class="code">(p,s)</code> other than zeros, libstdc++ does what you'd expect:
the first <code class="code">s</code> bytes of <code class="code">p</code> are used as a buffer,
which you must allocate and deallocate.)
</p><p>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.
</p></div></div><div class="navfooter"><hr/><table width="100%" summary="Navigation footer"><tr><td align="left"><a accesskey="p" href="io.html">Prev</a> </td><td align="center"><a accesskey="u" href="io.html">Up</a></td><td align="right"> <a accesskey="n" href="stringstreams.html">Next</a></td></tr><tr><td align="left" valign="top">Chapter 13.
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