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| <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>Stream Buffers</title><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><meta name="keywords" content="ISO C++, library" /><meta name="keywords" content="ISO C++, runtime, library" /><link rel="home" href="../index.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 width="20%" align="left"><a accesskey="p" href="io.html">Prev</a> </td><th width="60%" align="center">Chapter 13. |
| Input and Output |
| |
| </th><td width="20%" align="right"> <a accesskey="n" href="stringstreams.html">Next</a></td></tr></table><hr /></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="std.io.streambufs"></a>Stream Buffers</h2></div></div></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="io.streambuf.derived"></a>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" target="_top">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/" target="_top">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="filename">include/ext/*_filebuf.h</code>, and in the article |
| <a class="link" href="http://gabisoft.free.fr/articles/fltrsbf1.html" target="_top">Filtering |
| Streambufs</a> |
| by James Kanze. |
| </p></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="io.streambuf.buffering"></a>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 width="40%" align="left"><a accesskey="p" href="io.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="io.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="stringstreams.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Chapter 13. |
| Input and Output |
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