136 lines
8.9 KiB
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136 lines
8.9 KiB
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<?xml version="1.0" encoding="UTF-8" standalone="no"?>
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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-NS Stylesheets V1.78.1" /><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.
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Input and Output
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</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>
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</p><p>Creating your own stream buffers for I/O can be remarkably easy.
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If you are interested in doing so, we highly recommend two very
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excellent books:
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<a class="link" href="http://www.angelikalanger.com/iostreams.html" target="_top">Standard C++
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IOStreams and Locales</a> by Langer and Kreft, ISBN 0-201-18395-1, and
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<a class="link" href="http://www.josuttis.com/libbook/" target="_top">The C++ Standard Library</a>
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by Nicolai Josuttis, ISBN 0-201-37926-0. Both are published by
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Addison-Wesley, who isn't paying us a cent for saying that, honest.
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</p><p>Here is a simple example, io/outbuf1, from the Josuttis text. It
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transforms everything sent through it to uppercase. This version
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assumes many things about the nature of the character type being
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used (for more information, read the books or the newsgroups):
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</p><pre class="programlisting">
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#include <iostream>
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#include <streambuf>
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#include <locale>
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#include <cstdio>
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class outbuf : public std::streambuf
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{
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protected:
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/* central output function
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* - print characters in uppercase mode
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*/
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virtual int_type overflow (int_type c) {
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if (c != EOF) {
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// convert lowercase to uppercase
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c = std::toupper(static_cast<char>(c),getloc());
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// and write the character to the standard output
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if (putchar(c) == EOF) {
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return EOF;
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}
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}
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return c;
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}
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};
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int main()
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{
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// create special output buffer
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outbuf ob;
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// initialize output stream with that output buffer
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std::ostream out(&ob);
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out << "31 hexadecimal: "
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<< std::hex << 31 << std::endl;
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return 0;
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}
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</pre><p>Try it yourself! More examples can be found in 3.1.x code, in
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<code class="filename">include/ext/*_filebuf.h</code>, and in this article by James Kanze:
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<a class="link" href="http://kanze.james.neuf.fr/articles/fltrsbf1.html" target="_top">Filtering
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Streambufs</a>.
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</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
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the fact that C++ may not, in fact, have anything to do with it?
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</p><p>The rules for buffering can be a little odd, but they aren't any
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different from those of C. (Maybe that's why they can be a bit
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odd.) Many people think that writing a newline to an output
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stream automatically flushes the output buffer. This is true only
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when the output stream is, in fact, a terminal and not a file
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or some other device -- and <span class="emphasis"><em>that</em></span> may not even be true
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since C++ says nothing about files nor terminals. All of that is
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system-dependent. (The "newline-buffer-flushing only occurring
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on terminals" thing is mostly true on Unix systems, though.)
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</p><p>Some people also believe that sending <code class="code">endl</code> down an
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output stream only writes a newline. This is incorrect; after a
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newline is written, the buffer is also flushed. Perhaps this
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is the effect you want when writing to a screen -- get the text
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out as soon as possible, etc -- but the buffering is largely
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wasted when doing this to a file:
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</p><pre class="programlisting">
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output << "a line of text" << endl;
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output << some_data_variable << endl;
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output << "another line of text" << endl; </pre><p>The proper thing to do in this case to just write the data out
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and let the libraries and the system worry about the buffering.
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If you need a newline, just write a newline:
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</p><pre class="programlisting">
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output << "a line of text\n"
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<< some_data_variable << '\n'
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<< "another line of text\n"; </pre><p>I have also joined the output statements into a single statement.
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You could make the code prettier by moving the single newline to
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the start of the quoted text on the last line, for example.
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</p><p>If you do need to flush the buffer above, you can send an
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<code class="code">endl</code> if you also need a newline, or just flush the buffer
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yourself:
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</p><pre class="programlisting">
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output << ...... << flush; // can use std::flush manipulator
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output.flush(); // or call a member fn </pre><p>On the other hand, there are times when writing to a file should
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be like writing to standard error; no buffering should be done
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because the data needs to appear quickly (a prime example is a
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log file for security-related information). The way to do this is
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just to turn off the buffering <span class="emphasis"><em>before any I/O operations at
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all</em></span> have been done (note that opening counts as an I/O operation):
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</p><pre class="programlisting">
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std::ofstream os;
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std::ifstream is;
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int i;
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os.rdbuf()->pubsetbuf(0,0);
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is.rdbuf()->pubsetbuf(0,0);
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os.open("/foo/bar/baz");
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is.open("/qux/quux/quuux");
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...
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os << "this data is written immediately\n";
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is >> i; // and this will probably cause a disk read </pre><p>Since all aspects of buffering are handled by a streambuf-derived
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member, it is necessary to get at that member with <code class="code">rdbuf()</code>.
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Then the public version of <code class="code">setbuf</code> can be called. The
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arguments are the same as those for the Standard C I/O Library
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function (a buffer area followed by its size).
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</p><p>A great deal of this is implementation-dependent. For example,
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<code class="code">streambuf</code> does not specify any actions for its own
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<code class="code">setbuf()</code>-ish functions; the classes derived from
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<code class="code">streambuf</code> each define behavior that "makes
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sense" for that class: an argument of (0,0) turns off buffering
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for <code class="code">filebuf</code> but does nothing at all for its siblings
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<code class="code">stringbuf</code> and <code class="code">strstreambuf</code>, and specifying
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anything other than (0,0) has varying effects.
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User-defined classes derived from <code class="code">streambuf</code> can
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do whatever they want. (For <code class="code">filebuf</code> and arguments for
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<code class="code">(p,s)</code> other than zeros, libstdc++ does what you'd expect:
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the first <code class="code">s</code> bytes of <code class="code">p</code> are used as a buffer,
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which you must allocate and deallocate.)
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</p><p>A last reminder: there are usually more buffers involved than
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just those at the language/library level. Kernel buffers, disk
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buffers, and the like will also have an effect. Inspecting and
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changing those are system-dependent.
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</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.
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Input and Output
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</td><td width="20%" align="center"><a accesskey="h" href="../index.html">Home</a></td><td width="40%" align="right" valign="top"> Memory Based Streams</td></tr></table></div></body></html> |