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| author | upstream source tree <ports@midipix.org> | 2015-03-15 20:14:05 -0400 |
|---|---|---|
| committer | upstream source tree <ports@midipix.org> | 2015-03-15 20:14:05 -0400 |
| commit | 554fd8c5195424bdbcabf5de30fdc183aba391bd (patch) | |
| tree | 976dc5ab7fddf506dadce60ae936f43f58787092 /libstdc++-v3/doc/html/manual/streambufs.html | |
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1 files changed, 137 insertions, 0 deletions
diff --git a/libstdc++-v3/doc/html/manual/streambufs.html b/libstdc++-v3/doc/html/manual/streambufs.html new file mode 100644 index 000000000..92c148dfc --- /dev/null +++ b/libstdc++-v3/doc/html/manual/streambufs.html @@ -0,0 +1,137 @@ +<?xml version="1.0" encoding="UTF-8" standalone="no"?> +<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd"> +<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. + Input and Output + + </td><td align="center"><a accesskey="h" href="../spine.html">Home</a></td><td align="right" valign="top"> Memory Based Streams</td></tr></table></div></body></html> |