1 <?xml version=
"1.0" encoding=
"UTF-8" standalone=
"no"?>
2 <!DOCTYPE html PUBLIC
"-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
3 <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 V1.75.2" /><meta name=
"keywords" content=
" ISO C++ , library " /><link rel=
"home" href=
"../spine.html" title=
"The GNU C++ Library Documentation" /><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.
6 </th><td width=
"20%" align=
"right"> <a accesskey=
"n" href=
"stringstreams.html">Next
</a></td></tr></table><hr /></div><div class=
"sect1" title=
"Stream Buffers"><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=
"sect2" title=
"Derived streambuf Classes"><div class=
"titlepage"><div><div><h3 class=
"title"><a id=
"io.streambuf.derived"></a>Derived streambuf Classes
</h3></div></div></div><p>
7 </p><p>Creating your own stream buffers for I/O can be remarkably easy.
8 If you are interested in doing so, we highly recommend two very
10 <a class=
"ulink" href=
"http://www.angelikalanger.com/iostreams.html" target=
"_top">Standard C++
11 IOStreams and Locales
</a> by Langer and Kreft, ISBN
0-
201-
18395-
1, and
12 <a class=
"ulink" href=
"http://www.josuttis.com/libbook/" target=
"_top">The C++ Standard Library
</a>
13 by Nicolai Josuttis, ISBN
0-
201-
37926-
0. Both are published by
14 Addison-Wesley, who isn't paying us a cent for saying that, honest.
15 </p><p>Here is a simple example, io/outbuf1, from the Josuttis text. It
16 transforms everything sent through it to uppercase. This version
17 assumes many things about the nature of the character type being
18 used (for more information, read the books or the newsgroups):
19 </p><pre class=
"programlisting">
20 #include
<iostream
>
21 #include
<streambuf
>
22 #include
<locale
>
23 #include
<cstdio
>
25 class outbuf : public std::streambuf
28 /* central output function
29 * - print characters in uppercase mode
31 virtual int_type overflow (int_type c) {
33 // convert lowercase to uppercase
34 c = std::toupper(static_cast
<char
>(c),getloc());
36 // and write the character to the standard output
37 if (putchar(c) == EOF) {
47 // create special output buffer
49 // initialize output stream with that output buffer
50 std::ostream out(
&ob);
52 out
<< "31 hexadecimal: "
53 << std::hex
<< 31 << std::endl;
56 </pre><p>Try it yourself! More examples can be found in
3.1.x code, in
57 <code class=
"code">include/ext/*_filebuf.h
</code>, and in this article by James Kanze:
58 <a class=
"ulink" href=
"http://kanze.james.neuf.fr/articles/fltrsbf1.html" target=
"_top">Filtering
60 </p></div><div class=
"sect2" title=
"Buffering"><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? Chaptericularly
61 the fact that C++ may not, in fact, have anything to do with it?
62 </p><p>The rules for buffering can be a little odd, but they aren't any
63 different from those of C. (Maybe that's why they can be a bit
64 odd.) Many people think that writing a newline to an output
65 stream automatically flushes the output buffer. This is true only
66 when the output stream is, in fact, a terminal and not a file
67 or some other device -- and
<span class=
"emphasis"><em>that
</em></span> may not even be true
68 since C++ says nothing about files nor terminals. All of that is
69 system-dependent. (The
"newline-buffer-flushing only occurring
70 on terminals" thing is mostly true on Unix systems, though.)
71 </p><p>Some people also believe that sending
<code class=
"code">endl
</code> down an
72 output stream only writes a newline. This is incorrect; after a
73 newline is written, the buffer is also flushed. Perhaps this
74 is the effect you want when writing to a screen -- get the text
75 out as soon as possible, etc -- but the buffering is largely
76 wasted when doing this to a file:
77 </p><pre class=
"programlisting">
78 output
<< "a line of text" << endl;
79 output
<< some_data_variable
<< endl;
80 output
<< "another line of text" << endl;
</pre><p>The proper thing to do in this case to just write the data out
81 and let the libraries and the system worry about the buffering.
82 If you need a newline, just write a newline:
83 </p><pre class=
"programlisting">
84 output
<< "a line of text\n"
85 << some_data_variable
<< '\n'
86 << "another line of text\n";
</pre><p>I have also joined the output statements into a single statement.
87 You could make the code prettier by moving the single newline to
88 the start of the quoted text on the last line, for example.
89 </p><p>If you do need to flush the buffer above, you can send an
90 <code class=
"code">endl
</code> if you also need a newline, or just flush the buffer
92 </p><pre class=
"programlisting">
93 output
<< ......
<< flush; // can use std::flush manipulator
94 output.flush(); // or call a member fn
</pre><p>On the other hand, there are times when writing to a file should
95 be like writing to standard error; no buffering should be done
96 because the data needs to appear quickly (a prime example is a
97 log file for security-related information). The way to do this is
98 just to turn off the buffering
<span class=
"emphasis"><em>before any I/O operations at
99 all
</em></span> have been done (note that opening counts as an I/O operation):
100 </p><pre class=
"programlisting">
105 os.rdbuf()-
>pubsetbuf(
0,
0);
106 is.rdbuf()-
>pubsetbuf(
0,
0);
108 os.open(
"/foo/bar/baz");
109 is.open(
"/qux/quux/quuux");
111 os
<< "this data is written immediately\n";
112 is
>> i; // and this will probably cause a disk read
</pre><p>Since all aspects of buffering are handled by a streambuf-derived
113 member, it is necessary to get at that member with
<code class=
"code">rdbuf()
</code>.
114 Then the public version of
<code class=
"code">setbuf
</code> can be called. The
115 arguments are the same as those for the Standard C I/O Library
116 function (a buffer area followed by its size).
117 </p><p>A great deal of this is implementation-dependent. For example,
118 <code class=
"code">streambuf
</code> does not specify any actions for its own
119 <code class=
"code">setbuf()
</code>-ish functions; the classes derived from
120 <code class=
"code">streambuf
</code> each define behavior that
"makes
121 sense" for that class: an argument of (
0,
0) turns off buffering
122 for
<code class=
"code">filebuf
</code> but does nothing at all for its siblings
123 <code class=
"code">stringbuf
</code> and
<code class=
"code">strstreambuf
</code>, and specifying
124 anything other than (
0,
0) has varying effects.
125 User-defined classes derived from
<code class=
"code">streambuf
</code> can
126 do whatever they want. (For
<code class=
"code">filebuf
</code> and arguments for
127 <code class=
"code">(p,s)
</code> other than zeros, libstdc++ does what you'd expect:
128 the first
<code class=
"code">s
</code> bytes of
<code class=
"code">p
</code> are used as a buffer,
129 which you must allocate and deallocate.)
130 </p><p>A last reminder: there are usually more buffers involved than
131 just those at the language/library level. Kernel buffers, disk
132 buffers, and the like will also have an effect. Inspecting and
133 changing those are system-dependent.
134 </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.
137 </td><td width=
"20%" align=
"center"><a accesskey=
"h" href=
"../spine.html">Home
</a></td><td width=
"40%" align=
"right" valign=
"top"> Memory Based Streams
</td></tr></table></div></body></html>