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| 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="https://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="https://www.cppstdlib.com" 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 is 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 14. | |
| Input and Output | |
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