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2009-03-17 Benjamin Kosnik <bkoz@redhat.com> * doc/xml/manual/appendix_contributing.xml: Add docbook style sheet version information. Table-ize docbook element examples. * doc/xml/manual/using.xml: Human-readable header markup. Alphabetized. Add new headers. * doc/html: Regenerate. From-SVN: r144923
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91 lines
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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">
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<html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>CString (MFC)</title><meta name="generator" content="DocBook XSL Stylesheets V1.74.0" /><meta name="keywords" content=" ISO C++ , library " /><link rel="home" href="../spine.html" title="The GNU C++ Library Documentation" /><link rel="up" href="bk01pt05ch13.html" title="Chapter 13. String Classes" /><link rel="prev" href="bk01pt05ch13s05.html" title="Shrink to Fit" /><link rel="next" href="localization.html" title="Part VI. Localization" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">CString (MFC)</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="bk01pt05ch13s05.html">Prev</a> </td><th width="60%" align="center">Chapter 13. String Classes</th><td width="20%" align="right"> <a accesskey="n" href="localization.html">Next</a></td></tr></table><hr /></div><div class="sect1" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="strings.string.Cstring"></a>CString (MFC)</h2></div></div></div><p>
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</p><p>A common lament seen in various newsgroups deals with the Standard
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string class as opposed to the Microsoft Foundation Class called
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CString. Often programmers realize that a standard portable
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answer is better than a proprietary nonportable one, but in porting
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their application from a Win32 platform, they discover that they
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are relying on special functions offered by the CString class.
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</p><p>Things are not as bad as they seem. In
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<a class="ulink" href="http://gcc.gnu.org/ml/gcc/1999-04n/msg00236.html" target="_top">this
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message</a>, Joe Buck points out a few very important things:
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</p><div class="itemizedlist"><ul type="disc"><li><p>The Standard <code class="code">string</code> supports all the operations
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that CString does, with three exceptions.
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</p></li><li><p>Two of those exceptions (whitespace trimming and case
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conversion) are trivial to implement. In fact, we do so
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on this page.
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</p></li><li><p>The third is <code class="code">CString::Format</code>, which allows formatting
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in the style of <code class="code">sprintf</code>. This deserves some mention:
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</p></li></ul></div><p>
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The old libg++ library had a function called form(), which did much
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the same thing. But for a Standard solution, you should use the
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stringstream classes. These are the bridge between the iostream
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hierarchy and the string class, and they operate with regular
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streams seamlessly because they inherit from the iostream
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hierarchy. An quick example:
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</p><pre class="programlisting">
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#include <iostream>
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#include <string>
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#include <sstream>
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string f (string& incoming) // incoming is "foo N"
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{
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istringstream incoming_stream(incoming);
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string the_word;
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int the_number;
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incoming_stream >> the_word // extract "foo"
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>> the_number; // extract N
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ostringstream output_stream;
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output_stream << "The word was " << the_word
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<< " and 3*N was " << (3*the_number);
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return output_stream.str();
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} </pre><p>A serious problem with CString is a design bug in its memory
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allocation. Specifically, quoting from that same message:
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</p><pre class="programlisting">
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CString suffers from a common programming error that results in
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poor performance. Consider the following code:
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CString n_copies_of (const CString& foo, unsigned n)
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{
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CString tmp;
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for (unsigned i = 0; i < n; i++)
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tmp += foo;
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return tmp;
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}
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This function is O(n^2), not O(n). The reason is that each +=
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causes a reallocation and copy of the existing string. Microsoft
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applications are full of this kind of thing (quadratic performance
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on tasks that can be done in linear time) -- on the other hand,
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we should be thankful, as it's created such a big market for high-end
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ix86 hardware. :-)
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If you replace CString with string in the above function, the
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performance is O(n).
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</pre><p>Joe Buck also pointed out some other things to keep in mind when
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comparing CString and the Standard string class:
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</p><div class="itemizedlist"><ul type="disc"><li><p>CString permits access to its internal representation; coders
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who exploited that may have problems moving to <code class="code">string</code>.
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</p></li><li><p>Microsoft ships the source to CString (in the files
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MFC\SRC\Str{core,ex}.cpp), so you could fix the allocation
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bug and rebuild your MFC libraries.
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<span class="emphasis"><em><span class="emphasis"><em>Note:</em></span> It looks like the CString shipped
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with VC++6.0 has fixed this, although it may in fact have been
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one of the VC++ SPs that did it.</em></span>
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</p></li><li><p><code class="code">string</code> operations like this have O(n) complexity
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<span class="emphasis"><em>if the implementors do it correctly</em></span>. The libstdc++
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implementors did it correctly. Other vendors might not.
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</p></li><li><p>While parts of the SGI STL are used in libstdc++, their
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string class is not. The SGI <code class="code">string</code> is essentially
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<code class="code">vector<char></code> and does not do any reference
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counting like libstdc++'s does. (It is O(n), though.)
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So if you're thinking about SGI's string or rope classes,
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you're now looking at four possibilities: CString, the
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libstdc++ string, the SGI string, and the SGI rope, and this
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is all before any allocator or traits customizations! (More
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choices than you can shake a stick at -- want fries with that?)
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</p></li></ul></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="bk01pt05ch13s05.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="bk01pt05ch13.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="localization.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Shrink to Fit </td><td width="20%" align="center"><a accesskey="h" href="../spine.html">Home</a></td><td width="40%" align="right" valign="top"> Part VI. Localization</td></tr></table></div></body></html>
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