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diff --git a/libstdc++-v3/doc/html/manual/bk01pt05ch13s06.html b/libstdc++-v3/doc/html/manual/bk01pt05ch13s06.html new file mode 100644 index 00000000000..4725e2c012c --- /dev/null +++ b/libstdc++-v3/doc/html/manual/bk01pt05ch13s06.html @@ -0,0 +1,91 @@ +<?xml version="1.0" encoding="UTF-8" standalone="no"?> +<!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>CString (MFC)</title><meta name="generator" content="DocBook XSL Stylesheets V1.73.2" /><meta name="keywords" content=" ISO C++ , library " /><link rel="start" 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> + </p><p>A common lament seen in various newsgroups deals with the Standard + string class as opposed to the Microsoft Foundation Class called + CString. Often programmers realize that a standard portable + answer is better than a proprietary nonportable one, but in porting + their application from a Win32 platform, they discover that they + are relying on special functions offered by the CString class. + </p><p>Things are not as bad as they seem. In + <a class="ulink" href="http://gcc.gnu.org/ml/gcc/1999-04n/msg00236.html" target="_top">this + message</a>, Joe Buck points out a few very important things: + </p><div class="itemizedlist"><ul type="disc"><li><p>The Standard <code class="code">string</code> supports all the operations + that CString does, with three exceptions. + </p></li><li><p>Two of those exceptions (whitespace trimming and case + conversion) are trivial to implement. In fact, we do so + on this page. + </p></li><li><p>The third is <code class="code">CString::Format</code>, which allows formatting + in the style of <code class="code">sprintf</code>. This deserves some mention: + </p></li></ul></div><p> + The old libg++ library had a function called form(), which did much + the same thing. But for a Standard solution, you should use the + stringstream classes. These are the bridge between the iostream + hierarchy and the string class, and they operate with regular + streams seamlessly because they inherit from the iostream + hierarchy. An quick example: + </p><pre class="programlisting"> + #include <iostream> + #include <string> + #include <sstream> + + string f (string& incoming) // incoming is "foo N" + { + istringstream incoming_stream(incoming); + string the_word; + int the_number; + + incoming_stream >> the_word // extract "foo" + >> the_number; // extract N + + ostringstream output_stream; + output_stream << "The word was " << the_word + << " and 3*N was " << (3*the_number); + + return output_stream.str(); + } </pre><p>A serious problem with CString is a design bug in its memory + allocation. Specifically, quoting from that same message: + </p><pre class="programlisting"> + CString suffers from a common programming error that results in + poor performance. Consider the following code: + + CString n_copies_of (const CString& foo, unsigned n) + { + CString tmp; + for (unsigned i = 0; i < n; i++) + tmp += foo; + return tmp; + } + + This function is O(n^2), not O(n). The reason is that each += + causes a reallocation and copy of the existing string. Microsoft + applications are full of this kind of thing (quadratic performance + on tasks that can be done in linear time) -- on the other hand, + we should be thankful, as it's created such a big market for high-end + ix86 hardware. :-) + + If you replace CString with string in the above function, the + performance is O(n). + </pre><p>Joe Buck also pointed out some other things to keep in mind when + comparing CString and the Standard string class: + </p><div class="itemizedlist"><ul type="disc"><li><p>CString permits access to its internal representation; coders + who exploited that may have problems moving to <code class="code">string</code>. + </p></li><li><p>Microsoft ships the source to CString (in the files + MFC\SRC\Str{core,ex}.cpp), so you could fix the allocation + bug and rebuild your MFC libraries. + <span class="emphasis"><em><span class="emphasis"><em>Note:</em></span> It looks like the CString shipped + with VC++6.0 has fixed this, although it may in fact have been + one of the VC++ SPs that did it.</em></span> + </p></li><li><p><code class="code">string</code> operations like this have O(n) complexity + <span class="emphasis"><em>if the implementors do it correctly</em></span>. The libstdc++ + implementors did it correctly. Other vendors might not. + </p></li><li><p>While parts of the SGI STL are used in libstdc++, their + string class is not. The SGI <code class="code">string</code> is essentially + <code class="code">vector<char></code> and does not do any reference + counting like libstdc++'s does. (It is O(n), though.) + So if you're thinking about SGI's string or rope classes, + you're now looking at four possibilities: CString, the + libstdc++ string, the SGI string, and the SGI rope, and this + is all before any allocator or traits customizations! (More + choices than you can shake a stick at -- want fries with that?) + </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> |