<?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 Standardstring class as opposed to the Microsoft Foundation Class calledCString. Often programmers realize that a standard portableanswer is better than a proprietary nonportable one, but in portingtheir application from a Win32 platform, they discover that theyare 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">thismessage</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 operationsthat CString does, with three exceptions.</p></li><li><p>Two of those exceptions (whitespace trimming and caseconversion) are trivial to implement. In fact, we do soon this page.</p></li><li><p>The third is <code class="code">CString::Format</code>, which allows formattingin 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 muchthe same thing. But for a Standard solution, you should use thestringstream classes. These are the bridge between the iostreamhierarchy and the string class, and they operate with regularstreams seamlessly because they inherit from the iostreamhierarchy. 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 Nostringstream 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 memoryallocation. Specifically, quoting from that same message:</p><pre class="programlisting">CString suffers from a common programming error that results inpoor 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. Microsoftapplications are full of this kind of thing (quadratic performanceon 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-endix86 hardware. :-)If you replace CString with string in the above function, theperformance is O(n).</pre><p>Joe Buck also pointed out some other things to keep in mind whencomparing CString and the Standard string class:</p><div class="itemizedlist"><ul type="disc"><li><p>CString permits access to its internal representation; coderswho exploited that may have problems moving to <code class="code">string</code>.</p></li><li><p>Microsoft ships the source to CString (in the filesMFC\SRC\Str{core,ex}.cpp), so you could fix the allocationbug and rebuild your MFC libraries.<span class="emphasis"><em><span class="emphasis"><em>Note:</em></span> It looks like the CString shippedwith VC++6.0 has fixed this, although it may in fact have beenone 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++, theirstring class is not. The SGI <code class="code">string</code> is essentially<code class="code">vector<char></code> and does not do any referencecounting 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, thelibstdc++ string, the SGI string, and the SGI rope, and thisis all before any allocator or traits customizations! (Morechoices 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>