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4 </p><p>A common lament seen in various newsgroups deals with the Standard
5 string class as opposed to the Microsoft Foundation Class called
6 CString. Often programmers realize that a standard portable
7 answer is better than a proprietary nonportable one, but in porting
8 their application from a Win32 platform, they discover that they
9 are relying on special functions offered by the CString class.
10 </p><p>Things are not as bad as they seem. In
11 <a class="ulink" href="http://gcc.gnu.org/ml/gcc/1999-04n/msg00236.html" target="_top">this
12 message</a>, Joe Buck points out a few very important things:
13 </p><div class="itemizedlist"><ul type="disc"><li><p>The Standard <code class="code">string</code> supports all the operations
14 that CString does, with three exceptions.
15 </p></li><li><p>Two of those exceptions (whitespace trimming and case
16 conversion) are trivial to implement. In fact, we do so
18 </p></li><li><p>The third is <code class="code">CString::Format</code>, which allows formatting
19 in the style of <code class="code">sprintf</code>. This deserves some mention:
20 </p></li></ul></div><p>
21 The old libg++ library had a function called form(), which did much
22 the same thing. But for a Standard solution, you should use the
23 stringstream classes. These are the bridge between the iostream
24 hierarchy and the string class, and they operate with regular
25 streams seamlessly because they inherit from the iostream
26 hierarchy. An quick example:
27 </p><pre class="programlisting">
28 #include <iostream>
29 #include <string>
30 #include <sstream>
32 string f (string& incoming) // incoming is "foo N"
34 istringstream incoming_stream(incoming);
38 incoming_stream >> the_word // extract "foo"
39 >> the_number; // extract N
41 ostringstream output_stream;
42 output_stream << "The word was " << the_word
43 << " and 3*N was " << (3*the_number);
45 return output_stream.str();
46 } </pre><p>A serious problem with CString is a design bug in its memory
47 allocation. Specifically, quoting from that same message:
48 </p><pre class="programlisting">
49 CString suffers from a common programming error that results in
50 poor performance. Consider the following code:
52 CString n_copies_of (const CString& foo, unsigned n)
55 for (unsigned i = 0; i < n; i++)
60 This function is O(n^2), not O(n). The reason is that each +=
61 causes a reallocation and copy of the existing string. Microsoft
62 applications are full of this kind of thing (quadratic performance
63 on tasks that can be done in linear time) -- on the other hand,
64 we should be thankful, as it's created such a big market for high-end
67 If you replace CString with string in the above function, the
69 </pre><p>Joe Buck also pointed out some other things to keep in mind when
70 comparing CString and the Standard string class:
71 </p><div class="itemizedlist"><ul type="disc"><li><p>CString permits access to its internal representation; coders
72 who exploited that may have problems moving to <code class="code">string</code>.
73 </p></li><li><p>Microsoft ships the source to CString (in the files
74 MFC\SRC\Str{core,ex}.cpp), so you could fix the allocation
75 bug and rebuild your MFC libraries.
76 <span class="emphasis"><em><span class="emphasis"><em>Note:</em></span> It looks like the CString shipped
77 with VC++6.0 has fixed this, although it may in fact have been
78 one of the VC++ SPs that did it.</em></span>
79 </p></li><li><p><code class="code">string</code> operations like this have O(n) complexity
80 <span class="emphasis"><em>if the implementors do it correctly</em></span>. The libstdc++
81 implementors did it correctly. Other vendors might not.
82 </p></li><li><p>While parts of the SGI STL are used in libstdc++, their
83 string class is not. The SGI <code class="code">string</code> is essentially
84 <code class="code">vector<char></code> and does not do any reference
85 counting like libstdc++'s does. (It is O(n), though.)
86 So if you're thinking about SGI's string or rope classes,
87 you're now looking at four possibilities: CString, the
88 libstdc++ string, the SGI string, and the SGI rope, and this
89 is all before any allocator or traits customizations! (More
90 choices than you can shake a stick at -- want fries with that?)
91 </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.
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