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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 V1.73.2" /><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> |
