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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>
</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
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