+
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. +
Things are not as bad as they seem. In + this + message, Joe Buck points out a few very important things: +
The Standard string supports all the operations
+ that CString does, with three exceptions.
+
Two of those exceptions (whitespace trimming and case + conversion) are trivial to implement. In fact, we do so + on this page. +
The third is CString::Format, which allows formatting
+ in the style of sprintf. This deserves some mention:
+
+ 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: +
+ #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();
+ } A serious problem with CString is a design bug in its memory + allocation. Specifically, quoting from that same message: +
+ 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).
+ Joe Buck also pointed out some other things to keep in mind when + comparing CString and the Standard string class: +
CString permits access to its internal representation; coders
+ who exploited that may have problems moving to string.
+
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. + Note: 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. +
string operations like this have O(n) complexity
+ if the implementors do it correctly. The libstdc++
+ implementors did it correctly. Other vendors might not.
+
While parts of the SGI STL are used in libstdc++, their
+ string class is not. The SGI string is essentially
+ vector<char> 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?)
+