Chapter 21: Strings

Chapter 21 deals with the C++ strings library (a welcome relief).

Contents

• MFC's CString
• A case-insensitive string class
• Breaking a C++ string into tokens
• Simple transformations

MFC's CString

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:

   #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 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++-v3, their string class is not. The SGI string is essentially vector<char> and does not do any reference counting like libstdc++-v3'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?)

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A case-insensitive string class

The well-known-and-if-it-isn't-well-known-it-ought-to-be Guru of the Week discussions held on Usenet covered this topic in January of 1998. Briefly, the challenge was, "write a 'ci_string' class which is identical to the standard 'string' class, but is case-insensitive in the same way as the (common but nonstandard) C function stricmp():"

   ci_string s( "AbCdE" );

   // case insensitive
   assert( s == "abcde" );
   assert( s == "ABCDE" );

   // still case-preserving, of course
   assert( strcmp( s.c_str(), "AbCdE" ) == 0 );
   assert( strcmp( s.c_str(), "abcde" ) != 0 ); 

The solution is surprisingly easy. The original answer pages on the GotW website were removed into cold storage, in preparation for a published book of GotW notes. Before being put on the web, of course, it was posted on Usenet, and that posting containing the answer is available here.

See? Told you it was easy!

Added June 2000: The May issue of C++ Report contains a fascinating article by Matt Austern (yes, the Matt Austern) on why case-insensitive comparisons are not as easy as they seem, and why creating a class is the wrong way to go about it in production code. (The GotW answer mentions one of the principle difficulties; his article mentions more.)

Basically, this is "easy" only if you ignore some things, things which may be too important to your program to ignore. (I chose to ignore them when originally writing this entry, and am surprised that nobody ever called me on it...) The GotW question and answer remain useful instructional tools, however.

Added September 2000: James Kanze provided a link to a Unicode Technical Report discussing case handling, which provides some very good information.

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Breaking a C++ string into tokens

The Standard C (and C++) function strtok() leaves a lot to be desired in terms of user-friendliness. It's unintuitive, it destroys the character string on which it operates, and it requires you to handle all the memory problems. But it does let the client code decide what to use to break the string into pieces; it allows you to choose the "whitespace," so to speak.

A C++ implementation lets us keep the good things and fix those annoyances. The implementation here is more intuitive (you only call it once, not in a loop with varying argument), it does not affect the original string at all, and all the memory allocation is handled for you.

It's called stringtok, and it's a template function. It's given in this file in a less-portable form than it could be, to keep this example simple (for example, see the comments on what kind of string it will accept). The author uses a more general (but less readable) form of it for parsing command strings and the like. If you compiled and ran this code using it:

   std::list<string>  ls;
   stringtok (ls, " this  \t is\t\n  a test  ");
   for (std::list<string>const_iterator i = ls.begin();
        i != ls.end(); ++i)
   {
       std::cerr << ':' << (*i) << ":\n";
   }

   :this:
   :is:
   :a:
   :test:

As always, there is a price paid here, in that stringtok is not as fast as strtok. The other benefits usually outweight that, however. Another version of stringtok is given here, suggested by Chris King and tweaked by Petr Prikryl, and this one uses the transformation functions mentioned below. If you are comfortable with reading the new function names, this version is recommended as an example.

Added February 2001: Mark Wilden pointed out that the standard std::getline() function can be used with standard istringstreams to perform tokenizing as well. Build an istringstream from the input text, and then use std::getline with varying delimiters (the three-argument signature) to extract tokens into a string.

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Simple transformations

Here are Standard, simple, and portable ways to perform common transformations on a string instance, such as "convert to all upper case." The word transformations is especially apt, because the standard template function transform<> is used.

This code will go through some iterations (no pun). Here's the simplistic version usually seen on Usenet:

   #include <string>
   #include <algorithm>
   #include <cctype>      // old <ctype.h>

   std::string  s ("Some Kind Of Initial Input Goes Here");

   // Change everything into upper case
   std::transform (s.begin(), s.end(), s.begin(), toupper);

   // Change everything into lower case
   std::transform (s.begin(), s.end(), s.begin(), tolower);

   // Change everything back into upper case, but store the
   // result in a different string
   std::string  capital_s;
   capital_s.reserve(s.size());
   std::transform (s.begin(), s.end(), capital_s.begin(), tolower); 

At minimum, you can write short wrappers like

   char toLower (char c)
   {
      return tolower(static_cast<unsigned char>(c));
   }

The correct method is to use a facet for a particular locale and call its conversion functions. These are discussed more in Chapter 22; the specific part is here, which shows the final version of this code. (Thanks to James Kanze for assistance and suggestions on all of this.)

Another common operation is trimming off excess whitespace. Much like transformations, this task is trivial with the use of string's find family. These examples are broken into multiple statements for readability:

   std::string  str (" \t blah blah blah    \n ");

   // trim leading whitespace
   string::size_type  notwhite = str.find_first_not_of(" \t\n");
   str.erase(0,notwhite);

   // trim trailing whitespace
   notwhite = str.find_last_not_of(" \t\n"); 
   str.erase(notwhite+1); 

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