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6 <part id="manual.strings" xreflabel="Strings">
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22 <indexterm><primary>Strings</primary></indexterm>
25 <!-- Chapter 01 : Character Traits -->
27 <!-- Chapter 02 : String Classes -->
28 <chapter id="manual.strings.string" xreflabel="string">
29 <title>String Classes</title>
31 <sect1 id="strings.string.simple" xreflabel="Simple Transformations">
32 <title>Simple Transformations</title>
34 Here are Standard, simple, and portable ways to perform common
35 transformations on a <code>string</code> instance, such as
36 "convert to all upper case." The word transformations
37 is especially apt, because the standard template function
38 <code>transform<></code> is used.
41 This code will go through some iterations. Here's a simple
45 #include <string>
46 #include <algorithm>
47 #include <cctype> // old <ctype.h>
51 char operator() (char c) const { return std::tolower(c); }
56 char operator() (char c) const { return std::toupper(c); }
61 std::string s ("Some Kind Of Initial Input Goes Here");
63 // Change everything into upper case
64 std::transform (s.begin(), s.end(), s.begin(), ToUpper());
66 // Change everything into lower case
67 std::transform (s.begin(), s.end(), s.begin(), ToLower());
69 // Change everything back into upper case, but store the
70 // result in a different string
71 std::string capital_s;
72 capital_s.resize(s.size());
73 std::transform (s.begin(), s.end(), capital_s.begin(), ToUpper());
77 <emphasis>Note</emphasis> that these calls all
78 involve the global C locale through the use of the C functions
79 <code>toupper/tolower</code>. This is absolutely guaranteed to work --
80 but <emphasis>only</emphasis> if the string contains <emphasis>only</emphasis> characters
81 from the basic source character set, and there are <emphasis>only</emphasis>
82 96 of those. Which means that not even all English text can be
83 represented (certain British spellings, proper names, and so forth).
84 So, if all your input forevermore consists of only those 96
85 characters (hahahahahaha), then you're done.
87 <para><emphasis>Note</emphasis> that the
88 <code>ToUpper</code> and <code>ToLower</code> function objects
89 are needed because <code>toupper</code> and <code>tolower</code>
90 are overloaded names (declared in <code><cctype></code> and
91 <code><locale></code>) so the template-arguments for
92 <code>transform<></code> cannot be deduced, as explained in
93 <ulink url="http://gcc.gnu.org/ml/libstdc++/2002-11/msg00180.html">this
95 <!-- section 14.8.2.4 clause 16 in ISO 14882:1998 -->
96 At minimum, you can write short wrappers like
101 return std::tolower(c);
103 <para>(Thanks to James Kanze for assistance and suggestions on all of this.)
105 <para>Another common operation is trimming off excess whitespace. Much
106 like transformations, this task is trivial with the use of string's
107 <code>find</code> family. These examples are broken into multiple
108 statements for readability:
111 std::string str (" \t blah blah blah \n ");
113 // trim leading whitespace
114 string::size_type notwhite = str.find_first_not_of(" \t\n");
115 str.erase(0,notwhite);
117 // trim trailing whitespace
118 notwhite = str.find_last_not_of(" \t\n");
119 str.erase(notwhite+1); </programlisting>
120 <para>Obviously, the calls to <code>find</code> could be inserted directly
121 into the calls to <code>erase</code>, in case your compiler does not
122 optimize named temporaries out of existence.
126 <sect1 id="strings.string.case" xreflabel="Case Sensitivity">
127 <title>Case Sensitivity</title>
131 <para>The well-known-and-if-it-isn't-well-known-it-ought-to-be
132 <ulink url="http://www.gotw.ca/gotw/">Guru of the Week</ulink>
133 discussions held on Usenet covered this topic in January of 1998.
134 Briefly, the challenge was, <quote>write a 'ci_string' class which
135 is identical to the standard 'string' class, but is
136 case-insensitive in the same way as the (common but nonstandard)
137 C function stricmp()</quote>.
140 ci_string s( "AbCdE" );
143 assert( s == "abcde" );
144 assert( s == "ABCDE" );
146 // still case-preserving, of course
147 assert( strcmp( s.c_str(), "AbCdE" ) == 0 );
148 assert( strcmp( s.c_str(), "abcde" ) != 0 ); </programlisting>
150 <para>The solution is surprisingly easy. The original answer was
151 posted on Usenet, and a revised version appears in Herb Sutter's
152 book <emphasis>Exceptional C++</emphasis> and on his website as <ulink url="http://www.gotw.ca/gotw/029.htm">GotW 29</ulink>.
154 <para>See? Told you it was easy!</para>
156 <emphasis>Added June 2000:</emphasis> The May 2000 issue of C++
157 Report contains a fascinating <ulink
158 url="http://lafstern.org/matt/col2_new.pdf"> article</ulink> by
159 Matt Austern (yes, <emphasis>the</emphasis> Matt Austern) on why
160 case-insensitive comparisons are not as easy as they seem, and
161 why creating a class is the <emphasis>wrong</emphasis> way to go
162 about it in production code. (The GotW answer mentions one of
163 the principle difficulties; his article mentions more.)
165 <para>Basically, this is "easy" only if you ignore some things,
166 things which may be too important to your program to ignore. (I chose
167 to ignore them when originally writing this entry, and am surprised
168 that nobody ever called me on it...) The GotW question and answer
169 remain useful instructional tools, however.
171 <para><emphasis>Added September 2000:</emphasis> James Kanze provided a link to a
172 <ulink url="http://www.unicode.org/reports/tr21/tr21-5.html">Unicode
173 Technical Report discussing case handling</ulink>, which provides some
174 very good information.
178 <sect1 id="strings.string.character_types" xreflabel="Arbitrary Characters">
179 <title>Arbitrary Character Types</title>
183 <para>The <code>std::basic_string</code> is tantalizingly general, in that
184 it is parameterized on the type of the characters which it holds.
185 In theory, you could whip up a Unicode character class and instantiate
186 <code>std::basic_string<my_unicode_char></code>, or assuming
187 that integers are wider than characters on your platform, maybe just
188 declare variables of type <code>std::basic_string<int></code>.
190 <para>That's the theory. Remember however that basic_string has additional
191 type parameters, which take default arguments based on the character
192 type (called <code>CharT</code> here):
195 template <typename CharT,
196 typename Traits = char_traits<CharT>,
197 typename Alloc = allocator<CharT> >
198 class basic_string { .... };</programlisting>
199 <para>Now, <code>allocator<CharT></code> will probably Do The Right
200 Thing by default, unless you need to implement your own allocator
203 <para>But <code>char_traits</code> takes more work. The char_traits
204 template is <emphasis>declared</emphasis> but not <emphasis>defined</emphasis>.
205 That means there is only
208 template <typename CharT>
211 static void foo (type1 x, type2 y);
214 <para>and functions such as char_traits<CharT>::foo() are not
215 actually defined anywhere for the general case. The C++ standard
216 permits this, because writing such a definition to fit all possible
217 CharT's cannot be done.
219 <para>The C++ standard also requires that char_traits be specialized for
220 instantiations of <code>char</code> and <code>wchar_t</code>, and it
221 is these template specializations that permit entities like
222 <code>basic_string<char,char_traits<char>></code> to work.
224 <para>If you want to use character types other than char and wchar_t,
225 such as <code>unsigned char</code> and <code>int</code>, you will
226 need suitable specializations for them. For a time, in earlier
227 versions of GCC, there was a mostly-correct implementation that
228 let programmers be lazy but it broke under many situations, so it
229 was removed. GCC 3.4 introduced a new implementation that mostly
230 works and can be specialized even for <code>int</code> and other
233 <para>If you want to use your own special character class, then you have
234 <ulink url="http://gcc.gnu.org/ml/libstdc++/2002-08/msg00163.html">a lot
235 of work to do</ulink>, especially if you with to use i18n features
236 (facets require traits information but don't have a traits argument).
238 <para>Another example of how to specialize char_traits was given <ulink url="http://gcc.gnu.org/ml/libstdc++/2002-08/msg00260.html">on the
239 mailing list</ulink> and at a later date was put into the file <code>
240 include/ext/pod_char_traits.h</code>. We agree
241 that the way it's used with basic_string (scroll down to main())
242 doesn't look nice, but that's because <ulink url="http://gcc.gnu.org/ml/libstdc++/2002-08/msg00236.html">the
243 nice-looking first attempt</ulink> turned out to <ulink url="http://gcc.gnu.org/ml/libstdc++/2002-08/msg00242.html">not
244 be conforming C++</ulink>, due to the rule that CharT must be a POD.
245 (See how tricky this is?)
250 <sect1 id="strings.string.token" xreflabel="Tokenizing">
251 <title>Tokenizing</title>
254 <para>The Standard C (and C++) function <code>strtok()</code> leaves a lot to
255 be desired in terms of user-friendliness. It's unintuitive, it
256 destroys the character string on which it operates, and it requires
257 you to handle all the memory problems. But it does let the client
258 code decide what to use to break the string into pieces; it allows
259 you to choose the "whitespace," so to speak.
261 <para>A C++ implementation lets us keep the good things and fix those
262 annoyances. The implementation here is more intuitive (you only
263 call it once, not in a loop with varying argument), it does not
264 affect the original string at all, and all the memory allocation
267 <para>It's called stringtok, and it's a template function. Sources are
268 as below, in a less-portable form than it could be, to keep this
269 example simple (for example, see the comments on what kind of
270 string it will accept).
274 #include <string>
275 template <typename Container>
277 stringtok(Container &container, string const &in,
278 const char * const delimiters = " \t\n")
280 const string::size_type len = in.length();
281 string::size_type i = 0;
285 // Eat leading whitespace
286 i = in.find_first_not_of(delimiters, i);
287 if (i == string::npos)
288 return; // Nothing left but white space
290 // Find the end of the token
291 string::size_type j = in.find_first_of(delimiters, i);
294 if (j == string::npos)
296 container.push_back(in.substr(i));
300 container.push_back(in.substr(i, j-i));
302 // Set up for next loop
310 The author uses a more general (but less readable) form of it for
311 parsing command strings and the like. If you compiled and ran this
317 std::list<string> ls;
318 stringtok (ls, " this \t is\t\n a test ");
319 for (std::list<string>const_iterator i = ls.begin();
322 std::cerr << ':' << (*i) << ":\n";
324 <para>You would see this as output:
330 :test: </programlisting>
331 <para>with all the whitespace removed. The original <code>s</code> is still
332 available for use, <code>ls</code> will clean up after itself, and
333 <code>ls.size()</code> will return how many tokens there were.
335 <para>As always, there is a price paid here, in that stringtok is not
336 as fast as strtok. The other benefits usually outweigh that, however.
339 <para><emphasis>Added February 2001:</emphasis> Mark Wilden pointed out that the
340 standard <code>std::getline()</code> function can be used with standard
341 <code>istringstreams</code> to perform
342 tokenizing as well. Build an istringstream from the input text,
343 and then use std::getline with varying delimiters (the three-argument
344 signature) to extract tokens into a string.
349 <sect1 id="strings.string.shrink" xreflabel="Shrink to Fit">
350 <title>Shrink to Fit</title>
353 <para>From GCC 3.4 calling <code>s.reserve(res)</code> on a
354 <code>string s</code> with <code>res < s.capacity()</code> will
355 reduce the string's capacity to <code>std::max(s.size(), res)</code>.
357 <para>This behaviour is suggested, but not required by the standard. Prior
358 to GCC 3.4 the following alternative can be used instead
361 std::string(str.data(), str.size()).swap(str);
363 <para>This is similar to the idiom for reducing
364 a <code>vector</code>'s memory usage
365 (see <link linkend="faq.size_equals_capacity">this FAQ
366 entry</link>) but the regular copy constructor cannot be used
367 because libstdc++'s <code>string</code> is Copy-On-Write.
373 <sect1 id="strings.string.Cstring" xreflabel="CString (MFC)">
374 <title>CString (MFC)</title>
378 <para>A common lament seen in various newsgroups deals with the Standard
379 string class as opposed to the Microsoft Foundation Class called
380 CString. Often programmers realize that a standard portable
381 answer is better than a proprietary nonportable one, but in porting
382 their application from a Win32 platform, they discover that they
383 are relying on special functions offered by the CString class.
385 <para>Things are not as bad as they seem. In
386 <ulink url="http://gcc.gnu.org/ml/gcc/1999-04n/msg00236.html">this
387 message</ulink>, Joe Buck points out a few very important things:
390 <listitem><para>The Standard <code>string</code> supports all the operations
391 that CString does, with three exceptions.
393 <listitem><para>Two of those exceptions (whitespace trimming and case
394 conversion) are trivial to implement. In fact, we do so
397 <listitem><para>The third is <code>CString::Format</code>, which allows formatting
398 in the style of <code>sprintf</code>. This deserves some mention:
402 The old libg++ library had a function called form(), which did much
403 the same thing. But for a Standard solution, you should use the
404 stringstream classes. These are the bridge between the iostream
405 hierarchy and the string class, and they operate with regular
406 streams seamlessly because they inherit from the iostream
407 hierarchy. An quick example:
410 #include <iostream>
411 #include <string>
412 #include <sstream>
414 string f (string& incoming) // incoming is "foo N"
416 istringstream incoming_stream(incoming);
420 incoming_stream >> the_word // extract "foo"
421 >> the_number; // extract N
423 ostringstream output_stream;
424 output_stream << "The word was " << the_word
425 << " and 3*N was " << (3*the_number);
427 return output_stream.str();
429 <para>A serious problem with CString is a design bug in its memory
430 allocation. Specifically, quoting from that same message:
433 CString suffers from a common programming error that results in
434 poor performance. Consider the following code:
436 CString n_copies_of (const CString& foo, unsigned n)
439 for (unsigned i = 0; i < n; i++)
444 This function is O(n^2), not O(n). The reason is that each +=
445 causes a reallocation and copy of the existing string. Microsoft
446 applications are full of this kind of thing (quadratic performance
447 on tasks that can be done in linear time) -- on the other hand,
448 we should be thankful, as it's created such a big market for high-end
451 If you replace CString with string in the above function, the
454 <para>Joe Buck also pointed out some other things to keep in mind when
455 comparing CString and the Standard string class:
458 <listitem><para>CString permits access to its internal representation; coders
459 who exploited that may have problems moving to <code>string</code>.
461 <listitem><para>Microsoft ships the source to CString (in the files
462 MFC\SRC\Str{core,ex}.cpp), so you could fix the allocation
463 bug and rebuild your MFC libraries.
464 <emphasis><emphasis>Note:</emphasis> It looks like the CString shipped
465 with VC++6.0 has fixed this, although it may in fact have been
466 one of the VC++ SPs that did it.</emphasis>
468 <listitem><para><code>string</code> operations like this have O(n) complexity
469 <emphasis>if the implementors do it correctly</emphasis>. The libstdc++
470 implementors did it correctly. Other vendors might not.
472 <listitem><para>While parts of the SGI STL are used in libstdc++, their
473 string class is not. The SGI <code>string</code> is essentially
474 <code>vector<char></code> and does not do any reference
475 counting like libstdc++'s does. (It is O(n), though.)
476 So if you're thinking about SGI's string or rope classes,
477 you're now looking at four possibilities: CString, the
478 libstdc++ string, the SGI string, and the SGI rope, and this
479 is all before any allocator or traits customizations! (More
480 choices than you can shake a stick at -- want fries with that?)
487 <!-- Chapter 03 : Interacting with C -->