<?xml version="1.0" encoding="ISO-8859-1"?> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> <head> <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" /> <meta name="AUTHOR" content="pme@gcc.gnu.org (Phil Edwards)" /> <meta name="KEYWORDS" content="HOWTO, libstdc++, GCC, g++, libg++, STL" /> <meta name="DESCRIPTION" content="HOWTO for the libstdc++ chapter 21." /> <meta name="GENERATOR" content="vi and eight fingers" /> <title>libstdc++-v3 HOWTO: Chapter 21: Strings</title> <link rel="StyleSheet" href="../lib3styles.css" type="text/css" /> <link rel="Start" href="../documentation.html" type="text/html" title="GNU C++ Standard Library" /> <link rel="Prev" href="../20_util/howto.html" type="text/html" title="General Utilities" /> <link rel="Next" href="../22_locale/howto.html" type="text/html" title="Localization" /> <link rel="Copyright" href="../17_intro/license.html" type="text/html" /> <link rel="Help" href="../faq/index.html" type="text/html" title="F.A.Q." /> </head> <body> <h1 class="centered"><a name="top">Chapter 21: Strings</a></h1> <p>Chapter 21 deals with the C++ strings library (a welcome relief). </p> <!-- ####################################################### --> <hr /> <h1>Contents</h1> <ul> <li><a href="#1">MFC's CString</a></li> <li><a href="#2">A case-insensitive string class</a></li> <li><a href="#3">Breaking a C++ string into tokens</a></li> <li><a href="#4">Simple transformations</a></li> <li><a href="#5">Making strings of arbitrary character types</a></li> <li><a href="#6">Shrink-to-fit strings</a></li> </ul> <hr /> <!-- ####################################################### --> <h2><a name="1">MFC's CString</a></h2> <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 href="http://gcc.gnu.org/ml/gcc/1999-04n/msg00236.html">this message</a>, Joe Buck points out a few very important things: </p> <ul> <li>The Standard <code>string</code> supports all the operations that CString does, with three exceptions. </li> <li>Two of those exceptions (whitespace trimming and case conversion) are trivial to implement. In fact, we do so on this page. </li> <li>The third is <code>CString::Format</code>, which allows formatting in the style of <code>sprintf</code>. This deserves some mention: </li> </ul> <p><a name="1.1internal"> <!-- Coming from Chapter 27 --> 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: </a> </p> <pre> #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> 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> <ul> <li>CString permits access to its internal representation; coders who exploited that may have problems moving to <code>string</code>. </li> <li>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. <em><strong>Note:</strong> 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.</em> </li> <li><code>string</code> operations like this have O(n) complexity <em>if the implementors do it correctly</em>. The libstdc++ implementors did it correctly. Other vendors might not. </li> <li>While parts of the SGI STL are used in libstdc++-v3, their string class is not. The SGI <code>string</code> is essentially <code>vector<char></code> 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?) </li> </ul> <p>Return <a href="#top">to top of page</a> or <a href="../faq/index.html">to the FAQ</a>. </p> <hr /> <h2><a name="2">A case-insensitive string class</a></h2> <p>The well-known-and-if-it-isn't-well-known-it-ought-to-be <a href="http://www.gotw.ca/gotw/index.htm">Guru of the Week</a> 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():" </p> <pre> 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 ); </pre> <p>The solution is surprisingly easy. The original answer pages on the GotW website were removed into cold storage, in preparation for <a href="http://cseng.aw.com/bookpage.taf?ISBN=0-201-61562-2">a published book of GotW notes</a>. Before being put on the web, of course, it was posted on Usenet, and that posting containing the answer is <a href="gotw29a.txt">available here</a>. </p> <p>See? Told you it was easy!</p> <p><strong>Added June 2000:</strong> The May issue of <u>C++ Report</u> contains a fascinating article by Matt Austern (yes, <em>the</em> Matt Austern) on why case-insensitive comparisons are not as easy as they seem, and why creating a class is the <em>wrong</em> way to go about it in production code. (The GotW answer mentions one of the principle difficulties; his article mentions more.) </p> <p>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. </p> <p><strong>Added September 2000:</strong> James Kanze provided a link to a <a href="http://www.unicode.org/unicode/reports/tr21/">Unicode Technical Report discussing case handling</a>, which provides some very good information. </p> <p>Return <a href="#top">to top of page</a> or <a href="../faq/index.html">to the FAQ</a>. </p> <hr /> <h2><a name="3">Breaking a C++ string into tokens</a></h2> <p>The Standard C (and C++) function <code>strtok()</code> 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. </p> <p>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. </p> <p>It's called stringtok, and it's a template function. It's given <a href="stringtok_h.txt">in this file</a> 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: </p> <pre> 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"; } </pre> <p>You would see this as output: </p> <pre> :this: :is: :a: :test: </pre> <p>with all the whitespace removed. The original <code>s</code> is still available for use, <code>ls</code> will clean up after itself, and <code>ls.size()</code> will return how many tokens there were. </p> <p>As always, there is a price paid here, in that stringtok is not as fast as strtok. The other benefits usually outweight that, however. <a href="stringtok_std_h.txt">Another version of stringtok is given here</a>, 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. </p> <p><strong>Added February 2001:</strong> Mark Wilden pointed out that the standard <code>std::getline()</code> function can be used with standard <a href="../27_io/howto.html">istringstreams</a> 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. </p> <p>Return <a href="#top">to top of page</a> or <a href="../faq/index.html">to the FAQ</a>. </p> <hr /> <h2><a name="4">Simple transformations</a></h2> <p>Here are Standard, simple, and portable ways to perform common transformations on a <code>string</code> instance, such as "convert to all upper case." The word transformations is especially apt, because the standard template function <code>transform<></code> is used. </p> <p>This code will go through some iterations (no pun). Here's the simplistic version usually seen on Usenet: </p> <pre> #include <string> #include <algorithm> #include <cctype> // old <ctype.h> struct ToLower { char operator() (char c) const { return std::tolower(c); } }; struct ToUpper { char operator() (char c) const { return std::toupper(c); } }; int main() { 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.resize(s.size()); std::transform (s.begin(), s.end(), capital_s.begin(), ToUpper()); } </pre> <p><span class="larger"><strong>Note</strong></span> that these calls all involve the global C locale through the use of the C functions <code>toupper/tolower</code>. This is absolutely guaranteed to work -- but <em>only</em> if the string contains <em>only</em> characters from the basic source character set, and there are <em>only</em> 96 of those. Which means that not even all English text can be represented (certain British spellings, proper names, and so forth). So, if all your input forevermore consists of only those 96 characters (hahahahahaha), then you're done. </p> <p><span class="larger"><strong>Note</strong></span> that the <code>ToUpper</code> and <code>ToLower</code> function objects are needed because <code>toupper</code> and <code>tolower</code> are overloaded names (declared in <code><cctype></code> and <code><locale></code>) so the template-arguments for <code>transform<></code> cannot be deduced, as explained in <a href="http://gcc.gnu.org/ml/libstdc++/2002-11/msg00180.html">this message</a>. <!-- section 14.8.2.4 clause 16 in ISO 14882:1998 if you're into that sort of thing --> At minimum, you can write short wrappers like </p> <pre> char toLower (char c) { return std::tolower(c); } </pre> <p>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 <a href="../22_locale/howto.html#7">Correct Transformations</a>, which shows the final version of this code. (Thanks to James Kanze for assistance and suggestions on all of this.) </p> <p>Another common operation is trimming off excess whitespace. Much like transformations, this task is trivial with the use of string's <code>find</code> family. These examples are broken into multiple statements for readability: </p> <pre> 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); </pre> <p>Obviously, the calls to <code>find</code> could be inserted directly into the calls to <code>erase</code>, in case your compiler does not optimize named temporaries out of existence. </p> <p>Return <a href="#top">to top of page</a> or <a href="../faq/index.html">to the FAQ</a>. </p> <hr /> <h2><a name="5">Making strings of arbitrary character types</a></h2> <p>The <code>std::basic_string</code> is tantalizingly general, in that it is parameterized on the type of the characters which it holds. In theory, you could whip up a Unicode character class and instantiate <code>std::basic_string<my_unicode_char></code>, or assuming that integers are wider than characters on your platform, maybe just declare variables of type <code>std::basic_string<int></code>. </p> <p>That's the theory. Remember however that basic_string has additional type parameters, which take default arguments based on the character type (called CharT here): </p> <pre> template <typename CharT, typename Traits = char_traits<CharT>, typename Alloc = allocator<CharT> > class basic_string { .... };</pre> <p>Now, <code>allocator<CharT></code> will probably Do The Right Thing by default, unless you need to implement your own allocator for your characters. </p> <p>But <code>char_traits</code> takes more work. The char_traits template is <em>declared</em> but not <em>defined</em>. That means there is only </p> <pre> template <typename CharT> struct char_traits { static void foo (type1 x, type2 y); ... };</pre> <p>and functions such as char_traits<CharT>::foo() are not actually defined anywhere for the general case. The C++ standard permits this, because writing such a definition to fit all possible CharT's cannot be done. (For a time, in earlier versions of GCC, there was a mostly-correct implementation that let programmers be lazy. :-) But it broke under many situations, so it was removed. You are no longer allowed to be lazy and non-portable.) </p> <p>The C++ standard also requires that char_traits be specialized for instantiations of <code>char</code> and <code>wchar_t</code>, and it is these template specializations that permit entities like <code>basic_string<char,char_traits<char>></code> to work. </p> <p>If you want to use character types other than char and wchar_t, such as <code>unsigned char</code> and <code>int</code>, you will need to write specializations for them at the present time. If you want to use your own special character class, then you have <a href="http://gcc.gnu.org/ml/libstdc++/2002-08/msg00163.html">a lot of work to do</a>, especially if you with to use i18n features (facets require traits information but don't have a traits argument). </p> <p>One example of how to specialize char_traits is given <a href="http://gcc.gnu.org/ml/libstdc++/2002-08/msg00260.html">in this message</a>, which was then put into the file <code> include/ext/pod_char_traits.h</code> at a later date. We agree that the way it's used with basic_string (scroll down to main()) doesn't look nice, but that's because <a href="http://gcc.gnu.org/ml/libstdc++/2002-08/msg00236.html">the nice-looking first attempt</a> turned out to <a href="http://gcc.gnu.org/ml/libstdc++/2002-08/msg00242.html">not be conforming C++</a>, due to the rule that CharT must be a POD. (See how tricky this is?) </p> <p>Other approaches were suggested in that same thread, such as providing more specializations and/or some helper types in the library to assist users writing such code. So far nobody has had the time... <a href="../17_intro/contribute.html">do you?</a> </p> <p>Return <a href="#top">to top of page</a> or <a href="../faq/index.html">to the FAQ</a>. </p> <hr /> <h2><a name="6">Shrink-to-fit strings</a></h2> <!-- referenced by faq/index.html#5_9, update link if numbering changes --> <p>From GCC 3.4 calling <code>s.reserve(res)</code> on a <code>string s</code> with <code>res < s.capacity()</code> will reduce the string's capacity to <code>std::max(s.size(), res)</code>. </p> <p>This behaviour is suggested, but not required by the standard. Prior to GCC 3.4 the following alternative can be used instead </p> <pre> std::string(str.data(), str.size()).swap(str); </pre> <p>This is similar to the idiom for reducing a <code>vector</code>'s memory usage (see <a href='../faq/index.html#5_9'>FAQ 5.9</a>) but the regular copy constructor cannot be used because libstdc++'s <code>string</code> is Copy-On-Write. </p> <!-- ####################################################### --> <hr /> <p class="fineprint"><em> See <a href="../17_intro/license.html">license.html</a> for copying conditions. Comments and suggestions are welcome, and may be sent to <a href="mailto:libstdc++@gcc.gnu.org">the libstdc++ mailing list</a>. </em></p> </body> </html>