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diff --git a/libstdc++-v3/doc/xml/manual/strings.xml b/libstdc++-v3/doc/xml/manual/strings.xml new file mode 100644 index 000000000..4d9fc64f7 --- /dev/null +++ b/libstdc++-v3/doc/xml/manual/strings.xml @@ -0,0 +1,486 @@ +<chapter xmlns="http://docbook.org/ns/docbook" version="5.0" + xml:id="std.strings" xreflabel="Strings"> +<?dbhtml filename="strings.html"?> + +<info><title> + Strings + <indexterm><primary>Strings</primary></indexterm> +</title> + <keywordset> + <keyword> + ISO C++ + </keyword> + <keyword> + library + </keyword> + </keywordset> +</info> + +<!-- Sect1 01 : Character Traits --> + +<!-- Sect1 02 : String Classes --> +<section xml:id="std.strings.string" xreflabel="string"><info><title>String Classes</title></info> + + + <section xml:id="strings.string.simple" xreflabel="Simple Transformations"><info><title>Simple Transformations</title></info> + + <para> + 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. + </para> + <para> + This code will go through some iterations. Here's a simple + version: + </para> + <programlisting> + #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()); + } + </programlisting> + <para> + <emphasis>Note</emphasis> 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 <emphasis>only</emphasis> if the string contains <emphasis>only</emphasis> characters + from the basic source character set, and there are <emphasis>only</emphasis> + 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. + </para> + <para><emphasis>Note</emphasis> 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 + <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://gcc.gnu.org/ml/libstdc++/2002-11/msg00180.html">this + message</link>. + <!-- section 14.8.2.4 clause 16 in ISO 14882:1998 --> + At minimum, you can write short wrappers like + </para> + <programlisting> + char toLower (char c) + { + return std::tolower(c); + } </programlisting> + <para>(Thanks to James Kanze for assistance and suggestions on all of this.) + </para> + <para>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: + </para> + <programlisting> + 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); </programlisting> + <para>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. + </para> + + </section> + <section xml:id="strings.string.case" xreflabel="Case Sensitivity"><info><title>Case Sensitivity</title></info> + + <para> + </para> + + <para>The well-known-and-if-it-isn't-well-known-it-ought-to-be + <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://www.gotw.ca/gotw/">Guru of the Week</link> + discussions held on Usenet covered this topic in January of 1998. + Briefly, the challenge was, <quote>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()</quote>. + </para> + <programlisting> + 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 ); </programlisting> + + <para>The solution is surprisingly easy. The original answer was + posted on Usenet, and a revised version appears in Herb Sutter's + book <emphasis>Exceptional C++</emphasis> and on his website as <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://www.gotw.ca/gotw/029.htm">GotW 29</link>. + </para> + <para>See? Told you it was easy!</para> + <para> + <emphasis>Added June 2000:</emphasis> The May 2000 issue of C++ + Report contains a fascinating <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://lafstern.org/matt/col2_new.pdf"> article</link> by + Matt Austern (yes, <emphasis>the</emphasis> Matt Austern) on why + case-insensitive comparisons are not as easy as they seem, and + why creating a class is the <emphasis>wrong</emphasis> way to go + about it in production code. (The GotW answer mentions one of + the principle difficulties; his article mentions more.) + </para> + <para>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. + </para> + <para><emphasis>Added September 2000:</emphasis> James Kanze provided a link to a + <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://www.unicode.org/reports/tr21/tr21-5.html">Unicode + Technical Report discussing case handling</link>, which provides some + very good information. + </para> + + </section> + <section xml:id="strings.string.character_types" xreflabel="Arbitrary Characters"><info><title>Arbitrary Character Types</title></info> + + <para> + </para> + + <para>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>. + </para> + <para>That's the theory. Remember however that basic_string has additional + type parameters, which take default arguments based on the character + type (called <code>CharT</code> here): + </para> + <programlisting> + template <typename CharT, + typename Traits = char_traits<CharT>, + typename Alloc = allocator<CharT> > + class basic_string { .... };</programlisting> + <para>Now, <code>allocator<CharT></code> will probably Do The Right + Thing by default, unless you need to implement your own allocator + for your characters. + </para> + <para>But <code>char_traits</code> takes more work. The char_traits + template is <emphasis>declared</emphasis> but not <emphasis>defined</emphasis>. + That means there is only + </para> + <programlisting> + template <typename CharT> + struct char_traits + { + static void foo (type1 x, type2 y); + ... + };</programlisting> + <para>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. + </para> + <para>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. + </para> + <para>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 suitable specializations for them. 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. GCC 3.4 introduced a new implementation that mostly + works and can be specialized even for <code>int</code> and other + built-in types. + </para> + <para>If you want to use your own special character class, then you have + <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://gcc.gnu.org/ml/libstdc++/2002-08/msg00163.html">a lot + of work to do</link>, especially if you with to use i18n features + (facets require traits information but don't have a traits argument). + </para> + <para>Another example of how to specialize char_traits was given <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://gcc.gnu.org/ml/libstdc++/2002-08/msg00260.html">on the + mailing list</link> and at a later date was put into the file <code> + include/ext/pod_char_traits.h</code>. We agree + that the way it's used with basic_string (scroll down to main()) + doesn't look nice, but that's because <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://gcc.gnu.org/ml/libstdc++/2002-08/msg00236.html">the + nice-looking first attempt</link> turned out to <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://gcc.gnu.org/ml/libstdc++/2002-08/msg00242.html">not + be conforming C++</link>, due to the rule that CharT must be a POD. + (See how tricky this is?) + </para> + + </section> + + <section xml:id="strings.string.token" xreflabel="Tokenizing"><info><title>Tokenizing</title></info> + + <para> + </para> + <para>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. + </para> + <para>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. + </para> + <para>It's called stringtok, and it's a template function. Sources are + as below, 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). + </para> + +<programlisting> +#include <string> +template <typename Container> +void +stringtok(Container &container, string const &in, + const char * const delimiters = " \t\n") +{ + const string::size_type len = in.length(); + string::size_type i = 0; + + while (i < len) + { + // Eat leading whitespace + i = in.find_first_not_of(delimiters, i); + if (i == string::npos) + return; // Nothing left but white space + + // Find the end of the token + string::size_type j = in.find_first_of(delimiters, i); + + // Push token + if (j == string::npos) + { + container.push_back(in.substr(i)); + return; + } + else + container.push_back(in.substr(i, j-i)); + + // Set up for next loop + i = j + 1; + } +} +</programlisting> + + + <para> + 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: + </para> + + + <programlisting> + 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"; + } </programlisting> + <para>You would see this as output: + </para> + <programlisting> + :this: + :is: + :a: + :test: </programlisting> + <para>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. + </para> + <para>As always, there is a price paid here, in that stringtok is not + as fast as strtok. The other benefits usually outweigh that, however. + </para> + + <para><emphasis>Added February 2001:</emphasis> Mark Wilden pointed out that the + standard <code>std::getline()</code> function can be used with standard + <code>istringstreams</code> 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. + </para> + + + </section> + <section xml:id="strings.string.shrink" xreflabel="Shrink to Fit"><info><title>Shrink to Fit</title></info> + + <para> + </para> + <para>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>. + </para> + <para>This behaviour is suggested, but not required by the standard. Prior + to GCC 3.4 the following alternative can be used instead + </para> + <programlisting> + std::string(str.data(), str.size()).swap(str); + </programlisting> + <para>This is similar to the idiom for reducing + a <code>vector</code>'s memory usage + (see <link linkend="faq.size_equals_capacity">this FAQ + entry</link>) but the regular copy constructor cannot be used + because libstdc++'s <code>string</code> is Copy-On-Write. + </para> + <para>In <link linkend="status.iso.200x">C++0x</link> mode you can call + <code>s.shrink_to_fit()</code> to achieve the same effect as + <code>s.reserve(s.size())</code>. + </para> + + + </section> + + <section xml:id="strings.string.Cstring" xreflabel="CString (MFC)"><info><title>CString (MFC)</title></info> + + <para> + </para> + + <para>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. + </para> + <para>Things are not as bad as they seem. In + <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://gcc.gnu.org/ml/gcc/1999-04n/msg00236.html">this + message</link>, Joe Buck points out a few very important things: + </para> + <itemizedlist> + <listitem><para>The Standard <code>string</code> supports all the operations + that CString does, with three exceptions. + </para></listitem> + <listitem><para>Two of those exceptions (whitespace trimming and case + conversion) are trivial to implement. In fact, we do so + on this page. + </para></listitem> + <listitem><para>The third is <code>CString::Format</code>, which allows formatting + in the style of <code>sprintf</code>. This deserves some mention: + </para></listitem> + </itemizedlist> + <para> + 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: + </para> + <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(); + } </programlisting> + <para>A serious problem with CString is a design bug in its memory + allocation. Specifically, quoting from that same message: + </para> + <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). + </programlisting> + <para>Joe Buck also pointed out some other things to keep in mind when + comparing CString and the Standard string class: + </para> + <itemizedlist> + <listitem><para>CString permits access to its internal representation; coders + who exploited that may have problems moving to <code>string</code>. + </para></listitem> + <listitem><para>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. + <emphasis><emphasis>Note:</emphasis> 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.</emphasis> + </para></listitem> + <listitem><para><code>string</code> operations like this have O(n) complexity + <emphasis>if the implementors do it correctly</emphasis>. The libstdc++ + implementors did it correctly. Other vendors might not. + </para></listitem> + <listitem><para>While chapters of the SGI STL are used in libstdc++, 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++'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?) + </para></listitem> + </itemizedlist> + + </section> +</section> + +<!-- Sect1 03 : Interacting with C --> + +</chapter> |