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| author | upstream source tree <ports@midipix.org> | 2015-03-15 20:14:05 -0400 |
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| committer | upstream source tree <ports@midipix.org> | 2015-03-15 20:14:05 -0400 |
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| tree | 976dc5ab7fddf506dadce60ae936f43f58787092 /libstdc++-v3/doc/html/manual/strings.html | |
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diff --git a/libstdc++-v3/doc/html/manual/strings.html b/libstdc++-v3/doc/html/manual/strings.html new file mode 100644 index 000000000..a70edcdb8 --- /dev/null +++ b/libstdc++-v3/doc/html/manual/strings.html @@ -0,0 +1,366 @@ +<?xml version="1.0" encoding="UTF-8" standalone="no"?> +<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd"> +<html xmlns="http://www.w3.org/1999/xhtml"><head><title>Chapter 7. Strings</title><meta name="generator" content="DocBook XSL-NS Stylesheets V1.76.1"/><meta name="keywords" content=" ISO C++ , library "/><link rel="home" href="../spine.html" title="The GNU C++ Library"/><link rel="up" href="bk01pt02.html" title="Part II. Standard Contents"/><link rel="prev" href="traits.html" title="Traits"/><link rel="next" href="localization.html" title="Chapter 8. Localization"/></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Chapter 7. + Strings + +</th></tr><tr><td align="left"><a accesskey="p" href="traits.html">Prev</a> </td><th width="60%" align="center">Part II. + Standard Contents + </th><td align="right"> <a accesskey="n" href="localization.html">Next</a></td></tr></table><hr/></div><div class="chapter" title="Chapter 7. Strings"><div class="titlepage"><div><div><h2 class="title"><a id="std.strings"/>Chapter 7. + Strings + <a id="id475084" class="indexterm"/> +</h2></div></div></div><div class="toc"><p><strong>Table of Contents</strong></p><dl><dt><span class="section"><a href="strings.html#std.strings.string">String Classes</a></span></dt><dd><dl><dt><span class="section"><a href="strings.html#strings.string.simple">Simple Transformations</a></span></dt><dt><span class="section"><a href="strings.html#strings.string.case">Case Sensitivity</a></span></dt><dt><span class="section"><a href="strings.html#strings.string.character_types">Arbitrary Character Types</a></span></dt><dt><span class="section"><a href="strings.html#strings.string.token">Tokenizing</a></span></dt><dt><span class="section"><a href="strings.html#strings.string.shrink">Shrink to Fit</a></span></dt><dt><span class="section"><a href="strings.html#strings.string.Cstring">CString (MFC)</a></span></dt></dl></dd></dl></div><div class="section" title="String Classes"><div class="titlepage"><div><div><h2 class="title"><a id="std.strings.string"/>String Classes</h2></div></div></div><div class="section" title="Simple Transformations"><div class="titlepage"><div><div><h3 class="title"><a id="strings.string.simple"/>Simple Transformations</h3></div></div></div><p> + Here are Standard, simple, and portable ways to perform common + transformations on a <code class="code">string</code> instance, such as + "convert to all upper case." The word transformations + is especially apt, because the standard template function + <code class="code">transform<></code> is used. + </p><p> + This code will go through some iterations. Here's a simple + version: + </p><pre class="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()); + } + </pre><p> + <span class="emphasis"><em>Note</em></span> that these calls all + involve the global C locale through the use of the C functions + <code class="code">toupper/tolower</code>. This is absolutely guaranteed to work -- + but <span class="emphasis"><em>only</em></span> if the string contains <span class="emphasis"><em>only</em></span> characters + from the basic source character set, and there are <span class="emphasis"><em>only</em></span> + 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="emphasis"><em>Note</em></span> that the + <code class="code">ToUpper</code> and <code class="code">ToLower</code> function objects + are needed because <code class="code">toupper</code> and <code class="code">tolower</code> + are overloaded names (declared in <code class="code"><cctype></code> and + <code class="code"><locale></code>) so the template-arguments for + <code class="code">transform<></code> cannot be deduced, as explained in + <a class="link" href="http://gcc.gnu.org/ml/libstdc++/2002-11/msg00180.html">this + message</a>. + + At minimum, you can write short wrappers like + </p><pre class="programlisting"> + char toLower (char c) + { + return std::tolower(c); + } </pre><p>(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 class="code">find</code> family. These examples are broken into multiple + statements for readability: + </p><pre class="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); </pre><p>Obviously, the calls to <code class="code">find</code> could be inserted directly + into the calls to <code class="code">erase</code>, in case your compiler does not + optimize named temporaries out of existence. + </p></div><div class="section" title="Case Sensitivity"><div class="titlepage"><div><div><h3 class="title"><a id="strings.string.case"/>Case Sensitivity</h3></div></div></div><p> + </p><p>The well-known-and-if-it-isn't-well-known-it-ought-to-be + <a class="link" href="http://www.gotw.ca/gotw/">Guru of the Week</a> + discussions held on Usenet covered this topic in January of 1998. + Briefly, the challenge was, <span class="quote">“<span class="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()</span>”</span>. + </p><pre class="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 ); </pre><p>The solution is surprisingly easy. The original answer was + posted on Usenet, and a revised version appears in Herb Sutter's + book <span class="emphasis"><em>Exceptional C++</em></span> and on his website as <a class="link" href="http://www.gotw.ca/gotw/029.htm">GotW 29</a>. + </p><p>See? Told you it was easy!</p><p> + <span class="emphasis"><em>Added June 2000:</em></span> The May 2000 issue of C++ + Report contains a fascinating <a class="link" href="http://lafstern.org/matt/col2_new.pdf"> article</a> by + Matt Austern (yes, <span class="emphasis"><em>the</em></span> Matt Austern) on why + case-insensitive comparisons are not as easy as they seem, and + why creating a class is the <span class="emphasis"><em>wrong</em></span> 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><span class="emphasis"><em>Added September 2000:</em></span> James Kanze provided a link to a + <a class="link" href="http://www.unicode.org/reports/tr21/tr21-5.html">Unicode + Technical Report discussing case handling</a>, which provides some + very good information. + </p></div><div class="section" title="Arbitrary Character Types"><div class="titlepage"><div><div><h3 class="title"><a id="strings.string.character_types"/>Arbitrary Character Types</h3></div></div></div><p> + </p><p>The <code class="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 class="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 class="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 <code class="code">CharT</code> here): + </p><pre class="programlisting"> + template <typename CharT, + typename Traits = char_traits<CharT>, + typename Alloc = allocator<CharT> > + class basic_string { .... };</pre><p>Now, <code class="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 class="code">char_traits</code> takes more work. The char_traits + template is <span class="emphasis"><em>declared</em></span> but not <span class="emphasis"><em>defined</em></span>. + That means there is only + </p><pre class="programlisting"> + 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. + </p><p>The C++ standard also requires that char_traits be specialized for + instantiations of <code class="code">char</code> and <code class="code">wchar_t</code>, and it + is these template specializations that permit entities like + <code class="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 class="code">unsigned char</code> and <code class="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 class="code">int</code> and other + built-in types. + </p><p>If you want to use your own special character class, then you have + <a class="link" 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>Another example of how to specialize char_traits was given <a class="link" href="http://gcc.gnu.org/ml/libstdc++/2002-08/msg00260.html">on the + mailing list</a> and at a later date was put into the file <code class="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 <a class="link" href="http://gcc.gnu.org/ml/libstdc++/2002-08/msg00236.html">the + nice-looking first attempt</a> turned out to <a class="link" 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></div><div class="section" title="Tokenizing"><div class="titlepage"><div><div><h3 class="title"><a id="strings.string.token"/>Tokenizing</h3></div></div></div><p> + </p><p>The Standard C (and C++) function <code class="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. 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). + </p><pre class="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; + } +} +</pre><p> + 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 class="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"; + } </pre><p>You would see this as output: + </p><pre class="programlisting"> + :this: + :is: + :a: + :test: </pre><p>with all the whitespace removed. The original <code class="code">s</code> is still + available for use, <code class="code">ls</code> will clean up after itself, and + <code class="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 outweigh that, however. + </p><p><span class="emphasis"><em>Added February 2001:</em></span> Mark Wilden pointed out that the + standard <code class="code">std::getline()</code> function can be used with standard + <code class="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. + </p></div><div class="section" title="Shrink to Fit"><div class="titlepage"><div><div><h3 class="title"><a id="strings.string.shrink"/>Shrink to Fit</h3></div></div></div><p> + </p><p>From GCC 3.4 calling <code class="code">s.reserve(res)</code> on a + <code class="code">string s</code> with <code class="code">res < s.capacity()</code> will + reduce the string's capacity to <code class="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 class="programlisting"> + std::string(str.data(), str.size()).swap(str); + </pre><p>This is similar to the idiom for reducing + a <code class="code">vector</code>'s memory usage + (see <a class="link" href="../faq.html#faq.size_equals_capacity" title="7.8.">this FAQ + entry</a>) but the regular copy constructor cannot be used + because libstdc++'s <code class="code">string</code> is Copy-On-Write. + </p><p>In <a class="link" href="status.html#status.iso.200x" title="C++ 200x">C++0x</a> mode you can call + <code class="code">s.shrink_to_fit()</code> to achieve the same effect as + <code class="code">s.reserve(s.size())</code>. + </p></div><div class="section" title="CString (MFC)"><div class="titlepage"><div><div><h3 class="title"><a id="strings.string.Cstring"/>CString (MFC)</h3></div></div></div><p> + </p><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 class="link" href="http://gcc.gnu.org/ml/gcc/1999-04n/msg00236.html">this + message</a>, Joe Buck points out a few very important things: + </p><div class="itemizedlist"><ul class="itemizedlist"><li class="listitem"><p>The Standard <code class="code">string</code> supports all the operations + that CString does, with three exceptions. + </p></li><li class="listitem"><p>Two of those exceptions (whitespace trimming and case + conversion) are trivial to implement. In fact, we do so + on this page. + </p></li><li class="listitem"><p>The third is <code class="code">CString::Format</code>, which allows formatting + in the style of <code class="code">sprintf</code>. This deserves some mention: + </p></li></ul></div><p> + 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: + </p><pre class="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(); + } </pre><p>A serious problem with CString is a design bug in its memory + allocation. Specifically, quoting from that same message: + </p><pre class="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). + </pre><p>Joe Buck also pointed out some other things to keep in mind when + comparing CString and the Standard string class: + </p><div class="itemizedlist"><ul class="itemizedlist"><li class="listitem"><p>CString permits access to its internal representation; coders + who exploited that may have problems moving to <code class="code">string</code>. + </p></li><li class="listitem"><p>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. + <span class="emphasis"><em><span class="emphasis"><em>Note:</em></span> 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.</em></span> + </p></li><li class="listitem"><p><code class="code">string</code> operations like this have O(n) complexity + <span class="emphasis"><em>if the implementors do it correctly</em></span>. The libstdc++ + implementors did it correctly. Other vendors might not. + </p></li><li class="listitem"><p>While chapters of the SGI STL are used in libstdc++, their + string class is not. The SGI <code class="code">string</code> is essentially + <code class="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?) + </p></li></ul></div></div></div></div><div class="navfooter"><hr/><table width="100%" summary="Navigation footer"><tr><td align="left"><a accesskey="p" href="traits.html">Prev</a> </td><td align="center"><a accesskey="u" href="bk01pt02.html">Up</a></td><td align="right"> <a accesskey="n" href="localization.html">Next</a></td></tr><tr><td align="left" valign="top">Traits </td><td align="center"><a accesskey="h" href="../spine.html">Home</a></td><td align="right" valign="top"> Chapter 8. + Localization + +</td></tr></table></div></body></html> |