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<?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 "/><meta name="keywords" content=" ISO C++ , runtime , library "/><link rel="home" href="../index.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="id499563" 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.2011" title="C++ 2011">C++11</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="../index.html">Home</a></td><td align="right" valign="top"> Chapter 8. Localization </td></tr></table></div></body></html>
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