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+<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.
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-</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="shared_ptr.html">Prev</a> </td><th width="60%" align="center">The GNU C++ Library</th><td width="20%" align="right"> <a accesskey="n" href="bk01pt05ch13.html">Next</a></td></tr></table><hr /></div><div class="part" title="Part V. Strings"><div class="titlepage"><div><div><h1 class="title"><a id="manual.strings"></a>Part V.
+</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="id638392" class="indexterm"></a>
-</h1></div></div></div><div class="toc"><p><b>Table of Contents</b></p><dl><dt><span class="chapter"><a href="bk01pt05ch13.html">13. String Classes</a></span></dt><dd><dl><dt><span class="sect1"><a href="bk01pt05ch13.html#strings.string.simple">Simple Transformations</a></span></dt><dt><span class="sect1"><a href="bk01pt05ch13s02.html">Case Sensitivity</a></span></dt><dt><span class="sect1"><a href="bk01pt05ch13s03.html">Arbitrary Character Types</a></span></dt><dt><span class="sect1"><a href="bk01pt05ch13s04.html">Tokenizing</a></span></dt><dt><span class="sect1"><a href="bk01pt05ch13s05.html">Shrink to Fit</a></span></dt><dt><span class="sect1"><a href="bk01pt05ch13s06.html">CString (MFC)</a></span></dt></dl></dd></dl></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="shared_ptr.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="spine.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="bk01pt05ch13.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">shared_ptr </td><td width="20%" align="center"><a accesskey="h" href="../spine.html">Home</a></td><td width="40%" align="right" valign="top"> Chapter 13. String Classes</td></tr></table></div></body></html>
+ <a id="id457894" 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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