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367 lines
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<?xml version="1.0" encoding="UTF-8" standalone="no"?>
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
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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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Strings
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</th></tr><tr><td align="left"><a accesskey="p" href="traits.html">Prev</a> </td><th width="60%" align="center">Part II.
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Standard Contents
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</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.
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Strings
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<a id="id499563" class="indexterm"/>
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</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>
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Here are Standard, simple, and portable ways to perform common
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transformations on a <code class="code">string</code> instance, such as
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"convert to all upper case." The word transformations
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is especially apt, because the standard template function
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<code class="code">transform<></code> is used.
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</p><p>
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This code will go through some iterations. Here's a simple
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version:
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</p><pre class="programlisting">
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#include <string>
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#include <algorithm>
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#include <cctype> // old <ctype.h>
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struct ToLower
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{
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char operator() (char c) const { return std::tolower(c); }
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};
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struct ToUpper
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{
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char operator() (char c) const { return std::toupper(c); }
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};
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int main()
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{
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std::string s ("Some Kind Of Initial Input Goes Here");
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// Change everything into upper case
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std::transform (s.begin(), s.end(), s.begin(), ToUpper());
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// Change everything into lower case
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std::transform (s.begin(), s.end(), s.begin(), ToLower());
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// Change everything back into upper case, but store the
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// result in a different string
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std::string capital_s;
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capital_s.resize(s.size());
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std::transform (s.begin(), s.end(), capital_s.begin(), ToUpper());
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}
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</pre><p>
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<span class="emphasis"><em>Note</em></span> that these calls all
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involve the global C locale through the use of the C functions
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<code class="code">toupper/tolower</code>. This is absolutely guaranteed to work --
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but <span class="emphasis"><em>only</em></span> if the string contains <span class="emphasis"><em>only</em></span> characters
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from the basic source character set, and there are <span class="emphasis"><em>only</em></span>
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96 of those. Which means that not even all English text can be
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represented (certain British spellings, proper names, and so forth).
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So, if all your input forevermore consists of only those 96
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characters (hahahahahaha), then you're done.
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</p><p><span class="emphasis"><em>Note</em></span> that the
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<code class="code">ToUpper</code> and <code class="code">ToLower</code> function objects
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are needed because <code class="code">toupper</code> and <code class="code">tolower</code>
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are overloaded names (declared in <code class="code"><cctype></code> and
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<code class="code"><locale></code>) so the template-arguments for
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<code class="code">transform<></code> cannot be deduced, as explained in
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<a class="link" href="http://gcc.gnu.org/ml/libstdc++/2002-11/msg00180.html">this
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message</a>.
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At minimum, you can write short wrappers like
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</p><pre class="programlisting">
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char toLower (char c)
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{
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return std::tolower(c);
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} </pre><p>(Thanks to James Kanze for assistance and suggestions on all of this.)
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</p><p>Another common operation is trimming off excess whitespace. Much
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like transformations, this task is trivial with the use of string's
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<code class="code">find</code> family. These examples are broken into multiple
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statements for readability:
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</p><pre class="programlisting">
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std::string str (" \t blah blah blah \n ");
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// trim leading whitespace
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string::size_type notwhite = str.find_first_not_of(" \t\n");
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str.erase(0,notwhite);
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// trim trailing whitespace
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notwhite = str.find_last_not_of(" \t\n");
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str.erase(notwhite+1); </pre><p>Obviously, the calls to <code class="code">find</code> could be inserted directly
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into the calls to <code class="code">erase</code>, in case your compiler does not
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optimize named temporaries out of existence.
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</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>
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</p><p>The well-known-and-if-it-isn't-well-known-it-ought-to-be
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<a class="link" href="http://www.gotw.ca/gotw/">Guru of the Week</a>
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discussions held on Usenet covered this topic in January of 1998.
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Briefly, the challenge was, <span class="quote">“<span class="quote">write a 'ci_string' class which
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is identical to the standard 'string' class, but is
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case-insensitive in the same way as the (common but nonstandard)
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C function stricmp()</span>”</span>.
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</p><pre class="programlisting">
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ci_string s( "AbCdE" );
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// case insensitive
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assert( s == "abcde" );
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assert( s == "ABCDE" );
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// still case-preserving, of course
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assert( strcmp( s.c_str(), "AbCdE" ) == 0 );
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assert( strcmp( s.c_str(), "abcde" ) != 0 ); </pre><p>The solution is surprisingly easy. The original answer was
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posted on Usenet, and a revised version appears in Herb Sutter's
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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>.
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</p><p>See? Told you it was easy!</p><p>
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<span class="emphasis"><em>Added June 2000:</em></span> The May 2000 issue of C++
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Report contains a fascinating <a class="link" href="http://lafstern.org/matt/col2_new.pdf"> article</a> by
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Matt Austern (yes, <span class="emphasis"><em>the</em></span> Matt Austern) on why
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case-insensitive comparisons are not as easy as they seem, and
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why creating a class is the <span class="emphasis"><em>wrong</em></span> way to go
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about it in production code. (The GotW answer mentions one of
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the principle difficulties; his article mentions more.)
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</p><p>Basically, this is "easy" only if you ignore some things,
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things which may be too important to your program to ignore. (I chose
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to ignore them when originally writing this entry, and am surprised
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that nobody ever called me on it...) The GotW question and answer
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remain useful instructional tools, however.
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</p><p><span class="emphasis"><em>Added September 2000:</em></span> James Kanze provided a link to a
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<a class="link" href="http://www.unicode.org/reports/tr21/tr21-5.html">Unicode
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Technical Report discussing case handling</a>, which provides some
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very good information.
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</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>
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</p><p>The <code class="code">std::basic_string</code> is tantalizingly general, in that
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it is parameterized on the type of the characters which it holds.
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In theory, you could whip up a Unicode character class and instantiate
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<code class="code">std::basic_string<my_unicode_char></code>, or assuming
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that integers are wider than characters on your platform, maybe just
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declare variables of type <code class="code">std::basic_string<int></code>.
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</p><p>That's the theory. Remember however that basic_string has additional
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type parameters, which take default arguments based on the character
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type (called <code class="code">CharT</code> here):
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</p><pre class="programlisting">
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template <typename CharT,
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typename Traits = char_traits<CharT>,
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typename Alloc = allocator<CharT> >
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class basic_string { .... };</pre><p>Now, <code class="code">allocator<CharT></code> will probably Do The Right
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Thing by default, unless you need to implement your own allocator
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for your characters.
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</p><p>But <code class="code">char_traits</code> takes more work. The char_traits
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template is <span class="emphasis"><em>declared</em></span> but not <span class="emphasis"><em>defined</em></span>.
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That means there is only
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</p><pre class="programlisting">
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template <typename CharT>
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struct char_traits
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{
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static void foo (type1 x, type2 y);
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...
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};</pre><p>and functions such as char_traits<CharT>::foo() are not
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actually defined anywhere for the general case. The C++ standard
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permits this, because writing such a definition to fit all possible
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CharT's cannot be done.
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</p><p>The C++ standard also requires that char_traits be specialized for
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instantiations of <code class="code">char</code> and <code class="code">wchar_t</code>, and it
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is these template specializations that permit entities like
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<code class="code">basic_string<char,char_traits<char>></code> to work.
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</p><p>If you want to use character types other than char and wchar_t,
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such as <code class="code">unsigned char</code> and <code class="code">int</code>, you will
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need suitable specializations for them. For a time, in earlier
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versions of GCC, there was a mostly-correct implementation that
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let programmers be lazy but it broke under many situations, so it
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was removed. GCC 3.4 introduced a new implementation that mostly
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works and can be specialized even for <code class="code">int</code> and other
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built-in types.
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</p><p>If you want to use your own special character class, then you have
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<a class="link" href="http://gcc.gnu.org/ml/libstdc++/2002-08/msg00163.html">a lot
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of work to do</a>, especially if you with to use i18n features
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(facets require traits information but don't have a traits argument).
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</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
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mailing list</a> and at a later date was put into the file <code class="code">
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include/ext/pod_char_traits.h</code>. We agree
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that the way it's used with basic_string (scroll down to main())
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doesn't look nice, but that's because <a class="link" href="http://gcc.gnu.org/ml/libstdc++/2002-08/msg00236.html">the
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nice-looking first attempt</a> turned out to <a class="link" href="http://gcc.gnu.org/ml/libstdc++/2002-08/msg00242.html">not
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be conforming C++</a>, due to the rule that CharT must be a POD.
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(See how tricky this is?)
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</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>
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</p><p>The Standard C (and C++) function <code class="code">strtok()</code> leaves a lot to
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be desired in terms of user-friendliness. It's unintuitive, it
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destroys the character string on which it operates, and it requires
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you to handle all the memory problems. But it does let the client
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code decide what to use to break the string into pieces; it allows
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you to choose the "whitespace," so to speak.
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</p><p>A C++ implementation lets us keep the good things and fix those
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annoyances. The implementation here is more intuitive (you only
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call it once, not in a loop with varying argument), it does not
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affect the original string at all, and all the memory allocation
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is handled for you.
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</p><p>It's called stringtok, and it's a template function. Sources are
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as below, in a less-portable form than it could be, to keep this
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example simple (for example, see the comments on what kind of
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string it will accept).
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</p><pre class="programlisting">
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#include <string>
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template <typename Container>
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void
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stringtok(Container &container, string const &in,
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const char * const delimiters = " \t\n")
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{
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const string::size_type len = in.length();
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string::size_type i = 0;
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|
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while (i < len)
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{
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// Eat leading whitespace
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i = in.find_first_not_of(delimiters, i);
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if (i == string::npos)
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return; // Nothing left but white space
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|
|
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// Find the end of the token
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string::size_type j = in.find_first_of(delimiters, i);
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|
|
|
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// Push token
|
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if (j == string::npos)
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{
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container.push_back(in.substr(i));
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return;
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}
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else
|
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container.push_back(in.substr(i, j-i));
|
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|
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// Set up for next loop
|
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i = j + 1;
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}
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}
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</pre><p>
|
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|
|
The author uses a more general (but less readable) form of it for
|
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|
|
parsing command strings and the like. If you compiled and ran this
|
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|
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code using it:
|
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|
</p><pre class="programlisting">
|
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|
|
std::list<string> ls;
|
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stringtok (ls, " this \t is\t\n a test ");
|
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for (std::list<string>const_iterator i = ls.begin();
|
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i != ls.end(); ++i)
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{
|
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std::cerr << ':' << (*i) << ":\n";
|
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|
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} </pre><p>You would see this as output:
|
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|
|
</p><pre class="programlisting">
|
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|
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:this:
|
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:is:
|
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:a:
|
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|
|
:test: </pre><p>with all the whitespace removed. The original <code class="code">s</code> is still
|
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|
|
available for use, <code class="code">ls</code> will clean up after itself, and
|
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|
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<code class="code">ls.size()</code> will return how many tokens there were.
|
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|
|
</p><p>As always, there is a price paid here, in that stringtok is not
|
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|
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as fast as strtok. The other benefits usually outweigh that, however.
|
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|
|
</p><p><span class="emphasis"><em>Added February 2001:</em></span> Mark Wilden pointed out that the
|
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|
|
standard <code class="code">std::getline()</code> function can be used with standard
|
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|
|
<code class="code">istringstreams</code> to perform
|
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|
|
tokenizing as well. Build an istringstream from the input text,
|
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|
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and then use std::getline with varying delimiters (the three-argument
|
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signature) to extract tokens into a string.
|
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</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>
|
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|
|
</p><p>From GCC 3.4 calling <code class="code">s.reserve(res)</code> on a
|
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|
|
<code class="code">string s</code> with <code class="code">res < s.capacity()</code> will
|
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|
|
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>
|