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Strings
</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="traits.html">Prev</a> </td><th width="60%" align="center">Part II.
Standard Contents
</th><td width="20%" align="right"> <a accesskey="n" href="localization.html">Next</a></td></tr></table><hr /></div><div class="chapter"><div class="titlepage"><div><div><h2 class="title"><a id="std.strings"></a>Chapter 7.
Strings
<a id="id-1.3.4.5.1.1.1" class="indexterm"></a>
</h2></div></div></div><div class="toc"><p><strong>Table of Contents</strong></p><dl class="toc"><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"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="std.strings.string"></a>String Classes</h2></div></div></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="strings.string.simple"></a>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" target="_top">this
message</a>.
At minimum, you can write short wrappers like
</p><pre class="programlisting">
char toLower (char c)
{
// std::tolower(c) is undefined if c < 0 so cast to unsigned char.
return std::tolower((unsigned char)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"><div class="titlepage"><div><div><h3 class="title"><a id="strings.string.case"></a>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/" target="_top">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" target="_top">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" target="_top"> 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" target="_top">Unicode
Technical Report discussing case handling</a>, which provides some
very good information.
</p></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="strings.string.character_types"></a>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" target="_top">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" target="_top">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" target="_top">the
nice-looking first attempt</a> turned out to <a class="link" href="http://gcc.gnu.org/ml/libstdc++/2002-08/msg00242.html" target="_top">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"><div class="titlepage"><div><div><h3 class="title"><a id="strings.string.token"></a>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"><div class="titlepage"><div><div><h3 class="title"><a id="strings.string.shrink"></a>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 in GCC 3.
</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"><div class="titlepage"><div><div><h3 class="title"><a id="strings.string.Cstring"></a>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" target="_top">this
message</a>, Joe Buck points out a few very important things:
</p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><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" style="list-style-type: disc; "><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 parts 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 width="40%" align="left"><a accesskey="p" href="traits.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="std_contents.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="localization.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Traits </td><td width="20%" align="center"><a accesskey="h" href="../index.html">Home</a></td><td width="40%" align="right" valign="top"> Chapter 8.
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