Table of Contents
+ Here are Standard, simple, and portable ways to perform common
+ transformations on a string instance, such as
+ "convert to all upper case." The word transformations
+ is especially apt, because the standard template function
+ transform<> is used.
+
+ This code will go through some iterations. Here's a simple + version: +
+ #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());
+ }
+
+ Note that these calls all
+ involve the global C locale through the use of the C functions
+ toupper/tolower. This is absolutely guaranteed to work --
+ but only if the string contains only characters
+ from the basic source character set, and there are only
+ 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.
+
Note that the
+ ToUpper and ToLower function objects
+ are needed because toupper and tolower
+ are overloaded names (declared in <cctype> and
+ <locale>) so the template-arguments for
+ transform<> cannot be deduced, as explained in
+ this
+ message.
+
+ At minimum, you can write short wrappers like
+
+ char toLower (char c)
+ {
+ return std::tolower(c);
+ } (Thanks to James Kanze for assistance and suggestions on all of this.) +
Another common operation is trimming off excess whitespace. Much
+ like transformations, this task is trivial with the use of string's
+ find family. These examples are broken into multiple
+ statements for readability:
+
+ 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); Obviously, the calls to find could be inserted directly
+ into the calls to erase, in case your compiler does not
+ optimize named temporaries out of existence.
+
+
The well-known-and-if-it-isn't-well-known-it-ought-to-be + Guru of the Week + discussions held on Usenet covered this topic in January of 1998. + Briefly, the challenge was, âwrite 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()â. +
+ 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 );
The solution is surprisingly easy. The original answer was + posted on Usenet, and a revised version appears in Herb Sutter's + book Exceptional C++ and on his website as GotW 29. +
See? Told you it was easy!
+ Added June 2000: The May 2000 issue of C++ + Report contains a fascinating article by + Matt Austern (yes, the Matt Austern) on why + case-insensitive comparisons are not as easy as they seem, and + why creating a class is the wrong way to go + about it in production code. (The GotW answer mentions one of + the principle difficulties; his article mentions more.) +
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. +
Added September 2000: James Kanze provided a link to a + Unicode + Technical Report discussing case handling, which provides some + very good information. +
+
The std::basic_string 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
+ std::basic_string<my_unicode_char>, or assuming
+ that integers are wider than characters on your platform, maybe just
+ declare variables of type std::basic_string<int>.
+
That's the theory. Remember however that basic_string has additional
+ type parameters, which take default arguments based on the character
+ type (called CharT here):
+
+ template <typename CharT,
+ typename Traits = char_traits<CharT>,
+ typename Alloc = allocator<CharT> >
+ class basic_string { .... };Now, allocator<CharT> will probably Do The Right
+ Thing by default, unless you need to implement your own allocator
+ for your characters.
+
But char_traits takes more work. The char_traits
+ template is declared but not defined.
+ That means there is only
+
+ template <typename CharT>
+ struct char_traits
+ {
+ static void foo (type1 x, type2 y);
+ ...
+ };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. +
The C++ standard also requires that char_traits be specialized for
+ instantiations of char and wchar_t, and it
+ is these template specializations that permit entities like
+ basic_string<char,char_traits<char>> to work.
+
If you want to use character types other than char and wchar_t,
+ such as unsigned char and int, 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 int and other
+ built-in types.
+
If you want to use your own special character class, then you have + a lot + of work to do, especially if you with to use i18n features + (facets require traits information but don't have a traits argument). +
Another example of how to specialize char_traits was given on the
+ mailing list and at a later date was put into the file
+ include/ext/pod_char_traits.h. We agree
+ that the way it's used with basic_string (scroll down to main())
+ doesn't look nice, but that's because the
+ nice-looking first attempt turned out to not
+ be conforming C++, due to the rule that CharT must be a POD.
+ (See how tricky this is?)
+
+
The Standard C (and C++) function strtok() 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.
+
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. +
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). +
+#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;
+ }
+}
++ 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: +
+ 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";
+ } You would see this as output: +
+ :this: + :is: + :a: + :test:
with all the whitespace removed. The original s is still
+ available for use, ls will clean up after itself, and
+ ls.size() will return how many tokens there were.
+
As always, there is a price paid here, in that stringtok is not + as fast as strtok. The other benefits usually outweigh that, however. +
Added February 2001: Mark Wilden pointed out that the
+ standard std::getline() function can be used with standard
+ istringstreams 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.
+
+
From GCC 3.4 calling s.reserve(res) on a
+ string s with res < s.capacity() will
+ reduce the string's capacity to std::max(s.size(), res).
+
This behaviour is suggested, but not required by the standard. Prior + to GCC 3.4 the following alternative can be used instead +
+ std::string(str.data(), str.size()).swap(str); +
This is similar to the idiom for reducing
+ a vector's memory usage
+ (see this FAQ
+ entry) but the regular copy constructor cannot be used
+ because libstdc++'s string is Copy-On-Write.
+
In C++11 mode you can call
+ s.shrink_to_fit() to achieve the same effect as
+ s.reserve(s.size()).
+
+
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. +
Things are not as bad as they seem. In + this + message, Joe Buck points out a few very important things: +
The Standard string supports all the operations
+ that CString does, with three exceptions.
+
Two of those exceptions (whitespace trimming and case + conversion) are trivial to implement. In fact, we do so + on this page. +
The third is CString::Format, which allows formatting
+ in the style of sprintf. This deserves some mention:
+
+ 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: +
+ #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();
+ } A serious problem with CString is a design bug in its memory + allocation. Specifically, quoting from that same message: +
+ 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).
+ Joe Buck also pointed out some other things to keep in mind when + comparing CString and the Standard string class: +
CString permits access to its internal representation; coders
+ who exploited that may have problems moving to string.
+
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. + Note: 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. +
string operations like this have O(n) complexity
+ if the implementors do it correctly. The libstdc++
+ implementors did it correctly. Other vendors might not.
+
While chapters of the SGI STL are used in libstdc++, their
+ string class is not. The SGI string is essentially
+ vector<char> 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?)
+