c++ - example - wstring substr




std::wstring VS std::string (8)

string? wstring?

std::string is a basic_string templated on a char, and std::wstring on a wchar_t.

char vs. wchar_t

char is supposed to hold a character, usually an 8-bit character.
wchar_t is supposed to hold a wide character, and then, things get tricky:
On Linux, a wchar_t is 4 bytes, while on Windows, it's 2 bytes.

What about Unicode, then?

The problem is that neither char nor wchar_t is directly tied to unicode.

On Linux?

Let's take a Linux OS: My Ubuntu system is already unicode aware. When I work with a char string, it is natively encoded in UTF-8 (i.e. Unicode string of chars). The following code:

#include <cstring>
#include <iostream>

int main(int argc, char* argv[])
{
   const char text[] = "olé" ;


   std::cout << "sizeof(char)    : " << sizeof(char) << std::endl ;
   std::cout << "text            : " << text << std::endl ;
   std::cout << "sizeof(text)    : " << sizeof(text) << std::endl ;
   std::cout << "strlen(text)    : " << strlen(text) << std::endl ;

   std::cout << "text(ordinals)  :" ;

   for(size_t i = 0, iMax = strlen(text); i < iMax; ++i)
   {
      std::cout << " " << static_cast<unsigned int>(
                              static_cast<unsigned char>(text[i])
                          );
   }

   std::cout << std::endl << std::endl ;

   // - - - 

   const wchar_t wtext[] = L"olé" ;

   std::cout << "sizeof(wchar_t) : " << sizeof(wchar_t) << std::endl ;
   //std::cout << "wtext           : " << wtext << std::endl ; <- error
   std::cout << "wtext           : UNABLE TO CONVERT NATIVELY." << std::endl ;
   std::wcout << L"wtext           : " << wtext << std::endl;

   std::cout << "sizeof(wtext)   : " << sizeof(wtext) << std::endl ;
   std::cout << "wcslen(wtext)   : " << wcslen(wtext) << std::endl ;

   std::cout << "wtext(ordinals) :" ;

   for(size_t i = 0, iMax = wcslen(wtext); i < iMax; ++i)
   {
      std::cout << " " << static_cast<unsigned int>(
                              static_cast<unsigned short>(wtext[i])
                              );
   }

   std::cout << std::endl << std::endl ;

   return 0;
}

outputs the following text:

sizeof(char)    : 1
text            : olé
sizeof(text)    : 5
strlen(text)    : 4
text(ordinals)  : 111 108 195 169

sizeof(wchar_t) : 4
wtext           : UNABLE TO CONVERT NATIVELY.
wtext           : ol�
sizeof(wtext)   : 16
wcslen(wtext)   : 3
wtext(ordinals) : 111 108 233

You'll see the "olé" text in char is really constructed by four chars: 110, 108, 195 and 169 (not counting the trailing zero). (I'll let you study the wchar_t code as an exercise)

So, when working with a char on Linux, you should usually end up using Unicode without even knowing it. And as std::string works with char, so std::string is already unicode-ready.

Note that std::string, like the C string API, will consider the "olé" string to have 4 characters, not three. So you should be cautious when truncating/playing with unicode chars because some combination of chars is forbidden in UTF-8.

On Windows?

On Windows, this is a bit different. Win32 had to support a lot of application working with char and on different charsets/codepages produced in all the world, before the advent of Unicode.

So their solution was an interesting one: If an application works with char, then the char strings are encoded/printed/shown on GUI labels using the local charset/codepage on the machine. For example, "olé" would be "olé" in a French-localized Windows, but would be something different on an cyrillic-localized Windows ("olй" if you use Windows-1251). Thus, "historical apps" will usually still work the same old way.

For Unicode based applications, Windows uses wchar_t, which is 2-bytes wide, and is encoded in UTF-16, which is Unicode encoded on 2-bytes characters (or at the very least, the mostly compatible UCS-2, which is almost the same thing IIRC).

Applications using char are said "multibyte" (because each glyph is composed of one or more chars), while applications using wchar_t are said "widechar" (because each glyph is composed of one or two wchar_t. See MultiByteToWideChar and WideCharToMultiByte Win32 conversion API for more info.

Thus, if you work on Windows, you badly want to use wchar_t (unless you use a framework hiding that, like GTK+ or QT...). The fact is that behind the scenes, Windows works with wchar_t strings, so even historical applications will have their char strings converted in wchar_t when using API like SetWindowText() (low level API function to set the label on a Win32 GUI).

Memory issues?

UTF-32 is 4 bytes per characters, so there is no much to add, if only that a UTF-8 text and UTF-16 text will always use less or the same amount of memory than an UTF-32 text (and usually less).

If there is a memory issue, then you should know than for most western languages, UTF-8 text will use less memory than the same UTF-16 one.

Still, for other languages (chinese, japanese, etc.), the memory used will be either the same, or slightly larger for UTF-8 than for UTF-16.

All in all, UTF-16 will mostly use 2 and occassionally 4 bytes per characters (unless you're dealing with some kind of esoteric language glyphs (Klingon? Elvish?), while UTF-8 will spend from 1 to 4 bytes.

See http://en.wikipedia.org/wiki/UTF-8#Compared_to_UTF-16 for more info.

Conclusion

  1. When I should use std::wstring over std::string?

    On Linux? Almost never (§).
    On Windows? Almost always (§).
    On cross-platform code? Depends on your toolkit...

    (§) : unless you use a toolkit/framework saying otherwise

  2. Can std::string hold all the ASCII character set including special characters?

    Notice: A std::string is suitable for holding a 'binary' buffer, where a std::wstring is not!

    On Linux? Yes.
    On Windows? Only special characters available for the current locale of the Windows user.

    Edit (After a comment from Johann Gerell):
    a std::string will be enough to handle all char-based strings (each char being a number from 0 to 255). But:

    1. ASCII is supposed to go from 0 to 127. Higher chars are NOT ASCII.
    2. a char from 0 to 127 will be held correctly
    3. a char from 128 to 255 will have a signification depending on your encoding (unicode, non-unicode, etc.), but it will be able to hold all Unicode glyphs as long as they are encoded in UTF-8.
  3. Is std::wstring supported by almost all popular C++ compilers?

    Mostly, with the exception of GCC based compilers that are ported to Windows.
    It works on my g++ 4.3.2 (under Linux), and I used Unicode API on Win32 since Visual C++ 6.

  4. What is exactly a wide character?

    On C/C++, it's a character type written wchar_t which is larger than the simple char character type. It is supposed to be used to put inside characters whose indices (like Unicode glyphs) are larger than 255 (or 127, depending...).

I am not able to understand the differences between std::string and std::wstring. I know wstring supports wide characters such as Unicode characters. I have got the following questions:

  1. When should I use std::wstring over std::string?
  2. Can std::string hold the entire ASCII character set, including the special characters?
  3. Is std::wstring supported by all popular C++ compilers?
  4. What is exactly a "wide character"?

  1. When you want to have wide characters stored in your string. wide depends on the implementation. Visual C++ defaults to 16 bit if i remember correctly, while GCC defaults depending on the target. It's 32 bits long here. Please note wchar_t (wide character type) has nothing to do with unicode. It's merely guaranteed that it can store all the members of the largest character set that the implementation supports by its locales, and at least as long as char. You can store unicode strings fine into std::string using the utf-8 encoding too. But it won't understand the meaning of unicode code points. So str.size() won't give you the amount of logical characters in your string, but merely the amount of char or wchar_t elements stored in that string/wstring. For that reason, the gtk/glib C++ wrapper folks have developed a Glib::ustring class that can handle utf-8.

    If your wchar_t is 32 bits long, then you can use utf-32 as an unicode encoding, and you can store and handle unicode strings using a fixed (utf-32 is fixed length) encoding. This means your wstring's s.size() function will then return the right amount of wchar_t elements and logical characters.

  2. Yes, char is always at least 8 bit long, which means it can store all ASCII values.
  3. Yes, all major compilers support it.

  1. when you want to use Unicode strings and not just ascii, helpful for internationalisation
  2. yes, but it doesn't play well with 0
  3. not aware of any that don't
  4. wide character is the compiler specific way of handling the fixed length representation of a unicode character, for MSVC it is a 2 byte character, for gcc I understand it is 4 bytes. and a +1 for http://www.joelonsoftware.com/articles/Unicode.html

1) As mentioned by Greg, wstring is helpful for internationalization, that's when you will be releasing your product in languages other than english

4) Check this out for wide character http://en.wikipedia.org/wiki/Wide_character


Applications that are not satisfied with only 256 different characters have the options of either using wide characters (more than 8 bits) or a variable-length encoding (a multibyte encoding in C++ terminology) such as UTF-8. Wide characters generally require more space than a variable-length encoding, but are faster to process. Multi-language applications that process large amounts of text usually use wide characters when processing the text, but convert it to UTF-8 when storing it to disk.

The only difference between a string and a wstring is the data type of the characters they store. A string stores chars whose size is guaranteed to be at least 8 bits, so you can use strings for processing e.g. ASCII, ISO-8859-15, or UTF-8 text. The standard says nothing about the character set or encoding.

Practically every compiler uses a character set whose first 128 characters correspond with ASCII. This is also the case with compilers that use UTF-8 encoding. The important thing to be aware of when using strings in UTF-8 or some other variable-length encoding, is that the indices and lengths are measured in bytes, not characters.

The data type of a wstring is wchar_t, whose size is not defined in the standard, except that it has to be at least as large as a char, usually 16 bits or 32 bits. wstring can be used for processing text in the implementation defined wide-character encoding. Because the encoding is not defined in the standard, it is not straightforward to convert between strings and wstrings. One cannot assume wstrings to have a fixed-length encoding either.

If you don't need multi-language support, you might be fine with using only regular strings. On the other hand, if you're writing a graphical application, it is often the case that the API supports only wide characters. Then you probably want to use the same wide characters when processing the text. Keep in mind that UTF-16 is a variable-length encoding, meaning that you cannot assume length() to return the number of characters. If the API uses a fixed-length encoding, such as UCS-2, processing becomes easy. Converting between wide characters and UTF-8 is difficult to do in a portable way, but then again, your user interface API probably supports the conversion.


I frequently use std::string to hold utf-8 characters without any problems at all. I heartily recommend doing this when interfacing with API's which use utf-8 as the native string type as well.

For example, I use utf-8 when interfacing my code with the Tcl interpreter.

The major caveat is the length of the std::string, is no longer the number of characters in the string.


So, every reader here now should have a clear understanding about the facts, the situation. If not, then you must read paercebal's outstandingly comprehensive answer [btw: thanks!].

My pragmatical conclusion is shockingly simple: all that C++ (and STL) "character encoding" stuff is substantially broken and useless. Blame it on Microsoft or not, that will not help anyway.

My solution, after in-depth investigation, much frustration and the consequential experiences is the following:

  1. accept, that you have to be responsible on your own for the encoding and conversion stuff (and you will see that much of it is rather trivial)

  2. use std::string for any UTF-8 encoded strings (just a typedef std::string UTF8String)

  3. accept that such an UTF8String object is just a dumb, but cheap container. Do never ever access and/or manipulate characters in it directly (no search, replace, and so on). You could, but you really just really, really do not want to waste your time writing text manipulation algorithms for multi-byte strings! Even if other people already did such stupid things, don't do that! Let it be! (Well, there are scenarios where it makes sense... just use the ICU library for those).

  4. use std::wstring for UCS-2 encoded strings (typedef std::wstring UCS2String) - this is a compromise, and a concession to the mess that the WIN32 API introduced). UCS-2 is sufficient for most of us (more on that later...).

  5. use UCS2String instances whenever a character-by-character access is required (read, manipulate, and so on). Any character-based processing should be done in a NON-multibyte-representation. It is simple, fast, easy.

  6. add two utility functions to convert back & forth between UTF-8 and UCS-2:

    UCS2String ConvertToUCS2( const UTF8String &str );
    UTF8String ConvertToUTF8( const UCS2String &str );
    

The conversions are straightforward, google should help here ...

That's it. Use UTF8String wherever memory is precious and for all UTF-8 I/O. Use UCS2String wherever the string must be parsed and/or manipulated. You can convert between those two representations any time.

Alternatives & Improvements

  • conversions from & to single-byte character encodings (e.g. ISO-8859-1) can be realized with help of plain translation tables, e.g. const wchar_t tt_iso88951[256] = {0,1,2,...}; and appropriate code for conversion to & from UCS2.

  • if UCS-2 is not sufficient, than switch to UCS-4 (typedef std::basic_string<uint32_t> UCS2String)

ICU or other unicode libraries?

For advanced stuff.


There are some very good answers here, but I think there are a couple of things I can add regarding Windows/Visual Studio. Tis is based on my experience with VS2015. On Linux, basically the answer is to use UTF-8 encoded std::string everywhere. On Windows/VS it gets more complex. Here is why. Windows expects strings stored using chars to be encoded using the locale codepage. This is almost always the ASCII character set followed by 128 other special characters depending on your location. Let me just state that this in not just when using the Windows API, there are three other major places where these strings interact with standard C++. These are string literals, output to std::cout using << and passing a filename to std::fstream.

I will be up front here that I am a programmer, not a language specialist. I appreciate that USC2 and UTF-16 are not the same, but for my purposes they are close enough to be interchangeable and I use them as such here. I'm not actually sure which Windows uses, but I generally don't need to know either. I've stated UCS2 in this answer, so sorry in advance if I upset anyone with my ignorance of this matter and I'm happy to change it if I have things wrong.

String literals

If you enter string literals that contain only characters that can be represented by your codepage then VS stores them in your file with 1 byte per character encoding based on your codepage. Note that if you change your codepage or give your source to another developer using a different code page then I think (but haven't tested) that the character will end up different. If you run your code on a computer using a different code page then I'm not sure if the character will change too.

If you enter any string literals that cannot be represented by your codepage then VS will ask you to save the file as Unicode. The file will then be encoded as UTF-8. This means that all Non ASCII characters (including those which are on your codepage) will be represented by 2 or more bytes. This means if you give your source to someone else the source will look the same. However, before passing the source to the compiler, VS converts the UTF-8 encoded text to code page encoded text and any characters missing from the code page are replaced with ?.

The only way to guarantee correctly representing a Unicode string literal in VS is to precede the string literal with an L making it a wide string literal. In this case VS will convert the UTF-8 encoded text from the file into UCS2. You then need to pass this string literal into a std::wstring constructor or you need to convert it to utf-8 and put it in a std::string. Or if you want you can use the Windows API functions to encode it using your code page to put it in a std::string, but then you may as well have not used a wide string literal.

std::cout

When outputting to the console using << you can only use std::string, not std::wstring and the text must be encoded using your locale codepage. If you have a std::wstring then you must convert it using one of the Windows API functions and any characters not on your codepage get replaced by ? (maybe you can change the character, I can't remember).

std::fstream filenames

Windows OS uses UCS2/UTF-16 for its filenames so whatever your codepage, you can have files with any Unicode character. But this means that to access or create files with characters not on your codepage you must use std::wstring. There is no other way. This is a Microsoft specific extension to std::fstream so probably won't compile on other systems. If you use std::string then you can only utilise filenames that only include characters on your codepage.

Your options

If you are just working on Linux then you probably didn't get this far. Just use UTF-8 std::string everywhere.

If you are just working on Windows just use UCS2 std::wstring everywhere. Some purists may say use UTF8 then convert when needed, but why bother with the hassle.

If you are cross platform then it's a mess to be frank. If you try to use UTF-8 everywhere on Windows then you need to be really careful with your string literals and output to the console. You can easily corrupt your strings there. If you use std::wstring everywhere on Linux then you may not have access to the wide version of std::fstream, so you have to do the conversion, but there is no risk of corruption. So personally I think this is a better option. Many would disagree, but I'm not alone - it's the path taken by wxWidgets for example.

Another option could be to typedef unicodestring as std::string on Linux and std::wstring on Windows, and have a macro called UNI() which prefixes L on Windows and nothing on Linux, then the code

#include <fstream>
#include <string>
#include <iostream>
#include <Windows.h>

#ifdef _WIN32
typedef std::wstring unicodestring;
#define UNI(text) L ## text
std::string formatForConsole(const unicodestring &str)
{
    std::string result;
    //Call WideCharToMultiByte to do the conversion
    return result;
}
#else
typedef std::string unicodestring;
#define UNI(text) text
std::string formatForConsole(const unicodestring &str)
{
    return str;
}
#endif

int main()
{

    unicodestring fileName(UNI("fileName"));
    std::ofstream fout;
    fout.open(fileName);
    std::cout << formatForConsole(fileName) << std::endl;
    return 0;
}

would be fine on either platform I think.

Answers

So To answer your questions

1) If you are programming for Windows, then all the time, if cross platform then maybe all the time, unless you want to deal with possible corruption issues on Windows or write some code with platform specific #ifdefs to work around the differences, if just using Linux then never.

2)Yes. In addition on Linux you can use it for all Unicode too. On Windows you can only use it for all unicode if you choose to manually encode using UTF-8. But the Windows API and standard C++ classes will expect the std::string to be encoded using the locale codepage. This includes all ASCII plus another 128 characters which change depending on the codepage your computer is setup to use.

3)I believe so, but if not then it is just a simple typedef of a 'std::basic_string' using wchar_t instead of char

4)A wide character is a character type which is bigger than the 1 byte standard char type. On Windows it is 2 bytes, on Linux it is 4 bytes.





wstring