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%% Purpose: Overview of the Unicode support in wxWidgets
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%% Author: Vadim Zeitlin
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%% RCS-ID: $Id: tunicode.tex,v 1.18 2005/08/10 21:50:34 MW Exp $
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%% Copyright: (c) 1999 Vadim Zeitlin <zeitlin@dptmaths.ens-cachan.fr>
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%% Licence: wxWindows license
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\section{Unicode support in wxWidgets}\label{unicode}
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This section briefly describes the state of the Unicode support in wxWidgets.
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Read it if you want to know more about how to write programs able to work with
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characters from languages other than English.
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\subsection{What is Unicode?}\label{whatisunicode}
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Starting with release 2.1 wxWidgets has support for compiling in Unicode mode
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on the platforms which support it. Unicode is a standard for character
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encoding which addresses the shortcomings of the previous, 8 bit standards, by
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using at least 16 (and possibly 32) bits for encoding each character. This
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allows to have at least 65536 characters (what is called the BMP, or basic
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multilingual plane) and possible $2^{32}$ of them instead of the usual 256 and
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is sufficient to encode all of the world languages at once. More details about
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Unicode may be found at {\tt www.unicode.org}.
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% TODO expand on it, say that Unicode extends ASCII, mention ISO8859, ...
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As this solution is obviously preferable to the previous ones (think of
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incompatible encodings for the same language, locale chaos and so on), many
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modern operating systems support it. The probably first example is Windows NT
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which uses only Unicode internally since its very first version.
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Writing internationalized programs is much easier with Unicode and, as the
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support for it improves, it should become more and more so. Moreover, in the
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Windows NT/2000 case, even the program which uses only standard ASCII can profit
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from using Unicode because they will work more efficiently - there will be no
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need for the system to convert all strings the program uses to/from Unicode
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each time a system call is made.
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\subsection{Unicode and ANSI modes}\label{unicodeandansi}
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As not all platforms supported by wxWidgets support Unicode (fully) yet, in
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many cases it is unwise to write a program which can only work in Unicode
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environment. A better solution is to write programs in such way that they may
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be compiled either in ANSI (traditional) mode or in the Unicode one.
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This can be achieved quite simply by using the means provided by wxWidgets.
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Basically, there are only a few things to watch out for:
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\item Character type ({\tt char} or {\tt wchar\_t})
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\item Literal strings (i.e. {\tt "Hello, world!"} or {\tt '*'})
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\item String functions ({\tt strlen()}, {\tt strcpy()}, ...)
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\item Special preprocessor tokens ({\tt \_\_FILE\_\_}, {\tt \_\_DATE\_\_}
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and {\tt \_\_TIME\_\_})
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Let's look at them in order. First of all, each character in an Unicode
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program takes 2 bytes instead of usual one, so another type should be used to
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store the characters ({\tt char} only holds 1 byte usually). This type is
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called {\tt wchar\_t} which stands for {\it wide-character type}.
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Also, the string and character constants should be encoded using wide
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characters ({\tt wchar\_t} type) which typically take $2$ or $4$ bytes instead
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of {\tt char} which only takes one. This is achieved by using the standard C
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(and C++) way: just put the letter {\tt 'L'} after any string constant and it
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becomes a {\it long} constant, i.e. a wide character one. To make things a bit
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more readable, you are also allowed to prefix the constant with {\tt 'L'}
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instead of putting it after it.
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Of course, the usual standard C functions don't work with {\tt wchar\_t}
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strings, so another set of functions exists which do the same thing but accept
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{\tt wchar\_t *} instead of {\tt char *}. For example, a function to get the
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length of a wide-character string is called {\tt wcslen()} (compare with
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{\tt strlen()} - you see that the only difference is that the "str" prefix
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standing for "string" has been replaced with "wcs" standing for "wide-character
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And finally, the standard preprocessor tokens enumerated above expand to ANSI
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strings but it is more likely that Unicode strings are wanted in the Unicode
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build. wxWidgets provides the macros {\tt \_\_TFILE\_\_}, {\tt \_\_TDATE\_\_}
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and {\tt \_\_TTIME\_\_} which behave exactly as the standard ones except that
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they produce ANSI strings in ANSI build and Unicode ones in the Unicode build.
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To summarize, here is a brief example of how a program which can be compiled
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in both ANSI and Unicode modes could look like:
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const wchar_t *ws = L"Hello, world!";
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wprintf(L"Compiled at %s\n", __TDATE__);
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const char *s = "Hello, world!";
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printf("Compiled at %s\n", __DATE__);
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#endif // Unicode/ANSI
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Of course, it would be nearly impossibly to write such programs if it had to
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be done this way (try to imagine the number of {\tt \#ifdef UNICODE} an average
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program would have had!). Luckily, there is another way - see the next
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\subsection{Unicode support in wxWidgets}\label{unicodeinsidewxw}
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In wxWidgets, the code fragment from above should be written instead:
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wxChar ch = wxT('*');
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wxString s = wxT("Hello, world!");
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What happens here? First of all, you see that there are no more {\tt \#ifdef}s
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at all. Instead, we define some types and macros which behave differently in
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the Unicode and ANSI builds and allow us to avoid using conditional
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compilation in the program itself.
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We have a {\tt wxChar} type which maps either on {\tt char} or {\tt wchar\_t}
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depending on the mode in which program is being compiled. There is no need for
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a separate type for strings though, because the standard
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\helpref{wxString}{wxstring} supports Unicode, i.e. it stores either ANSI or
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Unicode strings depending on the compile mode.
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Finally, there is a special \helpref{wxT()}{wxt} macro which should enclose all
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literal strings in the program. As it is easy to see comparing the last
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fragment with the one above, this macro expands to nothing in the (usual) ANSI
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mode and prefixes {\tt 'L'} to its argument in the Unicode mode.
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The important conclusion is that if you use {\tt wxChar} instead of
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{\tt char}, avoid using C style strings and use {\tt wxString} instead and
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don't forget to enclose all string literals inside \helpref{wxT()}{wxt} macro, your
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program automatically becomes (almost) Unicode compliant!
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Just let us state once again the rules:
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\item Always use {\tt wxChar} instead of {\tt char}
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\item Always enclose literal string constants in \helpref{wxT()}{wxt} macro
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unless they're already converted to the right representation (another standard
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wxWidgets macro \helpref{\_()}{underscore} does it, for example, so there is no
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need for {\tt wxT()} in this case) or you intend to pass the constant directly
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to an external function which doesn't accept wide-character strings.
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\item Use {\tt wxString} instead of C style strings.
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\subsection{Unicode and the outside world}\label{unicodeoutsidewxw}
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We have seen that it was easy to write Unicode programs using wxWidgets types
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and macros, but it has been also mentioned that it isn't quite enough.
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Although everything works fine inside the program, things can get nasty when
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it tries to communicate with the outside world which, sadly, often expects
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ANSI strings (a notable exception is the entire Win32 API which accepts either
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Unicode or ANSI strings and which thus makes it unnecessary to ever perform
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any conversions in the program). GTK 2.0 only accepts UTF-8 strings.
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To get an ANSI string from a wxString, you may use the
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mb\_str() function which always returns an ANSI
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string (independently of the mode - while the usual
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\helpref{c\_str()}{wxstringcstr} returns a pointer to the internal
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representation which is either ASCII or Unicode). More rarely used, but still
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useful, is wc\_str() function which always returns
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Sometimes it is also necessary to go from ANSI strings to wxStrings.
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In this case, you can use the converter-constructor, as follows:
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const char* ascii_str = "Some text";
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wxString str(ascii_str, wxConvUTF8);
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This code also compiles fine under a non-Unicode build of wxWidgets,
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but in that case the converter is ignored.
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For more information about converters and Unicode see
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the \helpref{wxMBConv classes overview}{mbconvclasses}.
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% TODO describe fn_str(), wx_str(), wxCharBuf classes, ...
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\subsection{Unicode-related compilation settings}\label{unicodesettings}
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You should define {\tt wxUSE\_UNICODE} to $1$ to compile your program in
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Unicode mode. Note that it currently only works in Win32 and GTK 2.0 and
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wxWidgets are not Unicode-compliant yet. If you
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compile your program in ANSI mode you can still define {\tt wxUSE\_WCHAR\_T}
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to get some limited support for {\tt wchar\_t} type.
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This will allow your program to perform conversions between Unicode strings and
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ANSI ones (using \helpref{wxMBConv classes}{mbconvclasses})
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and construct wxString objects from Unicode strings (presumably read
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from some external file or elsewhere).