wxWidgets/docs/doxygen/overviews/unicode.h

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// Name:        unicode.h
// Purpose:     topic overview
// Author:      wxWidgets team
// RCS-ID:      $Id$
// Licence:     wxWindows license
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/*!

@page overview_unicode Unicode Support in wxWidgets

This section briefly describes the state of the Unicode support in wxWidgets.
Read it if you want to know more about how to write programs able to work with
characters from languages other than English.

@li @ref overview_unicode_what
@li @ref overview_unicode_ansi
@li @ref overview_unicode_supportin
@li @ref overview_unicode_supportout
@li @ref overview_unicode_settings
@li @ref overview_unicode_traps


<hr>


@section overview_unicode_what What is Unicode?

wxWidgets has support for compiling in Unicode mode on the platforms which
support it. Unicode is a standard for character encoding which addresses the
shortcomings of the previous, 8 bit standards, by using at least 16 (and
possibly 32) bits for encoding each character. This allows to have at least
65536 characters (what is called the BMP, or basic multilingual plane) and
possible 2^32 of them instead of the usual 256 and is sufficient to encode all
of the world languages at once. More details about Unicode may be found at
<http://www.unicode.org/>.

As this solution is obviously preferable to the previous ones (think of
incompatible encodings for the same language, locale chaos and so on), many
modern operating systems support it. The probably first example is Windows NT
which uses only Unicode internally since its very first version.

Writing internationalized programs is much easier with Unicode and, as the
support for it improves, it should become more and more so. Moreover, in the
Windows NT/2000 case, even the program which uses only standard ASCII can
profit from using Unicode because they will work more efficiently - there will
be no need for the system to convert all strings the program uses to/from
Unicode each time a system call is made.


@section overview_unicode_ansi Unicode and ANSI Modes

As not all platforms supported by wxWidgets support Unicode (fully) yet, in
many cases it is unwise to write a program which can only work in Unicode
environment. A better solution is to write programs in such way that they may
be compiled either in ANSI (traditional) mode or in the Unicode one.

This can be achieved quite simply by using the means provided by wxWidgets.
Basically, there are only a few things to watch out for:

- Character type (@c char or @c wchar_t)
- Literal strings (i.e. @c "Hello, world!" or @c '*')
- String functions (@c strlen(), @c strcpy(), ...)
- Special preprocessor tokens (@c __FILE__, @c __DATE__ and @c __TIME__)

Let's look at them in order. First of all, each character in an Unicode program
takes 2 bytes instead of usual one, so another type should be used to store the
characters (@c char only holds 1 byte usually). This type is called @c wchar_t
which stands for @e wide-character type.

Also, the string and character constants should be encoded using wide
characters (@c wchar_t type) which typically take 2 or 4 bytes instead of
@c char which only takes one. This is achieved by using the standard C (and
C++) way: just put the letter @c 'L' after any string constant and it becomes a
@e long constant, i.e. a wide character one. To make things a bit more
readable, you are also allowed to prefix the constant with @c 'L' instead of
putting it after it.

Of course, the usual standard C functions don't work with @c wchar_t strings,
so another set of functions exists which do the same thing but accept
@c wchar_t* instead of @c char*. For example, a function to get the length of a
wide-character string is called @c wcslen() (compare with @c strlen() - you see
that the only difference is that the "str" prefix standing for "string" has
been replaced with "wcs" standing for "wide-character string").

And finally, the standard preprocessor tokens enumerated above expand to ANSI
strings but it is more likely that Unicode strings are wanted in the Unicode
build. wxWidgets provides the macros @c __TFILE__, @c __TDATE__ and
@c __TTIME__ which behave exactly as the standard ones except that they produce
ANSI strings in ANSI build and Unicode ones in the Unicode build.

To summarize, here is a brief example of how a program which can be compiled
in both ANSI and Unicode modes could look like:

@code
#ifdef __UNICODE__
    wchar_t wch = L'*';
    const wchar_t *ws = L"Hello, world!";
    int len = wcslen(ws);

    wprintf(L"Compiled at %s\n", __TDATE__);
#else // ANSI
    char ch = '*';
    const char *s = "Hello, world!";
    int len = strlen(s);

    printf("Compiled at %s\n", __DATE__);
#endif // Unicode/ANSI
@endcode

Of course, it would be nearly impossibly to write such programs if it had to
be done this way (try to imagine the number of @ifdef UNICODE an average
program would have had!). Luckily, there is another way - see the next section.


@section overview_unicode_supportin Unicode Support in wxWidgets

In wxWidgets, the code fragment from above should be written instead:

@code
wxChar ch = wxT('*');
wxString s = wxT("Hello, world!");
int len = s.Len();
@endcode

What happens here? First of all, you see that there are no more UNICODE checks
at all. Instead, we define some types and macros which behave differently in
the Unicode and ANSI builds and allow us to avoid using conditional compilation
in the program itself.

We have a @c wxChar type which maps either on @c char or @c wchar_t depending
on the mode in which program is being compiled. There is no need for a separate
type for strings though, because the standard wxString supports Unicode, i.e.
it stores either ANSI or Unicode strings depending on the compile mode.

Finally, there is a special wxT() macro which should enclose all literal
strings in the program. As it is easy to see comparing the last fragment with
the one above, this macro expands to nothing in the (usual) ANSI mode and
prefixes @c 'L' to its argument in the Unicode mode.

The important conclusion is that if you use @c wxChar instead of @c char, avoid
using C style strings and use @c wxString instead and don't forget to enclose
all string literals inside wxT() macro, your program automatically becomes
(almost) Unicode compliant!

Just let us state once again the rules:

@li Always use wxChar instead of @c char
@li Always enclose literal string constants in wxT() macro unless they're
    already converted to the right representation (another standard wxWidgets
    macro _() does it, for example, so there is no need for wxT() in this case)
    or you intend to pass the constant directly to an external function which
    doesn't accept wide-character strings.
@li Use wxString instead of C style strings.


@section overview_unicode_supportout Unicode and the Outside World

We have seen that it was easy to write Unicode programs using wxWidgets types
and macros, but it has been also mentioned that it isn't quite enough. Although
everything works fine inside the program, things can get nasty when it tries to
communicate with the outside world which, sadly, often expects ANSI strings (a
notable exception is the entire Win32 API which accepts either Unicode or ANSI
strings and which thus makes it unnecessary to ever perform any conversions in
the program). GTK 2.0 only accepts UTF-8 strings.

To get an ANSI string from a wxString, you may use the mb_str() function which
always returns an ANSI string (independently of the mode - while the usual
c_str() returns a pointer to the internal representation which is either ASCII
or Unicode). More rarely used, but still useful, is wc_str() function which
always returns the Unicode string.

Sometimes it is also necessary to go from ANSI strings to wxStrings. In this
case, you can use the converter-constructor, as follows:

@code
const char* ascii_str = "Some text";
wxString str(ascii_str, wxConvUTF8);
@endcode

This code also compiles fine under a non-Unicode build of wxWidgets, but in
that case the converter is ignored.

For more information about converters and Unicode see the @ref overview_mbconv.


@section overview_unicode_settings Unicode Related Compilation Settings

You should define @c wxUSE_UNICODE to 1 to compile your program in Unicode
mode. This currently works for wxMSW, wxGTK, wxMac and wxX11. If you compile
your program in ANSI mode you can still define @c wxUSE_WCHAR_T to get some
limited support for @c wchar_t type.

This will allow your program to perform conversions between Unicode strings and
ANSI ones (using @ref overview_mbconv "wxMBConv") and construct wxString
objects from Unicode strings (presumably read from some external file or
elsewhere).


@section overview_unicode_traps Traps for the Unwary

@li Casting c_str() to void* is now char*, not wxChar*
@li Passing c_str(), mb_str() or wc_str() to variadic functions doesn't work.

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