@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.
+This section describes how does wxWidgets support Unicode and how can it affect
+your programs.
+Notice that Unicode support has changed radically in wxWidgets 3.0 and a lot of
+existing material pertaining to the previous versions of the library is not
+correct any more. Please see @ref overview_changes_unicode for the details of
+these changes.
+
+You can skip the first two sections if you're already familiar with Unicode and
+wish to jump directly in the details of its support in the library:
@li @ref overview_unicode_what
-@li @ref overview_unicode_ansi
+@li @ref overview_unicode_encodings
@li @ref overview_unicode_supportin
+@li @ref overview_unicode_pitfalls
@li @ref overview_unicode_supportout
@li @ref overview_unicode_settings
@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. A different approach is to encode all
-strings in UTF8 which does not require the use of wide characters and
-additionally is backwards compatible with 7-bit ASCII. The solution to
-use UTF8 is prefered under Linux and partially OS X.
-
-More details about Unicode may be found at <http://www.unicode.org/>.
-
-Writing internationalized programs is much easier with Unicode. Moreover
-even a program which uses only standard ASCII can benefit from using Unicode
-for string representation because there will be no need 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
-
-Until wxWidgets 3.0 it was possible to compile the library both in
-ANSI (=8-bit) mode as well as in wide char mode (16-bit per character
-on Windows and 32-but on most Unix versions, Linux and OS X). This
-has been changed in wxWidget with the removal of the ANSI mode,
-but much effort has been made so that most of the previous ANSI
-code should still compile and work as before.
+Unicode is a standard for character encoding which addresses the shortcomings
+of the previous standards (e.g. the ASCII standard), by using 8, 16 or 32 bits
+for encoding each character.
+This allows enough code points (see below for the definition) sufficient to
+encode all of the world languages at once.
+More details about Unicode may be found at http://www.unicode.org/.
+
+From a practical point of view, using Unicode is almost a requirement when
+writing applications for international audience. Moreover, any application
+reading files which it didn't produce or receiving data from the network from
+other services should be ready to deal with Unicode.
+
+
+@section overview_unicode_encodings Unicode Representations and Terminology
+
+When working with Unicode, it's important to define the meaning of some terms.
+
+A <b><em>glyph</em></b> is a particular image (usually part of a font) that
+represents a character or part of a character.
+Any character may have one or more glyph associated; e.g. some of the possible
+glyphs for the capital letter 'A' are:
+
+@image html overview_unicode_glyphs.png
+
+Unicode assigns each character of almost any existing alphabet/script a number,
+which is called <b><em>code point</em></b>; it's typically indicated in documentation
+manuals and in the Unicode website as @c U+xxxx where @c xxxx is an hexadecimal number.
+
+Note that typically one character is assigned exactly one code point, but there
+are exceptions; the so-called <em>precomposed characters</em>
+(see http://en.wikipedia.org/wiki/Precomposed_character) or the <em>ligatures</em>.
+In these cases a single "character" may be mapped to more than one code point or
+viceversa more characters may be mapped to a single code point.
+
+The Unicode standard divides the space of all possible code points in <b><em>planes</em></b>;
+a plane is a range of 65,536 (1000016) contiguous Unicode code points.
+Planes are numbered from 0 to 16, where the first one is the @e BMP, or Basic
+Multilingual Plane.
+The BMP contains characters for all modern languages, and a large number of
+special characters. The other planes in fact contain mainly historic scripts,
+special-purpose characters or are unused.
+
+Code points are represented in computer memory as a sequence of one or more
+<b><em>code units</em></b>, where a code unit is a unit of memory: 8, 16, or 32 bits.
+More precisely, a code unit is the minimal bit combination that can represent a
+unit of encoded text for processing or interchange.
+
+The <b><em>UTF</em></b> or Unicode Transformation Formats are algorithms mapping the Unicode
+code points to code unit sequences. The simplest of them is <b>UTF-32</b> where
+each code unit is composed by 32 bits (4 bytes) and each code point is always
+represented by a single code unit (fixed length encoding).
+(Note that even UTF-32 is still not completely trivial as the mapping is different
+for little and big-endian architectures). UTF-32 is commonly used under Unix systems for
+internal representation of Unicode strings.
+
+Another very widespread standard is <b>UTF-16</b> which is used by Microsoft Windows:
+it encodes the first (approximately) 64 thousands of Unicode code points
+(the BMP plane) using 16-bit code units (2 bytes) and uses a pair of 16-bit code
+units to encode the characters beyond this. These pairs are called @e surrogate.
+Thus UTF16 uses a variable number of code units to encode each code point.
+
+Finally, the most widespread encoding used for the external Unicode storage
+(e.g. files and network protocols) is <b>UTF-8</b> which is byte-oriented and so
+avoids the endianness ambiguities of UTF-16 and UTF-32.
+UTF-8 uses code units of 8 bits (1 byte); code points beyond the usual english
+alphabet are represented using a variable number of bytes, which makes it less
+efficient than UTF-32 for internal representation.
+
+As visual aid to understand the differences between the various concepts described
+so far, look at the different UTF representations of the same code point:
+
+@image html overview_unicode_codes.png
+
+In this particular case UTF8 requires more space than UTF16 (3 bytes instead of 2).
+
+Note that from the C/C++ programmer perspective the situation is further complicated
+by the fact that the standard type @c wchar_t which is usually used to represent the
+Unicode ("wide") strings in C/C++ doesn't have the same size on all platforms.
+It is 4 bytes under Unix systems, corresponding to the tradition of using
+UTF-32, but only 2 bytes under Windows which is required by compatibility with
+the OS which uses UTF-16.
+
+Typically when UTF8 is used, code units are stored into @c char types, since
+@c char are 8bit wide on almost all systems; when using UTF16 typically code
+units are stored into @c wchar_t types since @c wchar_t is at least 16bits on
+all systems. This is also the approach used by wxString.
+See @ref overview_string for more info.
+
+See also http://unicode.org/glossary/ for the official definitions of the
+terms reported above.
-@section overview_unicode_supportin Unicode Support in wxWidgets
-Since wxWidgets 3.0 Unicode support is always enabled meaning
-that the wxString class always uses Unicode to encode its content.
-Under Windows wxString uses UCS-2 (basically an array of 16-bit
-wchar_t). Under Unix, Linux and OS X however, wxString uses UTF8
-to encode its content.
+@section overview_unicode_supportin Unicode Support in wxWidgets
-For the programmer, the biggest change is that iterating over
-a string can be slower than before since wxString has to parse
-the entire string in order to find the n-th character in a
-string, meaning that iterating over a string should no longer
-be done by index but using iterators. Old code will still work
-but might be less efficient.
+Since wxWidgets 3.0 Unicode support is always enabled and building the library
+without it is not recommended any longer and will cease to be supported in the
+near future. This means that internally only Unicode strings are used and that,
+under Microsoft Windows, Unicode system API is used which means that wxWidgets
+programs require the Microsoft Layer for Unicode to run on Windows 95/98/ME.
+
+However, unlike the Unicode build mode of the previous versions of wxWidgets, this
+support is mostly transparent: you can still continue to work with the @b narrow
+(i.e. current locale-encoded @c char*) strings even if @b wide
+(i.e. UTF16-encoded @c wchar_t* or UTF8-encoded @c char*) strings are also
+supported. Any wxWidgets function accepts arguments of either type as both
+kinds of strings are implicitly converted to wxString, so both
+@code
+wxMessageBox("Hello, world!");
+@endcode
+and the somewhat less usual
+@code
+wxMessageBox(L"Salut \u00E0 toi!"); // U+00E0 is "Latin Small Letter a with Grave"
+@endcode
+work as expected.
-Old code like this:
+Notice that the narrow strings used with wxWidgets are @e always assumed to be
+in the current locale encoding, so writing
+@code
+wxMessageBox("Salut à toi!");
+@endcode
+wouldn't work if the encoding used on the user system is incompatible with
+ISO-8859-1 (or even if the sources were compiled under different locale
+in the case of gcc). In particular, the most common encoding used under
+modern Unix systems is UTF-8 and as the string above is not a valid UTF-8 byte
+sequence, nothing would be displayed at all in this case. Thus it is important
+to <b>never use 8-bit (instead of 7-bit) characters directly in the program source</b>
+but use wide strings or, alternatively, write:
+@code
+wxMessageBox(wxString::FromUTF8("Salut \xC3\xA0 toi!"));
+ // in UTF8 the character U+00E0 is encoded as 0xC3A0
+@endcode
+In a similar way, wxString provides access to its contents as either @c wchar_t or
+@c char character buffer. Of course, the latter only works if the string contains
+data representable in the current locale encoding. This will always be the case
+if the string had been initially constructed from a narrow string or if it
+contains only 7-bit ASCII data but otherwise this conversion is not guaranteed
+to succeed. And as with wxString::FromUTF8() example above, you can always use
+wxString::ToUTF8() to retrieve the string contents in UTF-8 encoding -- this,
+unlike converting to @c char* using the current locale, never fails.
+
+For more info about how wxString works, please see the @ref overview_string.
+
+To summarize, Unicode support in wxWidgets is mostly @b transparent for the
+application and if you use wxString objects for storing all the character data
+in your program there is really nothing special to do. However you should be
+aware of the potential problems covered by the following section.
+
+
+@section overview_unicode_pitfalls Potential Unicode Pitfalls
+
+The problems can be separated into three broad classes:
+
+@subsection overview_unicode_compilation_errors Unicode-Related Compilation Errors
+
+Because of the need to support implicit conversions to both @c char and
+@c wchar_t, wxString implementation is rather involved and many of its operators
+don't return the types which they could be naively expected to return.
+For example, the @c operator[] doesn't return neither a @c char nor a @c wchar_t
+but an object of a helper class wxUniChar or wxUniCharRef which is implicitly
+convertible to either. Usually you don't need to worry about this as the
+conversions do their work behind the scenes however in some cases it doesn't
+work. Here are some examples, using a wxString object @c s and some integer @c
+n:
+
+ - Writing @code switch ( s[n] ) @endcode doesn't work because the argument of
+ the switch statement must an integer expression so you need to replace
+ @c s[n] with @code s[n].GetValue() @endcode. You may also force the
+ conversion to @c char or @c wchar_t by using an explicit cast but beware that
+ converting the value to char uses the conversion to current locale and may
+ return 0 if it fails. Finally notice that writing @code (wxChar)s[n] @endcode
+ works both with wxWidgets 3.0 and previous library versions and so should be
+ used for writing code which should be compatible with both 2.8 and 3.0.
+
+ - Similarly, @code &s[n] @endcode doesn't yield a pointer to char so you may
+ not pass it to functions expecting @c char* or @c wchar_t*. Consider using
+ string iterators instead if possible or replace this expression with
+ @code s.c_str() + n @endcode otherwise.
+
+Another class of problems is related to the fact that the value returned by
+@c c_str() itself is also not just a pointer to a buffer but a value of helper
+class wxCStrData which is implicitly convertible to both narrow and wide
+strings. Again, this mostly will be unnoticeable but can result in some
+problems:
+
+ - You shouldn't pass @c c_str() result to vararg functions such as standard
+ @c printf(). Some compilers (notably g++) warn about this but even if they
+ don't, this @code printf("Hello, %s", s.c_str()) @endcode is not going to
+ work. It can be corrected in one of the following ways:
+
+ - Preferred: @code wxPrintf("Hello, %s", s) @endcode (notice the absence
+ of @c c_str(), it is not needed at all with wxWidgets functions)
+ - Compatible with wxWidgets 2.8: @code wxPrintf("Hello, %s", s.c_str()) @endcode
+ - Using an explicit conversion to narrow, multibyte, string:
+ @code printf("Hello, %s", (const char *)s.mb_str()) @endcode
+ - Using a cast to force the issue (listed only for completeness):
+ @code printf("Hello, %s", (const char *)s.c_str()) @endcode
+
+ - The result of @c c_str() can not be cast to @c char* but only to @c const @c
+ @c char*. Of course, modifying the string via the pointer returned by this
+ method has never been possible but unfortunately it was occasionally useful
+ to use a @c const_cast here to pass the value to const-incorrect functions.
+ This can be done either using new wxString::char_str() (and matching
+ wchar_str()) method or by writing a double cast:
+ @code (char *)(const char *)s.c_str() @endcode
+
+ - One of the unfortunate consequences of the possibility to pass wxString to
+ @c wxPrintf() without using @c c_str() is that it is now impossible to pass
+ the elements of unnamed enumerations to @c wxPrintf() and other similar
+ vararg functions, i.e.
+ @code
+ enum { Red, Green, Blue };
+ wxPrintf("Red is %d", Red);
+ @endcode
+ doesn't compile. The easiest workaround is to give a name to the enum.
+
+Other unexpected compilation errors may arise but they should happen even more
+rarely than the above-mentioned ones and the solution should usually be quite
+simple: just use the explicit methods of wxUniChar and wxCStrData classes
+instead of relying on their implicit conversions if the compiler can't choose
+among them.
+
+
+@subsection overview_unicode_data_loss Data Loss due To Unicode Conversion Errors
+
+wxString API provides implicit conversion of the internal Unicode string
+contents to narrow, char strings. This can be very convenient and is absolutely
+necessary for backwards compatibility with the existing code using wxWidgets
+however it is a rather dangerous operation as it can easily give unexpected
+results if the string contents isn't convertible to the current locale.
+
+To be precise, the conversion will always succeed if the string was created
+from a narrow string initially. It will also succeed if the current encoding is
+UTF-8 as all Unicode strings are representable in this encoding. However
+initializing the string using wxString::FromUTF8() method and then accessing it
+as a char string via its wxString::c_str() method is a recipe for disaster as the
+program may work perfectly well during testing on Unix systems using UTF-8 locale
+but completely fail under Windows where UTF-8 locales are never used because
+wxString::c_str() would return an empty string.
+
+The simplest way to ensure that this doesn't happen is to avoid conversions to
+@c char* completely by using wxString throughout your program. However if the
+program never manipulates 8 bit strings internally, using @c char* pointers is
+safe as well. So the existing code needs to be reviewed when upgrading to
+wxWidgets 3.0 and the new code should be used with this in mind and ideally
+avoiding implicit conversions to @c char*.
+
+
+@subsection overview_unicode_performance Unicode Performance Implications
+
+Under Unix systems wxString class uses variable-width UTF-8 encoding for
+internal representation and this implies that it can't guarantee constant-time
+access to N-th element of the string any longer as to find the position of this
+character in the string we have to examine all the preceding ones. Usually this
+doesn't matter much because most algorithms used on the strings examine them
+sequentially anyhow and because wxString implements a cache for iterating over
+the string by index but it can have serious consequences for algorithms
+using random access to string elements as they typically acquire O(N^2) time
+complexity instead of O(N) where N is the length of the string.
+
+Even despite caching the index, indexed access should be replaced with
+sequential access using string iterators. For example a typical loop:
@code
-wxString s = wxT("hello");
-size_t i;
-for (i = 0; i < s.Len(); i++)
+wxString s("hello");
+for ( size_t i = 0; i < s.length(); i++ )
{
- wxChar ch = s[i];
-
+ wchar_t ch = s[i];
+
// do something with it
}
@endcode
-
-should be replaced (especially in time critical places) with:
-
+should be rewritten as
@code
-wxString s = "hello";
-wxString::iterator i;
-for (i = s.begin(); i != s.end(); ++i)
+wxString s("hello");
+for ( wxString::const_iterator i = s.begin(); i != s.end(); ++i )
{
- wxUniChar uni_ch = *i;
- wxChar ch = uni_ch;
- // same as: wxChar ch = *i
-
+ wchar_t ch = *i
+
// do something with it
}
@endcode
-If you want to replace individual characters in the string you
-need to get a reference to that character:
-
+Another, similar, alternative is to use pointer arithmetic:
@code
-wxString s = "hello";
-wxString::iterator i;
-for (i = s.begin(); i != s.end(); ++i)
+wxString s("hello");
+for ( const wchar_t *p = s.wc_str(); *p; p++ )
{
- wxUniCharRef ch = *i;
- ch = 'a';
- // same as: *i = 'a';
+ wchar_t ch = *i
+
+ // do something with it
}
@endcode
+however this doesn't work correctly for strings with embedded @c NUL characters
+and the use of iterators is generally preferred as they provide some run-time
+checks (at least in debug build) unlike the raw pointers. But if you do use
+them, it is better to use @c wchar_t pointers rather than @c char ones to avoid the
+data loss problems due to conversion as discussed in the previous section.
-which will change the content of the wxString s from "hello" to "aaaaa".
-String literals are translated to Unicode when they are assigned to
-a wxString object so code can be written like this:
+@section overview_unicode_supportout Unicode and the Outside World
+Even though wxWidgets always uses Unicode internally, not all the other
+libraries and programs do and even those that do use Unicode may use a
+different encoding of it. So you need to be able to convert the data to various
+representations and the wxString methods wxString::ToAscii(), wxString::ToUTF8()
+(or its synonym wxString::utf8_str()), wxString::mb_str(), wxString::c_str() and
+wxString::wc_str() can be used for this.
+
+The first of them should be only used for the string containing 7-bit ASCII characters
+only, anything else will be replaced by some substitution character.
+wxString::mb_str() converts the string to the encoding used by the current locale
+and so can return an empty string if the string contains characters not representable in
+it as explained in @ref overview_unicode_data_loss. The same applies to wxString::c_str()
+if its result is used as a narrow string. Finally, wxString::ToUTF8() and wxString::wc_str()
+functions never fail and always return a pointer to char string containing the
+UTF-8 representation of the string or @c wchar_t string.
+
+wxString also provides two convenience functions: wxString::From8BitData() and
+wxString::To8BitData(). They can be used to create a wxString from arbitrary binary
+data without supposing that it is in current locale encoding, and then get it back,
+again, without any conversion or, rather, undoing the conversion used by
+wxString::From8BitData(). Because of this you should only use wxString::From8BitData()
+for the strings created using wxString::To8BitData(). Also notice that in spite
+of the availability of these functions, wxString is not the ideal class for storing
+arbitrary binary data as they can take up to 4 times more space than needed
+(when using @c wchar_t internal representation on the systems where size of
+wide characters is 4 bytes) and you should consider using wxMemoryBuffer
+instead.
+
+Final word of caution: most of these functions may return either directly the
+pointer to internal string buffer or a temporary wxCharBuffer or wxWCharBuffer
+object. Such objects are implicitly convertible to @c char and @c wchar_t pointers,
+respectively, and so the result of, for example, wxString::ToUTF8() can always be
+passed directly to a function taking <tt>const char*</tt>. However code such as
@code
-wxString s = "Hello, world!";
-int len = s.Len();
+const char *p = s.ToUTF8();
+...
+puts(p); // or call any other function taking const char *
@endcode
-
-wxWidgets provides wrappers around most Posix C functions (like printf(..))
-and the syntax has been adapted to support input with wxString, normal
-C-style strings and wchar_t strings:
-
+does @b not work because the temporary buffer returned by wxString::ToUTF8() is
+destroyed and @c p is left pointing nowhere. To correct this you should use
@code
-wxString s;
-s.Printf( "%s %s %s", "hello1", L"hello2", wxString("hello3") );
-wxPrintf( "Three times hello %s\n", s );
+const wxScopedCharBuffer p(s.ToUTF8());
+puts(p);
@endcode
+which does work.
-@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
-
-For more information about converters and Unicode see the @ref overview_mbconv.
-
+Similarly, wxWX2WCbuf can be used for the return type of wxString::wc_str().
+But, once again, none of these cryptic types is really needed if you just pass
+the return value of any of the functions mentioned in this section to another
+function directly.
@section overview_unicode_settings Unicode Related Compilation Settings
-You should define @c wxUSE_UNICODE to 1 to compile your program in Unicode
-mode. Since wxWidgets 3.0 this is always the case. When compiled in UTF8
-mode @c wxUSE_UNICODE_UTF8 is also defined.
+@c wxUSE_UNICODE is now defined as @c 1 by default to indicate Unicode support.
+If UTF-8 is used for the internal storage in wxString, @c wxUSE_UNICODE_UTF8 is
+also defined, otherwise @c wxUSE_UNICODE_WCHAR is.
+
+You are encouraged to always use the default build settings of wxWidgets; this avoids
+the need of different builds of the same application/library because of different
+"build modes".
*/
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