+@tableofcontents
+
+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.
+
+
+
+@section overview_unicode_what What is Unicode?
+
+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
+
+@subsection overview_unicode_support_default Unicode is Always Used by Default
+
+Since wxWidgets 3.0 Unicode support is always enabled and while building the
+library without it is still possible, 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.
+
+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.
+
+
+@subsection overview_unicode_support_utf Choosing Unicode Representation
+
+wxWidgets uses the system @c wchar_t in wxString implementation by default
+under all systems. Thus, under Microsoft Windows, UCS-2 (simplified version of
+UTF-16 without support for surrogate characters) is used as @c wchar_t is 2
+bytes on this platform. Under Unix systems, including Mac OS X, UCS-4 (also
+known as UTF-32) is used by default, however it is also possible to build
+wxWidgets to use UTF-8 internally by passing @c --enable-utf8 option to
+configure.
+
+The interface provided by wxString is the same independently of the format used
+internally. However different formats have specific advantages and
+disadvantages. Notably, under Unix, the underlying graphical toolkit (e.g.
+GTK+) usually uses UTF-8 encoded strings and using the same representations for
+the strings in wxWidgets allows to avoid conversion from UTF-32 to UTF-8 and
+vice versa each time a string is shown in the UI or retrieved from it. The
+overhead of such conversions is usually negligible for small strings but may be
+important for some programs. If you believe that it would be advantageous to
+use UTF-8 for the strings in your particular application, you may rebuild
+wxWidgets to use UTF-8 as explained above (notice that this is currently not
+supported under Microsoft Windows and arguably doesn't make much sense there as
+Windows itself uses UTF-16 and not UTF-8) but be sure to be aware of the
+performance implications (see @ref overview_unicode_performance) of using UTF-8
+in wxString before doing this!
+
+Generally speaking you should only use non-default UTF-8 build in specific
+circumstances e.g. building for resource-constrained systems where the overhead
+of conversions (and also reduced memory usage of UTF-8 compared to UTF-32 for
+the European languages) can be important. If the environment in which your
+program is running is under your control -- as is quite often the case in such
+scenarios -- consider ensuring that the system always uses UTF-8 locale and
+use @c --enable-utf8only configure option to disable support for the other
+locales and consider all strings to be in UTF-8. This further reduces the code
+size and removes the need for conversions in more cases.
+
+
+@subsection overview_unicode_settings Unicode Related Preprocessor Symbols
+
+@c wxUSE_UNICODE is defined as 1 now to indicate Unicode support. It can be
+explicitly set to 0 in @c setup.h under MSW or you can use @c --disable-unicode
+under Unix but doing this is strongly discouraged. By default, @c
+wxUSE_UNICODE_WCHAR is also defined as 1, however in UTF-8 build (described in
+the previous section), it is set to 0 and @c wxUSE_UNICODE_UTF8, which is
+usually 0, is set to 1 instead. In the latter case, @c wxUSE_UTF8_LOCALE_ONLY
+can also be set to 1 to indicate that all strings are considered to be in UTF-8.
+
+
+
+@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 be 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() cannot 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 Performance Implications of Using UTF-8
+
+As mentioned above, under Unix systems wxString class can use variable-width
+UTF-8 encoding for internal representation. In this case 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("hello");
+for ( size_t i = 0; i < s.length(); i++ )
+{
+ wchar_t ch = s[i];
+
+ // do something with it
+}
+@endcode
+should be rewritten as
+@code
+wxString s("hello");
+for ( wxString::const_iterator i = s.begin(); i != s.end(); ++i )
+{
+ wchar_t ch = *i
+
+ // do something with it
+}
+@endcode
+
+Another, similar, alternative is to use pointer arithmetic:
+@code
+wxString s("hello");
+for ( const wchar_t *p = s.wc_str(); *p; p++ )
+{
+ 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.
+
+
+@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
+const char *p = s.ToUTF8();
+...
+puts(p); // or call any other function taking const char *
+@endcode
+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
+const wxScopedCharBuffer p(s.ToUTF8());
+puts(p);
+@endcode
+which does work.
+
+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.
+
+*/
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