// Purpose: topic overview
// Author: wxWidgets team
// RCS-ID: $Id$
-// Licence: wxWindows license
+// Licence: wxWindows licence
/////////////////////////////////////////////////////////////////////////////
/**
@li @ref overview_unicode_supportin
@li @ref overview_unicode_pitfalls
@li @ref overview_unicode_supportout
-@li @ref overview_unicode_settings
<hr>
@section overview_unicode_what What is Unicode?
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
-(in 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/.
+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
other services should be ready to deal with Unicode.
-@section overview_unicode_encodings Unicode Representations
-
-Unicode provides a unique code to identify every character, however in practice
-these codes are not always used directly but encoded using one of the standard
-UTF or Unicode Transformation Formats which are algorithms mapping the Unicode
-codes to byte code sequences. The simplest of them is UTF-32 which simply maps
-the Unicode code to a 4 byte sequence representing this 32 bit number (although
-this 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
-UTF-16 which is used by Microsoft Windows: it encodes the first (approximately)
-64 thousands of Unicode characters using only 2 bytes and uses a pair of 16-bit
-codes to encode the characters beyond this. Finally, the most widespread
-encoding used for the external Unicode storage (e.g. files and network
-protocols) is UTF-8 which is byte-oriented and so avoids the endianness
-ambiguities of UTF-16 and UTF-32. However UTF-8 uses a variable number of bytes
-for representing Unicode characters which makes it less efficient than UTF-32
-for internal representation.
-
-From the C/C++ programmer perspective the situation is further complicated by
-the fact that the standard type @c wchar_t which is used to represent the
+@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 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.
+@subsection overview_unicode_support_default Unicode is Always Used by Default
-However, unlike Unicode build mode in the previous versions of wxWidgets, this
-support is mostly transparent: you can still continue to work with the narrow
-(i.e. @c char*) strings even if wide (i.e. @c wchar_t*) strings are also
+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 somewhat less usual
+and the somewhat less usual
@code
-wxMessageBox(L"Salut \u00e0 toi!"); // 00E0 is "Latin Small Letter a with Grave"
+wxMessageBox(L"Salut \u00E0 toi!"); // U+00E0 is "Latin Small Letter a with Grave"
@endcode
work as expected.
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 never use 8 bit characters directly in the program source but use wide
-strings or, alternatively, write
+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!"));
+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 wchar_t or
-char character buffer. Of course, the latter only works if the string contains
+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
wxString::ToUTF8() to retrieve the string contents in UTF-8 encoding -- this,
unlike converting to @c char* using the current locale, never fails.
-To summarize, Unicode support in wxWidgets is mostly transparent for the
+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
+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
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
+ 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 char or wchar_t by using an explicit cast but beware that
+ 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
- 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
+ - 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.
avoiding implicit conversions to @c char*.
-@subsection overview_unicode_performance Unicode Performance Implications
+@subsection overview_unicode_performance Performance Implications of Using UTF-8
-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.
+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:
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 wchar_t pointers rather than char ones to avoid the
+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.
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
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 wchar_t string.
+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
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 char and wchar_t pointers,
+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 @c const @c char*. However code such as
+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 may use
-@code
-wxCharBuffer p(s.ToUTF8());
-puts(p);
-@endcode
-which does work but results in an unnecessary copy of string data in the build
-configurations when wxString::ToUTF8() returns the pointer to internal string buffer.
-If this inefficiency is important you may write
+destroyed and @c p is left pointing nowhere. To correct this you should use
@code
-const wxUTF8Buf p(s.ToUTF8());
+const wxScopedCharBuffer p(s.ToUTF8());
puts(p);
@endcode
-where @c wxUTF8Buf is the type corresponding to the real return type of wxString::ToUTF8().
+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.
-@section overview_unicode_settings Unicode Related Compilation Settings
-
-@c wxUSE_UNICODE is now defined as 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.
-
*/
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