@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
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 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
+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
-to succeed. And as with @c FromUTF8() example above, you can always use @c
-ToUTF8() to retrieve the string contents in UTF-8 encoding -- this, unlike
-converting to @c char* using the current locale, never fails
+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 transparent for the
+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_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
- 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 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
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
+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:
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 FromUTF8() method and then accessing it as a char
-string via its 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 c_str() would
-return an empty string.
+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
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, but it can have serious consequences for the algorithms
-using indexed access to string elements as they typically acquire O(N^2) time
+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.
-To return to the linear complexity, indexed access should be replaced with
+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
for ( wxString::const_iterator i = s.begin(); i != s.end(); ++i )
{
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 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.
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 ToAscii(), ToUTF8() (or its synonym
-utf8_str()), mb_str(), c_str() and 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. 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 c_str()
-if its result is used as a narrow string. Finally, ToUTF8() and wc_str()
+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 wchar_t string.
+UTF-8 representation of the string or @c wchar_t string.
-wxString also provides two convenience functions: From8BitData() and
-To8BitData(). They can be used to create wxString from arbitrary binary data
-without supposing that it is in current locale encoding, and then get it back,
+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
-From8BitData(). Because of this you should only use From8BitData() for the
-strings created using To8BitData(). Also notice that in spite of the
-availability of these functions, wxString is not the ideal class for storing
+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
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,
-respectively, and so the result of, for example, ToUTF8() can always be passed
-directly to a function taking @c const @c char*. However code such as
+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 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 ToUTF8() returns the pointer to internal string buffer. If
-this inefficiency is important you may write
+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 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
-ToUTF8(). Similarly, wxWX2WCbuf can be used for the return type of wc_str().
+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.
+@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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