              
QTextCodec Class ReferenceThe QTextCodec class provides conversion between text encodings. More... Almost all the functions in this class are reentrant when Qt is built with thread support. The exceptions are ~QTextCodec(), setCodecForTr(), setCodecForCStrings(), and QTextCodec(). #include <qtextcodec.h> Inherited by QBig5Codec, QBig5hkscsCodec, QEucJpCodec, QEucKrCodec, QGb18030Codec, QJisCodec, QHebrewCodec, QSjisCodec, and QTsciiCodec. Public Members
Static Public Members
Protected Members
Static Protected Members
Detailed DescriptionThe QTextCodec class provides conversion between text encodings.Qt uses Unicode to store, draw and manipulate strings. In many situations you may wish to deal with data that uses a different encoding. For example, most Japanese documents are still stored in Shift-JIS or ISO2022, while Russian users often have their documents in KOI8-R or CP1251. Qt provides a set of QTextCodec classes to help with converting non-Unicode formats to and from Unicode. You can also create your own codec classes (see later). The supported encodings are:
QTextCodecs can be used as follows to convert some locally encoded string to Unicode. Suppose you have some string encoded in Russian KOI8-R encoding, and want to convert it to Unicode. The simple way to do this is:
QCString locallyEncoded = "..."; // text to convert
QTextCodec *codec = QTextCodec::codecForName("KOI8-R"); // get the codec for KOI8-R
QString unicodeString = codec->toUnicode( locallyEncoded );
After this, unicodeString holds the text converted to Unicode. Converting a string from Unicode to the local encoding is just as easy:
QString unicodeString = "..."; // any Unicode text
QTextCodec *codec = QTextCodec::codecForName("KOI8-R"); // get the codec for KOI8-R
QCString locallyEncoded = codec->fromUnicode( unicodeString );
Some care must be taken when trying to convert the data in chunks, for example, when receiving it over a network. In such cases it is possible that a multi-byte character will be split over two chunks. At best this might result in the loss of a character and at worst cause the entire conversion to fail. The approach to use in these situations is to create a QTextDecoder object for the codec and use this QTextDecoder for the whole decoding process, as shown below:
QTextCodec *codec = QTextCodec::codecForName( "Shift-JIS" );
QTextDecoder *decoder = codec->makeDecoder();
QString unicodeString;
while( receiving_data ) {
QByteArray chunk = new_data;
unicodeString += decoder->toUnicode( chunk.data(), chunk.length() );
}
The QTextDecoder object maintains state between chunks and therefore works correctly even if a multi-byte character is split between chunks. Support for new text encodings can be added to Qt by creating
QTextCodec subclasses.
Built-in codecs can be overridden by custom codecs since more
recently created QTextCodec objects take precedence over earlier
ones.
You may find it more convenient to make your codec class available
as a plugin; see the plugin
documentation for more details.
The abstract virtual functions describe the encoder to the
system and the coder is used as required in the different
text file formats supported by QTextStream, and under X11, for the
locale-specific character input and output.
To add support for another 8-bit encoding to Qt, make a subclass
of QTextCodec and implement at least the following methods:
If the encoding is multi-byte then it will have "state"; that is,
the interpretation of some bytes will be dependent on some preceding
bytes. For such encodings, you must implement:
If the encoding does not require state, you should implement:
The base QTextCodec class has default implementations of the above
two functions, but they are mutually recursive, so you must
re-implement at least one of them, or both for improved efficiency.
For conversion from Unicode to 8-bit encodings, it is rarely necessary
to maintain state. However, two functions similar to the two above
are used for encoding:
Again, these are mutually recursive so only one needs to be implemented,
or both if greater efficiency is possible.
Finally, you must implement:
A good model for this function is the
QWindowsLocalCodec::heuristicContentMatch function found in the Qt
sources.
A QTextCodec subclass might have improved performance if you also
re-implement:
Codecs can also be created as plugins.
See also Internationalization with Qt.
Warning: This function is not reentrant. Constructs a QTextCodec, and gives it the highest precedence. The
QTextCodec should always be constructed on the heap (i.e. with new). Qt takes ownership and will delete it when the application
terminates.
Warning: This function is not reentrant. Destroys the QTextCodec. Note that you should not delete codecs
yourself: once created they become Qt's responsibility.
s contains the string being tested for encode-ability.
Returns the codec used by QString to convert to and from const
char* and QCStrings. If this function returns 0 (the default),
QString assumes Latin-1.
See also setCodecForCStrings().
Note that this is often a poor choice, since character encodings
often use most of the available character sequences, and so only
by linguistic analysis could a true match be made.
chars contains the string to check, and len contains the
number of characters in the string to use.
See also heuristicContentMatch().
Example: qwerty/qwerty.cpp.
Example: qwerty/qwerty.cpp.
Example: qwerty/qwerty.cpp.
See also heuristicNameMatch().
Returns the codec used by QObject::tr() on its argument. If this
function returns 0 (the default), tr() assumes Latin-1.
See also setCodecForTr().
Warning: Do not call this function.
QApplication calls this function just before exiting to delete
any QTextCodec objects that may be lying around. Since various
other classes hold pointers to QTextCodec objects, it is not safe
to call this function earlier.
If you are using the utility classes (like QString) but not using
QApplication, calling this function at the very end of your
application may be helpful for chasing down memory leaks by
eliminating any QTextCodec objects.
Converts lenInOut characters (not bytes) from uc, producing
a QCString. lenInOut will be set to the length of the result (in bytes).
The default implementation makes an encoder with makeEncoder() and
converts the input with that. Note that the default makeEncoder()
implementation makes an encoder that simply calls this function,
hence subclasses must reimplement one function or the other to
avoid infinite recursion.
Reimplemented in QHebrewCodec.
uc is the unicode source string.
QTextCodec subclasses must reimplement this function. It examines
the first len bytes of chars and returns a value indicating
how likely it is that the string is a prefix of text encoded in
the encoding of the subclass. A negative return value indicates
that the text is detectably not in the encoding (e.g. it contains
characters undefined in the encoding). A return value of 0
indicates that the text should be decoded with this codec rather
than as ASCII, but there is no particular evidence. The value
should range up to len. Thus, most decoders will return -1, 0,
or -len.
The characters are not null terminated.
See also codecForContent().
A good match returns a positive number around the length of the
string. A bad match is negative.
The default implementation calls simpleHeuristicNameMatch() with
the name of the codec.
<code_set_name> name
<escape_char> character
% alias alias
CHARMAP
<token> /xhexbyte <Uunicode> ...
<token> /ddecbyte <Uunicode> ...
<token> /octbyte <Uunicode> ...
<token> /any/any... <Uunicode> ...
END CHARMAP
The resulting QTextCodec is returned (and also added to the global
list of codecs). The name() of the result is taken from the
code_set_name.
Note that a codec constructed in this way uses much more memory
and is slower than a hand-written QTextCodec subclass, since
tables in code are kept in memory shared by all Qt applications.
See also loadCharmapFile().
Example: qwerty/qwerty.cpp.
Example: i18n/main.cpp.
The caller is responsible for deleting the returned object.
The caller is responsible for deleting the returned object.
Subclasses of QTextCodec must reimplement this function. It
returns the MIBenum (see the
IANA character-sets encoding file for more information).
It is important that each QTextCodec subclass returns the correct
unique value for this function.
Reimplemented in QEucJpCodec.
Reimplemented in QEucJpCodec, QEucKrCodec, QJisCodec, QHebrewCodec, and QSjisCodec.
QTextCodec subclasses must reimplement this function. It returns
the name of the encoding supported by the subclass. When choosing
a name for an encoding, consider these points:
Example: qwerty/qwerty.cpp.
Warning: This function is not reentrant. Sets the codec used by QString to convert to and from const char*
and QCStrings. If c is 0 (the default), QString assumes Latin-1.
Warning: Some codecs do not preserve the characters in the ascii
range (0x00 to 0x7f). For example, the Japanese Shift-JIS
encoding maps the backslash character (0x5a) to the Yen character.
This leads to unexpected results when using the backslash
character to escape characters in strings used in e.g. regular
expressions. Use QString::fromLatin1() to preserve characters in
the ascii range when needed.
See also codecForCStrings() and setCodecForTr().
See also codecForLocale().
Warning: This function is not reentrant. Sets the codec used by QObject::tr() on its argument to c. If
c is 0 (the default), tr() assumes Latin-1.
If the literal quoted text in the program is not in the Latin-1
encoding, this function can be used to set the appropriate
encoding. For example, software developed by Korean programmers
might use eucKR for all the text in the program, in which case the
main() function might look like this:
Note that this is not the way to select the encoding that the user has chosen. For example, to convert an application containing
literal English strings to Korean, all that is needed is for the
English strings to be passed through tr() and for translation
files to be loaded. For details of internationalization, see the
Qt internationalization documentation.
See also codecForTr() and setCodecForCStrings().
The default implementation makes a decoder with makeDecoder() and
converts the input with that. Note that the default makeDecoder()
implementation makes a decoder that simply calls
this function, hence subclasses must reimplement one function or
the other to avoid infinite recursion.
a contains the source characters; len contains the number of
characters in a to use.
a contains the source characters.
a contains the source characters; len contains the number of
characters in a to use.
a contains the source characters.
chars contains the source characters.
This file is part of the Qt toolkit.
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