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QTextCodec Class Reference


Provides conversion between text encodings. More...

#include <qtextcodec.h>

List of all member functions.

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Detailed Description

Provides conversion between text encodings.

By making objects of subclasses of QTextCodec, support for new text encodings can be added to Qt.

The abstract virtual functions describe the encoder to the system and the coder is used as required in the different text file formats supported QTextStream and, under X11 for the locale-specific character input and output (under Windows NT codecs are not needed for GUI I/O since the system works with Unicode already, and Windows 95/98 has built-in convertors for the 8-bit local encoding).

More recently created QTextCodec objects take precedence over earlier ones.

To add support for another 8-bit encoding to Qt, make a subclass or QTextCodec and implement at least the following methods:

const char* name() const
Return the official name for the encoding.
int mibEnum() const
Return the MIB enum for the encoding if it is listed in the IANA character-sets encoding file.
If the encoding is multi-byte then it will have "state"; that is, the interpretation of some bytes will be dependent on some preceeding bytes. For such an encoding, you will need to implement
QTextDecoder* makeDecoder() const
Return a QTextDecoder that remembers incomplete multibyte sequence prefixes or other required state.
If the encoding does not require state, you should implement:
QString toUnicode(const char* chars, int len) const
Converts len characters from chars to Unicode.
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:

QTextEncoder* makeEncoder() const
Return a QTextDecoder.
QCString fromUnicode(const QString& uc, int& lenInOut ) const;
Converts lenInOut characters (of type QChar) from the start of the string uc, returning a QCString result, and also returning the length of the result in lenInOut.
Again, these are mutually recursive so only one needs to be implemented, or both if better efficiency is possible.

Finally, you must implement:

int heuristicContentMatch(const char* chars, int len) const
Gives a value indicating how likely it is that len chracters from chars are in the encoding.
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:

bool canEncode( QChar ) const
Test if a Unicode character can be encoded.
bool canEncode( const QString& ) const
Test if a string of Unicode characters can be encoded.
int heuristicNameMatch(const char* hint) const
Test if a possibly non-standard name is referring to the codec.

Member Function Documentation

QTextCodec::QTextCodec() [protected]

Constructs a QTextCodec, making it of highest precedence. The QTextCodec should always be constructed on the heap (with new), and once constructed it becomes the responsibility of Qt to delete it (which is done at QApplication destruction).

QTextCodec::~QTextCodec() [virtual]

Destructs the QTextCodec. Note that you should not delete codecs yourself - once created they become the responsibility of Qt to delete.

boolQTextCodec::canEncode(QCharch)const [virtual]

Returns TRUE if the unicode character ch can be fully encoded with this codec. The default implementation tests if the result of toUnicode(fromUnicode(ch)) is the original ch. Subclasses may be able to improve the efficiency.

boolQTextCodec::canEncode(constQString&s)const [virtual]

Returns TRUE if the unicode string s can be fully encoded with this codec. The default implementation tests if the result of toUnicode(fromUnicode(s)) is the original s. Subclasses may be able to improve the efficiency.

QTextCodec*QTextCodec::codecForContent(constchar*chars, intlen) [static]

Searches all installed QTextCodec objects, returning the one which most recognizes the given content. May return 0.

See also: heuristicContentMatch().

QTextCodec*QTextCodec::codecForIndex(inti) [static]

Returns the QTextCodec i places from the more recently inserted, or NULL if there is no such QTextCodec. Thus, codecForIndex(0) returns the most recently created QTextCodec.

QTextCodec*QTextCodec::codecForLocale() [static]

Returns a pointer to the codec most suitable for this locale.

QTextCodec*QTextCodec::codecForMib(intmib) [static]

Returns the QTextCodec which matches the MIBenum mib.

QTextCodec*QTextCodec::codecForName(constchar*hint, intaccuracy=0) [static]

Searches all installed QTextCodec objects, returning the one which best matches given name. Returns NULL if no codec has a match closeness above accuracy.

See also: heuristicNameMatch().

voidQTextCodec::deleteAllCodecs() [static]

Deletes all the created codecs.

Warning: Do not call this function.

QApplication calls this 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 can be helpful to chasing down memory leaks, as QTextCodec objects will not show up.

QCStringQTextCodec::fromUnicode(constQString&uc, int&lenInOut)const [virtual]

Subclasses of QTextCodec must override either this function or makeEncoder(). It converts the first lenInOut characters of uc from Unicode to the encoding of the subclass. If lenInOut is negative or too large, the length of uc is used instead.

The value returned is the property of the caller, which is responsible for deleting it with "delete []". The length of the resulting Unicode character sequence is returned in lenInOut.

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.

QCStringQTextCodec::fromUnicode(constQString&uc)const

This is an overloaded member function, provided for convenience. It differs from the above function only in what argument(s) it accepts.

intQTextCodec::heuristicContentMatch(constchar*chars, intlen)const [virtual]

Subclasses of QTextCodec must override 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. Any negative return value indicates that the text is detectably not in the encoding (eg. it contains undefined characters). 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().

intQTextCodec::heuristicNameMatch(constchar*hint)const [virtual]

Returns a value indicating how likely this decoder is for decoding some format that has the given name.

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.

QTextCodec*QTextCodec::loadCharmap(QIODevice*iod) [static]

Reads a POSIX2 charmap definition from iod. The parser recognises the following lines:

   &lt;code_set_name&gt; <i>name</i>
   &lt;escape_char&gt; <i>character</i>
   % alias <i>alias</i>
   CHARMAP
   &lt;<i>token</i>&gt; /x<i>hexbyte</i> &lt;U<i>unicode</i>&gt; ...
   &lt;<i>token</i>&gt; /d<i>decbyte</i> &lt;U<i>unicode</i>&gt; ...
   &lt;<i>token</i>&gt; /<i>octbyte</i> &lt;U<i>unicode</i>&gt; ...
   &lt;<i>token</i>&gt; /<i>any</i>/<i>any</i>... &lt;U<i>unicode</i>&gt; ...
   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 in memory shared by all applications simultaneously using Qt.

See also: loadCharmapFile().

QTextCodec*QTextCodec::loadCharmapFile(QStringfilename) [static]

A convenience function for QTextCodec::loadCharmap().

constchar*QTextCodec::locale() [static]

Returns a string representing the current language.

QTextDecoder*QTextCodec::makeDecoder()const [virtual]

Creates a QTextDecoder which stores enough state to decode chunks of char* data to create chunks of Unicode data. The default implementation creates a stateless decoder, which is sufficient for only the simplest encodings where each byte corresponds to exactly one Unicode character.

The caller is responsible for deleting the returned object.

QTextEncoder*QTextCodec::makeEncoder()const [virtual]

Creates a QTextEncoder which stores enough state to encode chunks of Unicode data as char* data. The default implementation creates a stateless encoder, which is sufficient for only the simplest encodings where each Unicode character corresponds to exactly one char.

The caller is responsible for deleting the returned object.

intQTextCodec::mibEnum()const [virtual]

Subclasses of QTextCodec must override this function. It returns the MIBenum (see the IANA character-sets encoding file for more information). It is important that each QTextCodec subclass return the correct unique value for this function.

constchar*QTextCodec::name()const [virtual]

Subclasses of QTextCodec must override this function. It returns the name of the encoding supported by the subclass. When choosing a name for an encoding, consider these points:

intQTextCodec::simpleHeuristicNameMatch(constchar*name, constchar*hint) [staticprotected]

A simple utility function for heuristicNameMatch() - it does some very minor character-skipping so that almost-exact matches score high.

QStringQTextCodec::toUnicode(constchar*chars, intlen)const [virtual]

Subclasses of QTextCodec must override this function or makeDecoder(). It converts the first len characters of chars to Unicode.

The default implementation makes an encoder 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.

QStringQTextCodec::toUnicode(constQByteArray&a)const

This is an overloaded member function, provided for convenience. It differs from the above function only in what argument(s) it accepts.

QStringQTextCodec::toUnicode(constQByteArray&a, intlen)const

This is an overloaded member function, provided for convenience. It differs from the above function only in what argument(s) it accepts.

QStringQTextCodec::toUnicode(constchar*chars)const

This is an overloaded member function, provided for convenience. It differs from the above function only in what argument(s) it accepts.


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