QTextCodec Class Reference

The QTextCodec class provides conversions between text encodings. More...

 #include <QTextCodec>

Note: All functions in this class are reentrant, except for setCodecForTr(), setCodecForCStrings(), and ~QTextCodec(), which are nonreentrant.

Public Types

class ConverterState
enum ConversionFlag { DefaultConversion, ConvertInvalidToNull, IgnoreHeader }
flags ConversionFlags

Public Functions

virtual QList<QByteArray> aliases () const
bool canEncode ( QChar ch ) const
bool canEncode ( const QString & s ) const
QByteArray fromUnicode ( const QString & str ) const
QByteArray fromUnicode ( const QChar * input, int number, ConverterState * state = 0 ) const
QTextDecoder * makeDecoder () const
QTextDecoder * makeDecoder ( ConversionFlags flags ) const
QTextEncoder * makeEncoder () const
QTextEncoder * makeEncoder ( ConversionFlags flags ) const
virtual int mibEnum () const = 0
virtual QByteArray name () const = 0
QString toUnicode ( const QByteArray & a ) const
QString toUnicode ( const char * input, int size, ConverterState * state = 0 ) const
QString toUnicode ( const char * chars ) const

Static Public Members

QList<QByteArray> availableCodecs ()
QList<int> availableMibs ()
QTextCodec * codecForCStrings ()
QTextCodec * codecForHtml ( const QByteArray & ba, QTextCodec * defaultCodec )
QTextCodec * codecForHtml ( const QByteArray & ba )
QTextCodec * codecForLocale ()
QTextCodec * codecForMib ( int mib )
QTextCodec * codecForName ( const QByteArray & name )
QTextCodec * codecForName ( const char * name )
QTextCodec * codecForTr ()
QTextCodec * codecForUtfText ( const QByteArray & ba, QTextCodec * defaultCodec )
QTextCodec * codecForUtfText ( const QByteArray & ba )
void setCodecForCStrings ( QTextCodec * codec )
void setCodecForLocale ( QTextCodec * c )
void setCodecForTr ( QTextCodec * c )

Protected Functions

QTextCodec ()
virtual ~QTextCodec ()
virtual QByteArray convertFromUnicode ( const QChar * input, int number, ConverterState * state ) const = 0
virtual QString convertToUnicode ( const char * chars, int len, ConverterState * state ) const = 0

Detailed Description

The QTextCodec class provides conversions 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 ISO 2022-JP, while Russian users often have their documents in KOI8-R or Windows-1251.

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.

The supported encodings are:

  • Apple Roman
  • Big5
  • Big5-HKSCS
  • CP949
  • EUC-JP
  • EUC-KR
  • GB18030-0
  • IBM 850
  • IBM 866
  • IBM 874
  • ISO 2022-JP
  • ISO 8859-1 to 10
  • ISO 8859-13 to 16
  • Iscii-Bng, Dev, Gjr, Knd, Mlm, Ori, Pnj, Tlg, and Tml
  • JIS X 0201
  • JIS X 0208
  • KOI8-R
  • KOI8-U
  • MuleLao-1
  • ROMAN8
  • Shift-JIS
  • TIS-620
  • TSCII
  • UTF-8
  • UTF-16
  • UTF-16BE
  • UTF-16LE
  • UTF-32
  • UTF-32BE
  • UTF-32LE
  • Windows-1250 to 1258
  • WINSAMI2

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 it is like this:

 QByteArray encodedString = "...";
 QTextCodec *codec = QTextCodec::codecForName("KOI8-R");
 QString string = codec->toUnicode(encodedString);

After this, string holds the text converted to Unicode. Converting a string from Unicode to the local encoding is just as easy:

 QString string = "...";
 QTextCodec *codec = QTextCodec::codecForName("KOI8-R");
 QByteArray encodedString = codec->fromUnicode(string);

To read or write files in various encodings, use QTextStream and its setCodec() function. See the Codecs example for an application of QTextCodec to file I/O.

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 string;
 while (new_data_available()) {
     QByteArray chunk = get_new_data();
     string += decoder->toUnicode(chunk);
 }
 delete decoder;

The QTextDecoder object maintains state between chunks and therefore works correctly even if a multi-byte character is split between chunks.

Creating Your Own Codec Class

Support for new text encodings can be added to Qt by creating QTextCodec subclasses.

The pure 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 encoding to Qt, make a subclass of QTextCodec and implement the functions listed in the table below.

FunctionDescription
name()Returns the official name for the encoding. If the encoding is listed in the IANA character-sets encoding file, the name should be the preferred MIME name for the encoding.
aliases()Returns a list of alternative names for the encoding. QTextCodec provides a default implementation that returns an empty list. For example, "ISO-8859-1" has "latin1", "CP819", "IBM819", and "iso-ir-100" as aliases.
mibEnum()Return the MIB enum for the encoding if it is listed in the IANA character-sets encoding file.
convertToUnicode()Converts an 8-bit character string to Unicode.
convertFromUnicode()Converts a Unicode string to an 8-bit character string.

You may find it more convenient to make your codec class available as a plugin; see How to Create Qt Plugins for details.

See also QTextStream, QTextDecoder, QTextEncoder, and Codecs Example.

Member Type Documentation

enum QTextCodec::ConversionFlag
flags QTextCodec::ConversionFlags

ConstantValueDescription
QTextCodec::DefaultConversion0No flag is set.
QTextCodec::ConvertInvalidToNull0x80000000If this flag is set, each invalid input character is output as a null character.
QTextCodec::IgnoreHeader0x1Ignore any Unicode byte-order mark and don't generate any.

The ConversionFlags type is a typedef for QFlags<ConversionFlag>. It stores an OR combination of ConversionFlag values.

Member Function Documentation

QTextCodec::QTextCodec () [protected]

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.

QTextCodec::~QTextCodec () [virtual protected]

Destroys the QTextCodec. Note that you should not delete codecs yourself: once created they become Qt's responsibility.

Warning: This function is not reentrant.

QList<QByteArray> QTextCodec::aliases () const [virtual]

Subclasses can return a number of aliases for the codec in question.

Standard aliases for codecs can be found in the IANA character-sets encoding file.

QList<QByteArray> QTextCodec::availableCodecs () [static]

Returns the list of all available codecs, by name. Call QTextCodec::codecForName() to obtain the QTextCodec for the name.

The list may contain many mentions of the same codec if the codec has aliases.

See also availableMibs(), name(), and aliases().

QList<int> QTextCodec::availableMibs () [static]

Returns the list of MIBs for all available codecs. Call QTextCodec::codecForMib() to obtain the QTextCodec for the MIB.

See also availableCodecs() and mibEnum().

bool QTextCodec::canEncode ( QChar ch ) const

Returns true if the Unicode character ch can be fully encoded with this codec; otherwise returns false.

bool QTextCodec::canEncode ( const QString & s ) const

This is an overloaded function.

s contains the string being tested for encode-ability.

QTextCodec * QTextCodec::codecForCStrings () [static]

Returns the codec used by QString to convert to and from const char * and QByteArrays. If this function returns 0 (the default), QString assumes Latin-1.

See also setCodecForCStrings().

QTextCodec * QTextCodec::codecForHtml ( const QByteArray & ba, QTextCodec * defaultCodec ) [static]

Tries to detect the encoding of the provided snippet of HTML in the given byte array, ba, by checking the BOM (Byte Order Mark) and the content-type meta header and returns a QTextCodec instance that is capable of decoding the html to unicode. If the codec cannot be detected from the content provided, defaultCodec is returned.

This function was introduced in Qt 4.4.

See also codecForUtfText().

QTextCodec * QTextCodec::codecForHtml ( const QByteArray & ba ) [static]

This is an overloaded function.

Tries to detect the encoding of the provided snippet of HTML in the given byte array, ba, by checking the BOM (Byte Order Mark) and the content-type meta header and returns a QTextCodec instance that is capable of decoding the html to unicode. If the codec cannot be detected, this overload returns a Latin-1 QTextCodec.

QTextCodec * QTextCodec::codecForLocale () [static]

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

On Windows, the codec will be based on a system locale. On Unix systems, starting with Qt 4.2, the codec will be using the iconv library. Note that in both cases the codec's name will be "System".

See also setCodecForLocale().

QTextCodec * QTextCodec::codecForMib ( int mib ) [static]

Returns the QTextCodec which matches the MIBenum mib.

QTextCodec * QTextCodec::codecForName ( const QByteArray & name ) [static]

Searches all installed QTextCodec objects and returns the one which best matches name; the match is case-insensitive. Returns 0 if no codec matching the name name could be found.

QTextCodec * QTextCodec::codecForName ( const char * name ) [static]

Searches all installed QTextCodec objects and returns the one which best matches name; the match is case-insensitive. Returns 0 if no codec matching the name name could be found.

QTextCodec * QTextCodec::codecForTr () [static]

Returns the codec used by QObject::tr() on its argument. If this function returns 0 (the default), tr() assumes Latin-1.

See also setCodecForTr().

QTextCodec * QTextCodec::codecForUtfText ( const QByteArray & ba, QTextCodec * defaultCodec ) [static]

Tries to detect the encoding of the provided snippet ba by using the BOM (Byte Order Mark) and returns a QTextCodec instance that is capable of decoding the text to unicode. If the codec cannot be detected from the content provided, defaultCodec is returned.

This function was introduced in Qt 4.6.

See also codecForHtml().

QTextCodec * QTextCodec::codecForUtfText ( const QByteArray & ba ) [static]

This is an overloaded function.

Tries to detect the encoding of the provided snippet ba by using the BOM (Byte Order Mark) and returns a QTextCodec instance that is capable of decoding the text to unicode. If the codec cannot be detected, this overload returns a Latin-1 QTextCodec.

See also codecForHtml().

QByteArray QTextCodec::convertFromUnicode ( const QChar * input, int number, ConverterState * state ) const [pure virtual protected]

QTextCodec subclasses must reimplement this function.

Converts the first number of characters from the input array from Unicode to the encoding of the subclass, and returns the result in a QByteArray.

state can be 0 in which case the conversion is stateless and default conversion rules should be used. If state is not 0, the codec should save the state after the conversion in state, and adjust the remainingChars and invalidChars members of the struct.

QString QTextCodec::convertToUnicode ( const char * chars, int len, ConverterState * state ) const [pure virtual protected]

QTextCodec subclasses must reimplement this function.

Converts the first len characters of chars from the encoding of the subclass to Unicode, and returns the result in a QString.

state can be 0, in which case the conversion is stateless and default conversion rules should be used. If state is not 0, the codec should save the state after the conversion in state, and adjust the remainingChars and invalidChars members of the struct.

QByteArray QTextCodec::fromUnicode ( const QString & str ) const

Converts str from Unicode to the encoding of this codec, and returns the result in a QByteArray.

QByteArray QTextCodec::fromUnicode ( const QChar * input, int number, ConverterState * state = 0 ) const

Converts the first number of characters from the input array from Unicode to the encoding of this codec, and returns the result in a QByteArray.

The state of the convertor used is updated.

QTextDecoder * QTextCodec::makeDecoder () const

Creates a QTextDecoder which stores enough state to decode chunks of char * data to create chunks of Unicode data.

The caller is responsible for deleting the returned object.

QTextDecoder * QTextCodec::makeDecoder ( ConversionFlags flags ) const

Creates a QTextDecoder with a specified flags to decode chunks of char * data to create chunks of Unicode data.

The caller is responsible for deleting the returned object.

This function was introduced in Qt 4.7.

QTextEncoder * QTextCodec::makeEncoder () const

Creates a QTextEncoder which stores enough state to encode chunks of Unicode data as char * data.

The caller is responsible for deleting the returned object.

QTextEncoder * QTextCodec::makeEncoder ( ConversionFlags flags ) const

Creates a QTextEncoder with a specified flags to encode chunks of Unicode data as char * data.

The caller is responsible for deleting the returned object.

This function was introduced in Qt 4.7.

int QTextCodec::mibEnum () const [pure virtual]

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

QByteArray QTextCodec::name () const [pure virtual]

QTextCodec subclasses must reimplement this function. It returns the name of the encoding supported by the subclass.

If the codec is registered as a character set in the IANA character-sets encoding file this method should return the preferred mime name for the codec if defined, otherwise its name.

void QTextCodec::setCodecForCStrings ( QTextCodec * codec ) [static]

Sets the codec used by QString to convert to and from const char * and QByteArrays. If the codec 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. To avoid undesirable side-effects, we recommend avoiding such codecs with setCodecsForCString().

Warning: This function is not reentrant.

See also codecForCStrings() and setCodecForTr().

void QTextCodec::setCodecForLocale ( QTextCodec * c ) [static]

Set the codec to c; this will be returned by codecForLocale(). If c is a null pointer, the codec is reset to the default.

This might be needed for some applications that want to use their own mechanism for setting the locale.

See also codecForLocale().

void QTextCodec::setCodecForTr ( QTextCodec * c ) [static]

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:

 int main(int argc, char *argv[])
 {
     QApplication app(argc, argv);
     QTextCodec::setCodecForTr(QTextCodec::codecForName("eucKR"));
     ...
 }

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 Internationalization with Qt.

Warning: This function is not reentrant.

See also codecForTr() and setCodecForCStrings().

QString QTextCodec::toUnicode ( const QByteArray & a ) const

Converts a from the encoding of this codec to Unicode, and returns the result in a QString.

QString QTextCodec::toUnicode ( const char * input, int size, ConverterState * state = 0 ) const

Converts the first size characters from the input from the encoding of this codec to Unicode, and returns the result in a QString.

The state of the convertor used is updated.

QString QTextCodec::toUnicode ( const char * chars ) const

This is an overloaded function.

chars contains the source characters.

Notes provided by the Qt Community
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Votes: 0

Coverage: Qt 4.8

Picture of David Villalobos David Villalobos

Ant Farmer
19 notes

Set codecs can be called at app start up.

Set codecs can be called at app start up.

  1. int main(int argc, char *argv[])
  2. {
  3.   QApplication app(argc, argv);
  4.   QTextCodec::setCodecForCStrings(QTextCodec::codecForName("UTF-8"));
  5.   QTextCodec::setCodecForTr(QTextCodec::codecForName("UTF-8"));
  6.   MainWindow *mainWindow = new MainWindow;
  7.   mainWindow->show();
  8.   return app.exec();
  9. }

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