QTextCodec Class

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

Header:	#include <QTextCodec>
qmake:	QT += core

• List of all members, including inherited members
• Obsolete members

Note: All functions in this class are reentrant.

• setCodecForLocale(QTextCodec *c)
• ~QTextCodec()

Note: These functions are also thread-safe:

• codecForName(const QByteArray &name)
• codecForMib(int mib)
• availableCodecs()
• availableMibs()
• codecForLocale()

Public Types

	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
bool	canEncode(QStringView s) const
QByteArray	fromUnicode(const QString &str) const
QByteArray	fromUnicode(QStringView str) const
QByteArray	fromUnicode(const QChar *input, int number, QTextCodec::ConverterState *state = nullptr) const
QTextDecoder *	makeDecoder(QTextCodec::ConversionFlags flags = DefaultConversion) const
QTextEncoder *	makeEncoder(QTextCodec::ConversionFlags flags = DefaultConversion) const
virtual int	mibEnum() const = 0
virtual QByteArray	name() const = 0
QString	toUnicode(const QByteArray &a) const
QString	toUnicode(const char *chars) const
QString	toUnicode(const char *input, int size, QTextCodec::ConverterState *state = nullptr) const

Static Public Members

QList<QByteArray>	availableCodecs()
QList<int>	availableMibs()
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 *	codecForUtfText(const QByteArray &ba, QTextCodec *defaultCodec)
QTextCodec *	codecForUtfText(const QByteArray &ba)
void	setCodecForLocale(QTextCodec *c)

Protected Functions

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

Detailed Description

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:

• Big5
• Big5-HKSCS
• CP949
• EUC-JP
• EUC-KR
• GB18030
• HP-ROMAN8
• 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
• KOI8-R
• KOI8-U
• Macintosh
• Shift-JIS
• TIS-620
• TSCII
• UTF-8
• UTF-16
• UTF-16BE
• UTF-16LE
• UTF-32
• UTF-32BE
• UTF-32LE
• Windows-1250 to 1258

If Qt is compiled with ICU support enabled, most codecs supported by ICU will also be available to the application.

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.

Function	Description
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.

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

Member Type Documentation

enum QTextCodec::ConversionFlagflags QTextCodec::ConversionFlags

Constant	Value	Description
QTextCodec::DefaultConversion	0	No flag is set.
QTextCodec::ConvertInvalidToNull	0x80000000	If this flag is set, each invalid input character is output as a null character.
QTextCodec::IgnoreHeader	0x1	Ignore 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

[protected] QTextCodec::QTextCodec()

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.

[virtual protected] QTextCodec::~QTextCodec()

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

Warning: This function is not reentrant.

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

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.

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

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.

Note: This function is thread-safe.

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

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

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

Note: This function is thread-safe.

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.

bool QTextCodec::canEncode(QStringView s) const

This is an overloaded function.

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

This function was introduced in Qt 5.10.

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

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().

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

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.

[static] QTextCodec *QTextCodec::codecForLocale()

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

The codec will be retrieved from ICU where that backend is in use, otherwise it may be obtained from an OS-specific API. In the latter case, the codec's name may be "System".

Note: This function is thread-safe.

See also setCodecForLocale().

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

Returns the QTextCodec which matches the MIBenum mib.

Note: This function is thread-safe.

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

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.

Note: This function is thread-safe.

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

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.

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

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. This function can detect one of the following codecs:

• UTF-32 Little Endian
• UTF-32 Big Endian
• UTF-16 Little Endian
• UTF-16 Big Endian
• UTF-8

If the codec cannot be detected from the content provided, defaultCodec is returned.

This function was introduced in Qt 4.6.

See also codecForHtml().

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

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. This function can detect one of the following codecs:

• UTF-32 Little Endian
• UTF-32 Big Endian
• UTF-16 Little Endian
• UTF-16 Big Endian
• UTF-8

If the codec cannot be detected from the content provided, this overload returns a Latin-1 QTextCodec.

See also codecForHtml().

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

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 nullptr 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.

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

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 nullptr, 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(QStringView str) const

This is an overloaded function.

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

This function was introduced in Qt 5.10.

QByteArray QTextCodec::fromUnicode(const QChar *input, int number, QTextCodec::ConverterState *state = nullptr) 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(QTextCodec::ConversionFlags flags = DefaultConversion) 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(QTextCodec::ConversionFlags flags = DefaultConversion) 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.

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

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.

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

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.

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

Set the codec to c; this will be returned by codecForLocale(). If c is nullptr, 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.

Warning: This function is not reentrant.

See also codecForLocale().

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 *chars) const

This is an overloaded function.

chars contains the source characters.

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