QStringConverter Class

Detailed Description

Qt uses UTF-16 to store, draw and manipulate strings. In many situations you may wish to deal with data that uses a different encoding. Most text data transferred over files and network connections is encoded in UTF-8.

The QStringConverter class is a base class for the QStringEncoder and QStringDecoder classes that help with converting between different text encodings. QStringDecoder can decode a string from an encoded representation into UTF-16, the format Qt uses internally. QStringEncoder does the opposite operation, encoding UTF-16 encoded data (usually in the form of a QString) to the requested encoding.

The supported encodings are:

• UTF-8
• UTF-16
• UTF-16BE
• UTF-16LE
• UTF-32
• UTF-32BE
• UTF-32LE
• ISO-8859-1 (Latin-1)
• The system encoding

QStringConverters can be used as follows to convert some encoded string to and from UTF-16.

Suppose you have some string encoded in UTF-8, and want to convert it to a QString. The simple way to do it is to use a QStringDecoder like this:

 QByteArray encodedString = "...";
 auto toUtf16 = QStringDecoder(QStringDecoder::Utf8);
 QString string = toUtf16(encodedString);

After this, string holds the text in decoded form. Converting a string from Unicode to the local encoding is just as easy using the QStringEncoder class:

 QString string = "...";
 auto fromUtf16 = QStringEncoder(QStringEncoder::Utf8);
 QByteArray encodedString = fromUtf16(string);

To read or write text files in various encodings, use QTextStream and its setEncoding() function.

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.

Both QStringEncoder and QStringDecoder make this easy, by tracking this in an internal state. So simply calling the encoder or decoder again with the next chunk of data will automatically continue encoding or decoding the data correctly:

 auto toUtf16 = QStringDecoder(QStringDecoder::Utf8);

 QString string;
 while (new_data_available()) {
     QByteArray chunk = get_new_data();
     string += toUtf16(chunk);
 }

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

QStringConverter objects can't be copied because of their internal state, but can be moved.