In Qt how do QTextCodec::codecForName("UTF-16") and codecForName("UTF-32") decide the endianness to use?

Question

In the Qt documentation it states that (among others) the following Unicode string encodings are supported:

• UTF-8
• UTF-16
• UTF-16BE
• UTF-16LE
• UTF-32
• UTF-32BE
• UTF-32LE

Due to the three different codecs listed for 2 and 4 octet encoded Unicode, I was wondering: how do the two non-endian codecs ("UTF-16" and "UTF-32") decide which endianness to use?

Answer 1

Based on the source code in src/corelibs/codecs/, it seems Qt uses the byte ordering of the host for UTF-16 and UTF-32.

If you use QTextCodec to read an existing Unicode string that has a BOM, and you didn't explicitly ask to ignore the header, the byte ordering detected in the string is used.

• In *qutfcodec_p.h* both QUtf16Codec::e and QUtf32Codec::e are initialized with the value DetectEndianness (an enum).

• In qutfcodec.cpp, near the beginning of the functions convertFromUnicode and convertToUnicode from the classes QUtf16 and QUtf32 (used by QUtf16Codec and QUtf32Codec), you can find the line:

  endian = (QSysInfo::ByteOrder == QSysInfo::BigEndian) 
      ? BigEndianness : LittleEndianness;