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A value greater than \U0000FFFF may be represented by a single wchar_t if the UTF-32 encoding is used, or two if UTF-16 is used. Importantly, the universal character name \u00C0 always denotes the character "À", regardless of what kind of string literal it is used in, or the encoding in use. The octal and hex escape sequences always denote ...
The BOM for little-endian UTF-32 is the same pattern as a little-endian UTF-16 BOM followed by a UTF-16 NUL character, an unusual example of the BOM being the same pattern in two different encodings. Programmers using the BOM to identify the encoding will have to decide whether UTF-32 or UTF-16 with a NUL first character is more likely.
UTF-32 (32-bit Unicode Transformation Format), sometimes called UCS-4, is a fixed-length encoding used to encode Unicode code points that uses exactly 32 bits (four bytes) per code point (but a number of leading bits must be zero as there are far fewer than 2 32 Unicode code points, needing actually only 21 bits). [1]
Part of the C standard since C11, [17] in <uchar.h>, a type capable of holding 32 bits even if wchar_t is another size. If the macro __STDC_UTF_32__ is defined as 1, the type is used for UTF-32 on that system. This is always the case in C23. [15] C++ does not define such a macro, but the type is always used for UTF-32 in that language. [16 ...
Not only that, it is wrong. Even Microsoft will recognize UTF-16 without the BOM, and I kind of doubt much software recognizes UTF-32 using it (as it will look like UTF-16 starting with a BOM and a NUL). Therefore there are two reasons: identify byte order in UTF-16 and UTF-32, and as a marker to distinguish UTF-8 from legacy 8-bit encodings.
UTF-8, UTF-16, UTF-32 and UTF-EBCDIC have these important properties but UTF-7 and GB 18030 do not. Fixed-size characters can be helpful, but even if there is a fixed byte count per code point (as in UTF-32), there is not a fixed byte count per displayed character due to combining characters. Considering these incompatibilities and other quirks ...
In the other encodings, each code point may be represented by a variable number of code units. UTF-32 is widely used as an internal representation of text in programs (as opposed to stored or transmitted text), since every Unix operating system that uses the gcc compilers to generate software uses it as the standard "wide character" encoding
Improved Unicode support based on the C Unicode Technical Report ISO/IEC TR 19769:2004 (char16_t and char32_t types for storing UTF-16/UTF-32 encoded data, including conversion functions in <uchar.h> and the corresponding u and U string literal prefixes, as well as the u8 prefix for UTF-8 encoded literals).