Origin of the U+nnnn notation (original) (raw)

From: Hohberger, Clive (CHohberger@zebra.com)
Date: Tue Nov 08 2005 - 09:00:45 CST

Next message: Philippe Verdy: "Re: Origin of the U+nnnn notation"

Adding to Philippe's excellent description, I think of the set {U+nnnn}
as a set of ordinal numbers, as they represent positions in a table. The
construct U-nnnn therefore is meaningless as an ordinal number.
Clive

-----Original Message-----
From: unicode-bounce@unicode.org [mailto:unicode-bounce@unicode.org] On
Behalf Of Philippe Verdy
Sent: Tuesday, November 08, 2005 8:05 AM
To: Dominikus Scherkl; 'Jukka K. Korpela'; unicode@unicode.org
Subject: Re: Origin of the U+nnnn notation

From: "Dominikus Scherkl" <lyratelle@gmx.de>
>> I have been unable to hunt down the historical origin of the
>> notation U+nnnn (where nnnn are hexadecimal digits) that we
>> use to refer to characters (and code points).
>> Presumably "U" stands for "UCS" or for "Unicode", but where
>> does the plus sign come from?
> Maybe it was thought of as an offset from the unit (character null)
> like in ETA+5 minutes (expected time of arrival was passed five
minutes
> ago - an euphemism for beeing 5 minutes late).

U-nnnn already exists (or I should say, it has existed). It was refering
to
16-bit code units, not really to characters and was a fixed-width
notation
(with 4 hexadecimal digits). The "U" meant "Unicode" (1.0 and before).

U+[n...n]nnnn was created to avoid the confusion with the past 16-bit
only
Unicode 1.0 standard (which was not fully compatible with ISO/IEC 10646
code
points). It is a variable-width notation that refers to ISO/IEC 10646
code
points. The "U" means "UCS" or "Universal Character Set". At that time,
the
UCS code point range was up to 31 bits wide.

The U-nnnn notation is abandoned now, except for references to Unicode
1.0.
If one uses it, it will refer to one or more 16-bit code units needed to

encode each codepoint (possibly with surrogate pairs). It does not
designates abstract characters or codepoints unambiguously.

Later, the variable-width U+[n...n]nnnn notation was restricted to allow

only codepoints in the 17 first planes of the joined ISO/IEC 10646-1 and

Unicode standards (so the only standard codepoints are between U+0000
and
U+10FFFF, some of them being permanently assigned to non-characters).

The references to larger code points with U+[n...n]nnnn is discouraged,
as
they no longer designate valid code points in both standards. Their
definition and use is then application-specific.

There are '''no''' negative codepoints in either standards (U-0001 does
not
designate the 32-bit code unit that you could store in a signed
wide-char
datatype, but in past standard it designated the same codepoint as
U+0001
now). Using "+" makes the statement about signs clear: standard code
points
all have positive values.

So if you want a representation for negative code units, you need
another
notation (for example N-0001 to represent the negative code unit with
negative value -1): this notation is application-specific.

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