Address wording issues related to ISO 10646

Document Number: P1139R0
Date: 2019-01-21
Audience: SG16, CWG
Author: R. Martinho Fernandes
Reply-to: cpp@rmf.io

Motivation

Review of some editorial fixes following the recent update of the normative reference to ISO 10646 has unearthed a series of wording issues around the subject. This paper intends to fix those issues by rewording relevant paragraphs.

Proposal

This paper addresses all of the following issues:

  1. The current wording in [lex.charset] does not specify what the behaviour is for a universal-character-name without a corresponding short identifier in ISO 10646.

    For example, \U99004141 and \U00110000. Neither of these designates a code point in ISO 10646, but the standard is silent about this, which makes the behaviour undefined by omission.

    This paper addresses this by making such uses ill-formed, maintaining consistency with the current treatment of surrogate values (\U0000D800 is already ill-formed).

  2. The current wording in [lex.charset] uses “hexadecimal value”, which is confusing because a value is just a number, and hexadecimal is just a way to represent numbers; “value” alone should suffice.

    This paper addresses this by removing the need for this term.

  3. There is some interest in using the U+ notation (as in U+0041 or U+1F34A) to refer to Unicode code points across the entire standard.

    This paper changes all the relevant wording to use U+ notation.

  4. The current text includes explanations of terms from ISO 10646 (like “surrogate code point” or “control character”) in normative text, which is undesirable.

    This paper moves such explanations to non-normative text, and clarifies some existing explanations.

Technical Specifications

In this description, text that should be deleted is marked red and striked out; text that should be added is marked green and underlined. Apply these changes on top of the editorial fix provided in PR #2201.

Edit 5.3 [lex.charset], paragraph 2 as follows.

2 The universal-character-name construct provides a way to name other characters.

    hex-quad:         hexadecimal-digit hexadecimal-digit hexadecimal-digit hexadecimal-digit

    universal-character-name:         *

Edit 5.13.3 [lex.ccon], paragraph 3 as follows.

3 A character literal that begins with u8, such as u8'w', is a character literal of type char, known as a UTF-8 character literal. The value of a UTF-8 character literal is equal to its ISO 10646 code point value, provided that the code point value is representable with a single UTF-8 code unit (that is, provided it is in the C0 Controls and Basic Latin Unicode block)[Note—that is, provided it is in the range 0x0-0x7F, inclusive—end note]. If the value is not representable with a single UTF-8 code unit, the program is ill-formed. A UTF-8 character literal containing multiple c-chars is ill-formed.

Edit 5.13.3 [lex.ccon], paragraph 4 as follows.

4 A character literal that begins with the letter u, such as u'x', is a character literal of type char16_t. The value of a char16_t character literal containing a single c-char is equal to its ISO 10646 code point value, provided that the code point value is representable with a single 16-bit code unit ([Note—that is, provided it is in the basic multi-lingual planethe range 0x0-0xFFFF, inclusive)—end note]. If the value is not representable with a single 16-bit code unit, the program is ill-formed. A char16_t character literal containing multiple c-chars is ill-formed.

Edit 5.13.3 [lex.string], paragraph 10 as follows.

10 A string-literal that begins with u, such as u"asdf", is a char16_t string literal. A char16_t string literal has type “array of n const char16_t”, where n is the size of the string as defined below; it is initialized with the given characters. A single c-char may produce more than one char16_t character in the form of surrogate pairs [Note— a surrogate pair is a representation for a single character as a sequence of two 16-bit code units—end note].

Edit 19.8 [cpp.predefined], item (2.4) as follows.

(2.4) —__STDC_ISO_10646__
An integer literal of the form yyyymmL (for example, 199712L). If this symbol is defined, then every character in the Unicode required set, when stored in an object of type wchar_t, has the same value as the short identifiercode point of that character. The Unicode required set consists of all the characters that are defined by ISO/IEC 10646, along with all amendments and technical corrigenda as of the specified year and month.