ISO/IEC JTC1 SC22 WG21 P2314R1
Author: Jens Maurer
Target audience: SG16, EWG
2021-03-15

P2314R1: Character sets and encodings

Introduction

This paper implements the following changes: This paper resolves the following core issues:

Changes since R0

Terminology changes

The following terms are defined by this paper: The term "basic / extended source character set" is removed.

Behavior changes

The core behavior change is that universal-character-names are no longer formed in translation phase 1. Instead, all Unicode input characters are retained throughout the translation.

This changes the specified behavior of the stringizing preprocessor operator [cpp.stringize] as follows
C++20this paper
#define S(x) # x
const char * s1 = S(Köppe);       // "K\\u00f6ppe"
const char * s2 = S(K\u00f6ppe);  // "K\\u00f6ppe"
      
#define S(x) # x
const char * s1 = S(Köppe);       // "Köppe"
const char * s2 = S(K\u00f6ppe);  // "Köppe"
      
However, it turns out that all major implementations already implement what this paper specifies, i.e. no implementation provides an escaped UCN.

Not all string-literals are the same

In C++, string literals can appear in the following contexts:

ContextDestination
asm-declarationbuild environment
#include "fn" or #include <fn>file name
language linkagetranslation
operator "" [over.literal]translation
#line directivediagnostic
argument for [[nodiscard]] and [[deprecated]]diagnostic
#error, static_assertdiagnostic
__FILE__, __func__literal encoding
std::typeinfo::name()literal encoding
character-literal or string-literal appearing elsewhereliteral encoding
user-defined-literalliteral encoding

The destinations have the following meaning:

The existing text in 5.13.5 [lex.string] already specifies that the initialization of a string literal object (as needed when using a string-literal as a primary expression) is the point where the string-literal is encoded. In other contexts, no such encoding happens. No wording changes in this area are intended by this paper.

Comparison with P2297R0

This paper P2314 refomulates the core language rules around lexing of non-basic characters, while keeping the actual semantic changes to a minimum. This makes it more likely that the paper can either directly proceed to CWG or be reviewed by EWG with minimal effort.

The paper P2297R0 "Wording improvements for encodings and character sets" by Corentin Jabot has overlap with this paper. The main differences are:

Wording changes

Change in 3.35 [defns.multibyte]:
multibyte character

sequence of one or more bytes representing a member of the extended character set of either the source or the execution environment the code unit sequence for an encoded character of the execution character set

[Note 1 to entry: The extended character set is a superset of the basic character set (5.3). — end note]

Change in 5.2 [lex.phases] paragraph 1:
1. Physical source file characters are mapped, in an implementation-defined manner, to the basic source translation character set (introducing new-line characters for end-of-line indicators) if necessary. The set of physical source file characters accepted is implementation-defined. Any source file character not in the basic source character set (5.3 [lex.charset]) is replaced by the universal-character-name that designates that character. An implementation may use any internal encoding, so long as an actual extended character encountered in the source file, and the same extended character expressed in the source file as a universal-character-name (e.g., using the \uXXXX notation), are handled equivalently except where this replacement is reverted (5.4 [lex.pptoken]) in a raw string literal.

...

3. The source file is decomposed into preprocessing tokens (5.4 [lex.pptoken]) and sequences of white-space characters (including comments). A source file shall not end in a partial preprocessing token or in a partial comment. [ Footnote: ... ] Each comment is replaced by one space character. New-line characters are retained. Whether each nonempty sequence of white-space characters other than new-line is retained or replaced by one space character is unspecified. Each universal-character-name outside of a header-name or a character or string literal is replaced by the designated element of the translation character set ([lex.charset]). The process of dividing a source file’s characters into preprocessing tokens is context-dependent. [Example: See the handling of < within a #include preprocessing directive. — end example]

4. Preprocessing directives are executed, macro invocations are expanded, and _Pragma unary operator expressions are executed. If a character sequence that matches the syntax of a universal-character-name is produced by token concatenation (15.6.3 [lex.concat]), the behavior is undefined. A #include preprocessing directive causes the named header or source file to be processed from phase 1 through phase 4, recursively. All preprocessing directives are then deleted.

5. For a sequence of two or more adjacent string-literal tokens, a common encoding-prefix is determined as specified in 5.13.5 [lex.string]. Each such string-literal token is then considered to have that common encoding-prefix. Each basic source character set member in a character-literal or a string-literal, as well as each escape sequence and universal-character-name in a character-literal or a non-raw string literal, is converted to the corresponding member of the execution character set (5.13.3, 5.13.5); if there is no corresponding member, it is converted to an implementation-defined member other than the null (wide) character. [ Footnote: An implementation need not convert all non-corresponding source characters to the same execution character. ]

6. Adjacent string literal string-literal tokens are concatenated (5.13.5 [lex.string]).

Replace all of 5.3 [lex.charset] (paragraphs 1-3):
1 The translation character set consists of the following elements: [ Note: ISO/IEC 10646 code points are integers in the range [0, 10FFFF] (hexadecimal). A surrogate code point is a value in the range [D800, DFFF] (hexadecimal). A UCS scalar value is any code point that is not a surrogate code point. -- end note ]

2 The basic character set is a subset of the translation character set, consisting of 96 characters as specified in table X. [ Note: Unicode short names are given only as a means to identifying the character; the numerical value has no other meaning in this context. -- end note ]

U+0009 CHARACTER TABULATION
U+000B LINE TABULATION
U+000C FORM FEED (FF)
U+0020 SPACE
U+000A LINE FEED (LF)new-line
U+0021 EXCLAMATION MARK!
U+0022 QUOTATION MARK"
U+0023 NUMBER SIGN#
U+0025 PERCENT SIGN%
U+0026 AMPERSAND&
U+0027 APOSTROPHE'
U+0028 LEFT PARENTHESIS(
U+0029 RIGHT PARENTHESIS)
U+002A ASTERISK*
U+002B PLUS SIGN+
U+002C COMMA,
U+002D HYPHEN-MINUS-
U+002E FULL STOP.
U+002F SOLIDUS/
U+0030 .. U+0039 DIGIT ZERO .. NINE0 1 2 3 4 5 6 7 8 9
U+003A COLON:
U+003B SEMICOLON;
U+003C LESS-THAN SIGN<
U+003D EQUALS SIGN=
U+003E GREATER-THAN SIGN>
U+003F QUESTION MARK?
U+0041 .. U+005A LATIN CAPITAL LETTER A .. ZA B C D E F G H I J K L M
N O P Q R S T U V W X Y Z
U+005B LEFT SQUARE BRACKET[
U+005C REVERSE SOLIDUS\
U+005D RIGHT SQUARE BRACKET]
U+005E CIRCUMFLEX ACCENT^
U+005F LOW LINE_
U+0061 .. U+007A LATIN SMALL LETTER A .. Za b c d e f g h i j k l m
n o p q r s t u v w x y z
U+007B LEFT CURLY BRACKET{
U+007C VERTICAL LINE|
U+007D RIGHT CURLY BRACKET}
U+007E TILDE~

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 :
    \u hex-quad
    \U hex-quad hex-quad
A universal-character-name designates the character in ISO/IEC 10646 (if any) the translation character set whose Unicode code point UCS scalar value is the hexadecimal number represented by the sequence of hexadecimal-digits in the universal-character-name. The program is ill-formed if that number is not a Unicode code point or if it is a surrogate code point UCS scalar value. Noncharacter code points and reserved code points are considered to designate separate characters distinct from any ISO/IEC 10646 character. If a universal-character-name outside the c-char-sequence, s-char-sequence, or r-char-sequence of a character-literal or string-literal (in either case, including within a user-defined-literal) corresponds to a control character or to a character in the basic source character set, the program is ill-formed. [ Footnote: Note: A sequence of characters resembling a universal-character-name in an r-char-sequence (5.13.5) does not form a universal-character-name. ] [Note: ISO/IEC 10646 code points are integers in the range [0, 10FFFF] (hexadecimal). A surrogate code point is a value in the range [D800, DFFF] (hexadecimal). A control character is a character whose code point is in either of the ranges [0, 1F] or [7F, 9F] (hexadecimal). — end note]
The basic literal character set consists of all characters of the basic character set, plus the control characters specified in table Y.
U+0000NULL
U+0007BELL
U+0008BACKSPACE
U+000DCARRIAGE RETURN (CR)

A code unit is an integer value of character type (6.8.1 [basic.fundamental]). Characters in a character-literal other than a multicharacter or non-encodable character literal or in a string-literal are encoded as a sequence of one or more code units, as determined by the encoding-prefix ([lex.ccon], [lex.string]); this is termed the respective literal encoding. The ordinary literal encoding is the encoding applied to an ordinary character or string literal. The wide literal encoding is the encoding applied to a wide character or string literal.

A literal encoding encodes each element of the basic literal character set as a single code unit with non-negative value, distinct from the code unit for any other such element. [ Note: A character not in the basic literal character set can be encoded with more than one code unit; the value of such a code unit can be the same as that of a code unit for an element of the basic literal character set. -- end note ]. The U+0000 NULL character is encoded as the value 0. No other element of the translation character set is encoded with a code unit of value 0. The code unit value of each decimal digit character after the digit 0 (U+0030) shall be one greater than the value of the previous. The ordinary and wide literal encodings are otherwise implementation-defined. For a UTF-8, UTF-16, or UTF-32 literal, the UCS scalar value corresponding to each character of the translation character set is encoded as specified in ISO/IEC 10646 for the respective UCS encoding form.

The basic execution character set and the basic execution wide-character set shall each contain all the members of the basic source character set, plus control characters representing alert, backspace, and carriage return, plus a null character (respectively, null wide character), whose value is 0. For each basic execution character set, the values of the members shall be non-negative and distinct from one another. In both the source and execution basic character sets, the value of each character after 0 in the above list of decimal digits shall be one greater than the value of the previous. The execution character set and the execution wide-character set are implementation-defined supersets of the basic execution character set and the basic execution wide-character set, respectively. The values of the members of the execution character sets and the sets of additional members are locale-specific.
Change in 5.4 [lex.pptoken] paragraph 2:
A preprocessing token is the minimal lexical element of the language in translation phases 3 through 6. In this document, glyphs are used to identify elements of the basic character set ([lex.charset]). The categories of preprocessing token are: header names, placeholder tokens produced by preprocessing import and module directives (import-keyword, module-keyword, and export-keyword), identifiers, preprocessing numbers, character literals (including user-defined character literals), string literals (including user-defined string literals), preprocessing operators and punctuators, and single non-whitespace characters that do not lexically match the other preprocessing token categories. If a ' or a "U+0027 APOSTROPHE or a U+0022 QUOTATION MARK character matches the last category, the behavior is undefined. Preprocessing tokens can be separated by whitespace; this consists of comments (5.7), or whitespace characters (space, horizontal tabU+0020 SPACE, U+0009 CHARACTER TABULATION, new-line, vertical tab, and form-feedU+000B LINE TABULATION, and U+000C FORM FEED), or both. ...
Change in 5.4 [lex.pptoken] paragraph 3 bullet 1:
Change in 5.8 [lex.header] paragraph 1:
h-char:
    any member of the source translation character set except new-line and > U+003E GREATER-THAN SIGN
...
q-char:
    any member of the source translation character set except new-line and " U+0022 QUOTATION MARK
Change in 5.13.3 [lex.ccon] before paragraph 1:
basic-c-char:
    any member of the basic source translation character set
    except the single-quote ’, backslash \ U+0027 APOSTROPHE, U+005C REVERSE SOLIDUS, or new-line character
...
conditional-escape-sequence-char:
    any member of the basic source character set that is not an octal-digit, a simple-escape-sequence-char, or
    the characters u, U, or x
Change in 5.13.3 [lex.ccon] paragraph 2:
[Note 1 : The associated character encoding for ordinary and wide character literals determines encodability, but does not determine the value of non-encodable ordinary or wide character literals or ordinary or wide multicharacter literals. The examples in Table 9 for non-encodable ordinary and wide character literals assume that the specified character lacks representation in the execution character set ordinary literal encoding or execution wide-character set wide literal encoding, respectively, or that encoding it would require more than one code unit. — end note]
Change in 5.13.3 [lex.ccon] table tab:lex.ccon.literal:
Encoding prefix...Associated character encoding
none... encoding of the execution character set ordinary literal encoding
L... encoding of the execution wide-character set wide literal encoding
Replace 5.13.3 [lex.ccon] table tab:lex.ccon.esc:
The character specified by a simple-escape-sequence is specified in Table 10.

charactersimple-escape-sequence
U+000ALINE FEED (LF)\n
U+0009CHARACTER TABULATION\t
U+000BLINE TABULATION\v
U+0008BACKSPACE\b
U+000DCARRIAGE RETURN (CR)\r
U+000CFORM FEED (FF)\f
U+0007BELL\a
U+005CREVERSE SOLIDUS\\\
U+003FQUESTION MARK"\?
U+0027APOSTROPHE'\'
U+0022QUOTATION MARK?\"

Change in 5.13.5 [lex.string] before paragraph 1:
basic-s-char:
    any member of the basic source translation character set
    except the double-quote ", backslash \U+0022 QUOTATION MARK, U+005C REVERSE SOLIDUS, or new-line character
...
r-char:
    any member of the source translation character set, except a right parenthesis ) U+0029 RIGHT PARENTHESIS followed by
    the initial d-char-sequence (which may be empty) followed by a double quote " U+0022 QUOTATION MARK.
...
d-char:
    any member of the basic source character set except:
    space, the left parenthesis (, the right parenthesis ), the backslash \, and the control characters
    representing horizontal tab, vertical tab, form feed
    U+0020 SPACE, U+0028 LEFT PARENTHESIS, U+0029 RIGHT PARENTHESIS, U+005C REVERSE SOLIDUS,
    U+0009 CHARACTER TABULATION, U+000B LINE TABULATION, U+000C FORM FEED (FF), and new-line

and the control characters
representing horizontal tab, vertical tab, form feed, and newline.

Change in 5.13.5 [lex.string] table tab:lex.string.literal:
Encoding prefix...Associated character encoding
none... encoding of the execution character set ordinary literal encoding
L... encoding of the execution widecharacter set wide literal encoding
Change in 5.13.5 [lex.string] paragraphs 7 and 8:
- 7 - In translation phase 6 (5.2 [lex.phases]), adjacent string-literals are concatenated. The common encoding-prefix for a sequence of adjacent string-literals is determined pairwise as follows: If both two string-literals have the same encoding-prefix, the resulting concatenated string-literal has common encoding-prefix is that encoding-prefix. If one string-literal has no encoding-prefix, it is treated as a string-literal of the same encoding-prefix as the common encoding-prefix is that of the other operand string-literal. If a UTF-8 string literal token is adjacent to a wide string literal token, the program is ill-formed. Any other concatenations combinations are conditionally-supported with implementation-defined behavior. [Note: This concatenation is an interpretation, not a conversion. Because the interpretation happens in translation phase 6 (after each character from a string-literal has been translated into a value from the appropriate character set), a A string-literal’s initial rawness has no effect on the interpretation or well-formedness of the concatenation determination of the common encoding-prefix. -- end note]

Table 13 has some examples of valid concatenations.

- 8 - In translation phase 6 (5.2 [lex.phases]), adjacent string-literals are concatenated. The lexical structure of the contents of the individual string-literals is retained. Characters in concatenated strings are kept distinct. [Example:

"\xA" "B"
contains the two characters represents the code unit ’\xA’ and the character ’B’ after concatenation (and not the single hexadecimal character code unit ’\xAB’). Similarly,
R"(\u00)" "41"
represents six characters, starting with a backslash and ending with the digit 1 (and not the single character "A" specified by a universal-character-name).

Table 13 has some examples of valid concatenations. — end example]

In translation phase 6 (5.2), after adjacent string-literals are concatenated, a null character is appended to the result.

Change in 5.13.5 [lex.string] paragraph 10 and de-bulletize:
String literal objects are initialized with the sequence of code unit values corresponding to the string-literal's sequence of s-chars (for a non-raw string literal) and r-chars (for a raw string literal), plus a terminating U+0000 NULL character, in order as follows:
Change in 5.13.8 [lex.ext] paragraph 3:
[ Note: The sequence c1 c2 ...ck can only contain characters from the basic source character set. — end note]
Change in 5.13.8 [lex.ext] paragraph 4:
[ Note: The sequence c1 c2 ...ck can only contain characters from the basic source character set. — end note]
Change in 6.7.1 [intro.memory] paragraph 1:
The fundamental storage unit in the memory model is the byte. A byte is at least large enough to contain any member the ordinary literal encoding of any element of the basic execution literal character set (5.3) and the eight-bit code units of the Unicode UTF-8 encoding form and is composed of a contiguous sequence of bits, [ Footnote: ... ] the number of which is implementation-defined.
Change in 6.8.2 [basic.fundamental] paragraph 7:
Type char is a distinct type that has an implementation-defined choice of “signed char” or “unsigned char” as its underlying type. The values of type char can represent distinct codes for all members of the implementation’s basic character set. ...
Editing note: The strike-out above is already stated in the definition of "byte", above. If desired, we can add a note that a char takes exactly one byte.

Change in 6.8.2 [basic.fundamental] paragraph 8:

Type wchar_t is a distinct type that has an implementation-defined signed or unsigned integer type as its underlying type. The values of type wchar_t can represent distinct codes for all members of the largest extended any character set specified among the supported locales (28.3.1).
Change in 6.8.2 [basic.fundamental] paragraph 11:
The types char, wchar_t, char8_t, char16_t, char32_t are collectively called character types. The character types, Types bool, char, wchar_t, char8_t, char16_t, char32_t, and the signed and unsigned integer types are collectively called integral types. A synonym for integral type is integer type. [Note: Enumerations (9.7.1) are not integral; however, unscoped enumerations can be promoted to integral types as specified in 7.3.6. — end note]
Change in 7.5.1 [expr.prim.literal] paragraph 1:
A literal is a primary expression. The type of a literal is determined based on its form as specified in 5.13 [lex.literal]. A string-literal is an lvalue designating the corresponding string literal object ([lex.string]), a user-defined-literal has the same value category as the corresponding operator call expression described in 5.13.8 [lex.ext], and any other literal is a prvalue.
Change in 15.2 [cpp.cond] paragraph 12:
The resulting tokens comprise the controlling constant expression which is evaluated according to the rules of 7.7 using arithmetic that has at least the ranges specified in 17.3. For the purposes of this token conversion and evaluation all signed and unsigned integer types act as if they have the same representation as, respectively, intmax_t or uintmax_t (17.4). [Note: ... -- end note] This includes interpreting character-literals, which may involve converting escape sequences into execution character set members interpreting escape-sequences and universal-character-names (5.13.3 [lex.ccon]). Whether the numeric value for these character-literals matches the value obtained when an identical character-literal occurs in an expression (other than within a #if or #elif directive) is implementation-defined. [Note: ... -- end note] Also, whether a single-character character-literal may have a negative value is implementation-defined. Each subexpression with type bool is subjected to integral promotion before processing continues.
Change in 15.6.3 [lex.concat] paragraph 3:
For both object-like and function-like macro invocations, before the replacement list is reexamined for more macro names to replace, each instance of a ## preprocessing token in the replacement list (not from an argument) is deleted and the preceding preprocessing token is concatenated with the following preprocessing token. Placemarker preprocessing tokens are handled specially: concatenation of two placemarkers results in a single placemarker preprocessing token, and concatenation of a placemarker with a non-placemarker preprocessing token results in the non-placemarker preprocessing token. If the result is not a valid preprocessing token, the behavior is undefined. If the result matches the syntax of a universal-character-name, the behavior is undefined. The resulting token is available for further macro replacement. The order of evaluation of ## operators is unspecified.
Change in 16.3.3.3.5.1 [character.seq] paragraph 1:
The C standard library makes widespread use of characters and character sequences that follow a few uniform conventions:
Change in 16.3.3.3.5.2 [multibyte.strings] paragraph 1:
A null-terminated multibyte string, or ntmbs, is an ntbs that constitutes a sequence of valid multibyte characters, beginning and ending in the initial shift state. [ Footnote: An NTBS that contains characters only from the basic execution literal character set is also an NTMBS. Each multibyte character then consists of a single byte. ]
Change in 27.13 [time.parse] table [tab:time.parse.spec]:
%ZThe time zone abbreviation or name. A single word is parsed. This word can only contain characters from the basic source character set (5.3 [lex.charset]) that are alphanumeric, or one of ’_’, ’/’, ’-’, or ’+’.
Change in 28.4.2.2.3 [locale.ctype.virtuals] paragraphs 11 and 13:
The only characters for which unique transformations are required are those in the basic source character set (5.3 [lex.charset]).

[...]

For any character c in the basic source character set (5.3 [lex.charset]) the transformation is such that

do_widen(do_narrow(c, 0)) == c
Change in C.2.3 [diff.cpp14.lex]:
Affected subclause: 5.2
Change: Removal of trigraph support as a required feature.
Rationale: Prevents accidental uses of trigraphs in non-raw string literals and comments. Effect on original feature: Valid C ++ 2014 code that uses trigraphs may not be valid or may have different semantics in this revision of C ++ . Implementations may choose to translate trigraphs as specified in C ++ 2014 if they appear outside of a raw string literal, as part of the implementation-defined mapping from physical source file characters to the basic source character set.

Acknowledgements

Thanks to Corentin Jabot and his related paper P2297R0 for detailed discussions.