Proposed resolution for core issues 411, 1656, and 2333; numeric and universal character escapes in character and string literals

• Introduction
• Changes from P2029R1
• Proposed resolution overview
• Implementation impact
  • Semantics of numeric-escape-sequences in UTF-8 literals
  • Character literal type for characters not representable in the execution character set
• Acknowledgements
• Proposed resolution
  • [lex.phases]
  • [lex.ccon]
  • [lex.string]

Introduction

This paper proposes substantial changes to [lex.phases], [lex.ccon], and [lex.string] intended to address the following core issues as well as several other more minor issues.

• Core Issue 411: Use of universal-character-name in character versus string literals
  
• Core Issue 1656: Encoding of numerically-escaped characters
  
• Core Issue 2333: Escape sequences in UTF-8 character literals

This paper follows a prior proposed resolution that only attempted to address CWG 2333. That proposed resolution was discussed on the core mailing list and at the June 24th, 2019 core issues processing teleconference. The resolution proposed in this paper addresses the feedback provided during those discussions and further review at the January 16th, 2020 core issues processing teleconference, the March 23rd, 2020 core issues processing teleconference, discussion on the core mailing list originating on June 28th, 2020, and further discussions on the core mailing list originating on July 7th, 2020.

The notes for CWG 2333 in the current active issues list (revision 100) state that discussion at the August 14th, 2017 core issues processing teleconference resulted in a determination that numeric escape sequences in UTF-8 character literals should be ill-formed. The issue has remained in drafting status since then.

SG16 discussed this issue during its October 17th, 2018 teleconference. The SG16 consensus was for a different resolution than is currently described in the active issues list. The SG16 consensus was based on the following observations:

• The current resolution in the active issues list contradicts existing practice. gcc, Clang, and Visual C++ all allow octal and hexadecimal escape sequences in UTF-8 literals.
• Octal and hexadecimal escape sequences in UTF-8 literals are useful for a number of purposes:
  • Embedding null characters: u8"\0"
  • Creating ill-formed code unit sequences for testing purposes.
  • Creating Modified UTF-8, CESU-8, and WTF-8 string literals. This entails two abilities:
    • Embedding U+0000 as an overlong UTF-8 sequence: u8"\xC0\x80"
    • Embedding lone surrogate code points as individual UTF-8 code unit sequences. For example, encoding U+D800 as u8"\xED\xA0\x80". (Note that use of \u escapes specifying surrogate code points is ill-formed).
  • Compatibility with existing log/debug systems that output literals with non-printable characters represented with escapes so as to facilitate copy/paste of such output into code.

The proposed resolution reflects the SG16 consensus.

CWG 411 is addressed by specifying different behavior for character literals vs string literals for characters that are not representable by a single code unit. For example, when the execution character set is UTF-8, '\u0153' is conditionally-supported, has type int and an implementation-defined value, but "\u0153" is a character array of length 3 containing the UTF-8 encoding of U+0153 (LATIN SMALL LIGATURE OE) and a null character (\xC5\x93\x00).

CWG 1656 is addressed by clarifying that numeric escape sequences denote code unit values in the execution character set; that the values are not subject to conversion from the encoding of the source file to the execution character set.

This paper does not attempt to address the existing issue concerning the order in which adjacent string literals are encoded and concatenated as reported at https://lists.isocpp.org/core/2020/07/9537.php (as of 2020-07-07, a core issue number has not been assigned for this issue).

Changes from P2029R1

• Rebased on N4861.
• Addressed core wording changes requested during the March 23rd, 2020 core issues processing teleconference.
  • Removed italics for uses of defined terms. Ialics are now consistently used when defining terms or when defining or using grammar terms.
  • [lex.phases]p1.5: Inlined the referenced footnote and removed the external link.
  • [lex.phases]p1.5: Updated for removal of character-escape-sequence.
  • [lex.phases]p1.6: Moved appending of a null character to string literals to translation phase 6.
  • [lex.ccon]: Modified conditional-escape-sequence-char to avoid ambiguity with universal-character-name, octal-escape-sequence, and hexadecimal-escape-sequence.
  • [lex.ccon]: Removed character-escape-sequence by making conditional-escape-sequence a peer of simple-escape-sequence and numeric-escape-sequence.
  • [lex.ccon]pX: Corrected sentence capitalization.
  • [lex.ccon]pX: Swapped the order of the encoding prefix and kind columns in the table.
  • [lex.ccon]p5: Inlined the referenced footnote and removed the external link.
  • [lex.ccon]pZ: Corrected the list introduction text to end with a colon rather than a period.
  • [lex.ccon]pZ: Corrected the subparagraph numbers to align with the top of their associated text.
  • [lex.ccon]pZ: Updated for removal of character-escape-sequence and to allow conditional-escape-sequences to specify non-character values.
  • [lex.ccon]pZ: Reworded to eliminate ambiguous subparagraph applicability.
  • [lex.ccon]pZ: Moved example qualification text to a footnote.
  • [lex.ccon]p6: Inlined the referenced footnote and removed the external link.
  • [lex.ccon]p6: Moved wording for conditional-escape-sequences to [lex.ccon]pZ.
  • [lex.string]pX: Removed the empty string literal examples.
  • [lex.string]pX: Corrected a link to the string-literal grammar term.
  • [lex.string]pX: Swapped the order of the encoding prefix and kind columns in the table.
  • [lex.string]pX: Added additional introductory text for the table of string literal kinds.
  • [lex.string]p12: Moved appending of a null character to string literals to translation phase 6.
  • [lex.string]pZ: Corrected the list introduction text to end with a colon rather than a period.
  • [lex.string]pZ: Corrected the subparagraph numbers to align with the top of their associated text.
  • [lex.string]pZ: Removed the reference to translation phase 7 now that appending a null character to string literals has been moved to translation phase 6.
  • [lex.string]pZ: Updated wording to clarify that a sequence of characters is encoded as a group rather than one at a time.
  • [lex.string]pZ: Updated wording to clarify the expected behavior for stateful character encodings.
  • [lex.string]pZ: Updated for removal of character-escape-sequence and to allow conditional-escape-sequences to specify non-character values.
• Addressed changes requested during discussion on the core mailing list originating on June 28th, 2020:
  • Added content to the introduction stating that this paper does not attempt to address the existing issues concerning the order in which adjacent string literals are encoded and concatenated.
  • [lex.ccon]pW, [lex.ccon]pX, [lex.ccon]pZ: Added the new paragraph W and reworded the others based on suggestions by Jens. These changes removed the need to define separate character literal kinds for ordinary vs wide non-encodable and multicharacter literals. This also restores the ability for a character literal to be both an ordinary character literal and a multicharacter literal.
  • [lex.ccon]pX, [lex.ccon]pZ: Replaced "applicable associated character encoding" with "literal's associated character encoding".
  • [lex.ccon]pX: Renamed conditional character literal to non-encodable character literal.
  • [lex.ccon]pX: Changed the associated character encoding for the non-encodable and multicharacter literal kinds from implementation-defined to match the encoding of the applicable execution character set. This avoided some otherwise ambiguous or incorrect references to associated character encodings.
  • [lex.ccon]pX: In table Y, merged adjacent cells that had the same contents.
  • [lex.string]pZ: Removed uses of "should" from the notes regarding handling of stateful encodings.
  • [lex.string]pZ: Added a note to clarify that character sequences may be encoded as a group such that the resulting code unit sequence may differ from the sequence that would be obtained by encoding each character independently.
• Addressed changes requested during further discussions on the core mailing list originating on July 7th, 2020:
  • [lex.phases]p6, [lex.string]p12: Adjusted wording to minimize duplicate normative wording regarding string literal null character appendment.
  • [lex.ccon]: Modified the wording for the conditional-escape-sequence-char grammar production to avoid referring to "members" of grammar productions.
• [lex.phases]p1.5: Removed the pre-existing footnote.
• [lex.string]pZ: Removed redundant wording for behavior that is implicit in the grammar for octal and hexadecimal escape sequences.
• Updated the acknowledgements section to recognize additional contributors.

Proposed resolution overview

The proposed wording changes are intended to resolve CWG 411, CWG 1656, and CWG 2333 by:

• Clarifying that hexadecimal and octal escape sequences:
  • are valid in u8, u, and U character literals. (CWG 2333)
  • specify values that need not correspond to valid code unit values for the literal's associated character encoding. (CWG 2333)
  • specify the execution-time value of character literals or individual code unit values of string literals; that the value expressed is not subject to conversion to the literal's associated character encoding. (CWG 1656)
  • specify values that may result in string literals that are ill-formed according to the literal's associated character encoding. (CWG 2333)
• Clarifying that characters that cannot be specified in a character literal because they require more than one code unit in the literal's associated character encoding may be specified in string literals. (CWG 411)

Additionally, the wording updates are intended to:

• Specify behavior for non-ordinary character and string literals when a character lacks representation in the literal's associated character encoding (wording was previously missing).
• Broaden use of the term multicharacter literal to allow it to be applicable to both ordinary and wide character literals.
• Specify that wide multicharacter literals are conditionally supported (consistent with multicharacter literals).
• Specify that ordinary and wide string literals that specify characters that are not representable in the literal's associated character encoding are conditionally supported (gcc 10 rejects).
• Specify that wide character literals that have a c-char-sequence that specifies a character that is not representable in the execution wide-character set or that cannot be encoded in a single code unit are conditionally supported (consistent with ordinary character literals).
• Remove non-normative wording that states that wchar_t is able to represent all members of the execution wide-character set as this contradicts long standing existing practice for some implementations (Visual C++ has a 16-bit wchar_t type, but encodes wide strings as UTF-16; some characters require multiple code units).
• Modernize the wording with current style preferences and terminology.
• Move appending of a null character to string literals from translation phase 7 to translation phase 6.

The wording changes introduce the following:

• The concept of an "associated character encoding" for character and string literals; this enables wording to be independent of the particular kind of literal (ordinary, wide or Unicode).
• New basic-c-char, basic-s-char, numeric-escape-sequence, and simple-escape-sequence-char grammar non-terminals; these factor the existing c-char, s-char, escape-sequence, and simple-escape-sequence non-terminals to give names to their productions for reference purposes.
• New conditional-escape-sequence and conditional-escape-sequence-char grammar non-terminals; these address previously missing grammar support for conditionally-supported implementation-defined escape sequences.
• A new non-encodable character literal kind; this provides a name for conditionally-supported implementation-defined ordinary and wide character literals that have c-char-sequences that specify characters that either lack representation in the literal's associated character encoding or that cannot be encoded as a single code unit.

Implementation impact

The author intends the proposed wording to reflect existing practice for the gcc and Clang compilers. If this proposal is adopted, neither of those compilers are expected to require updates. However, there is implementation impact to the Microsoft Visual C++ compiler.

Semantics of numeric-escape-sequences in UTF-8 literals

Consider the following (C++20) code:

constexpr const char8_t c[] = u8"\xc3\x80"; // UTF-8 encoding of U+00C0 {LATIN CAPITAL LETTER A WITH GRAVE}
#if defined(_MSC_VER)
  // Microsoft Visual C++:
  static_assert(c[0] == 0xC3); // UTF-8 encoding of U+00C3 {LATIN CAPITAL LETTER A WITH TILDE}
  static_assert(c[1] == 0x83);
  static_assert(c[2] == 0xC2); // UTF-8 encoding of U+0080 {<control>}
  static_assert(c[3] == 0x80);
  static_assert(c[4] == 0x00); // null
#else
  // Gcc and Clang:
  static_assert(c[0] == 0xC3); // UTF-8 encoding of U+00C0 {LATIN CAPITAL LETTER A WITH GRAVE}
  static_assert(c[1] == 0x80);
  static_assert(c[2] == 0x00); // null
#endif

The author does not know if the Visual C++ behavior exhibited for UTF-8 literals is intentional or reflective of a defect. The behavior is inconsistent for UTF-8 and UTF-16 literals. For UTF-8 literals, numeric escape sequences that specify values outside the range of char8_t are accepted as code point values and encoded as UTF-8. However, for UTF-16 literals, numeric escape sequences that specify values outside the range of char16_t are rejected rather than being considered a code point value and encoded as a UTF-16 surrogate pair.

Character literal type for characters not representable in the execution character set

Consider the following code assuming that the execution character set does not have representation for the specified Unicode code points.

auto c1 = '\uFF10';
extern char c1;
#ifdef _MSC_VER
static_assert('\uFF10' == '?');
#endif
auto c2 = '\U0001F235';
extern int c2;
#ifdef _MSC_VER
static_assert('\U0001F235' == '??');
#endif

<source>(1): warning C4566: character represented by universal-character-name '\uFF10' cannot be represented in the current code page (1252)
<source>(4): warning C4566: character represented by universal-character-name '\uFF10' cannot be represented in the current code page (1252)
<source>(6): warning C4566: character represented by universal-character-name '\U0001F235' cannot be represented in the current code page (1252)
<source>(9): warning C4566: character represented by universal-character-name '\U0001F235' cannot be represented in the current code page (1252)

Gcc and Clang both reject the above code regardless of whether those Unicode characters have representation in the execution character set. If they are representable, then the code is rejected (as permitted) because the characters cannot be encoded in a single code unit. If they are not representable (which only happens for gcc since Clang always targets UTF-8), then the code is rejected because the redeclaration of c1 as char does not match the deduced int type for its first declaration.

Acknowledgements

Thank you to Jens Maurer, Steve Downey, Alisdair Meredith, Corentin Jabot, and Richard Smith for their tireless feedback on numerous drafts of this paper!

Proposed resolution

These changes are relative to N4861.

The changes to [lex.ccon] and [lex.string] are rather pervasive. For ease of review, unchanged paragraphs in these sections are retained in the wording below. These paragraphs are introduced with "No changes to ..." and are highlighted with a blue background.

[lex.phases]

Change in 5.2 [lex.phases] paragraph 1.5:

Each basic source character set memberbasic-c-char, basic-s-char, and r-char in a character-literal or a string-literal, as well as each escape sequenceescape-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 ([lex.ccon], [lex.string]); is encoded in the literal's associated character encoding as specified in [lex.ccon] and [lex.string]. 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. ]}

Change in 5.2 [lex.phases] paragraph 1.6:
Drafting note: This addition duplicates wording in [lex.string], but seems important to include here.

Adjacent string literal tokensstring-literals are concatenated and a null character is appended to each string-literal as specified in [lex.string].

[lex.ccon]

Change in 5.13.3 [lex.ccon]:

character-literal:

encoding-prefix_opt ' c-char-sequence '

encoding-prefix: one of

u8  u  U  L

c-char-sequence:

c-char

c-char-sequence c-char

c-char:

any member of the basic source character set except the single-quote ', backslash \, or new-line character

basic-c-char

escape-sequence

universal-character-name

basic-c-char:

any member of the basic source character set except the single-quote ', backslash \, or new-line character

escape-sequence:

simple-escape-sequence

octal-escape-sequence

hexadecimal-escape-sequence

numeric-escape-sequence

conditional-escape-sequence

simple-escape-sequence: one of

\'  \"  \?  \\
\a  \b  \f  \n  \r  \t  \v 
\ simple-escape-sequence-char

simple-escape-sequence-char: one of

'  "  ?  \
a  b  f  n  r  t  v

numeric-escape-sequence:

octal-escape-sequence

hexadecimal-escape-sequence

octal-escape-sequence:

\ octal-digit

\ octal-digit octal-digit

\ octal-digit octal-digit octal-digit

hexadecimal-escape-sequence:

\x hexadecimal-digit

hexadecimal-escape-sequence hexadecimal-digit

conditional-escape-sequence:

\ conditional-escape-sequence-char

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

Delete 5.13.3 [lex.ccon] paragraph 1:
Drafting Note: The contents of paragraphs 1-5 were incorporated into new paragraphs.

A character-literal that does not begin with u8, u, U, or L is an ordinary character literal. An ordinary character literal that contains a single c-char representable in the execution character set has type char, with value equal to the numerical value of the encoding of the c-char in the execution character set. An ordinary character literal that contains more than one c-char is a multicharacter literal. A multicharacter literal, or an ordinary character literal containing a single c-char not representable in the execution character set, is conditionally-supported, has type int, and has an implementation-defined value.

Delete 5.13.3 [lex.ccon] paragraph 2:
Drafting Note: The contents of paragraphs 1-5 were incorporated into new paragraphs. The note regarding the range of single code unit values was removed.

A character-literal that begins with u8, such as u8'w', is a character-literal of type char8_t, known as a UTF-8 character literal. The value of a UTF-8 character literal is equal to its ISO/IEC 10646 code point value, provided that the code point value can be encoded as a single UTF-8 code unit. [ Note: That is, provided the code point value is in the range [0,7F] (hexadecimal). — 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.

Delete 5.13.3 [lex.ccon] paragraph 3:
Drafting Note: The contents of paragraphs 1-5 were incorporated into new paragraphs. The note regarding the range of singled code unit values was removed.

A character-literal that begins with the letter u, such as u'x', is a character-literal of type char16_t, known as a UTF-16 character literal. The value of a UTF-16 character literal is equal to its ISO/IEC 10646 code point value, provided that the code point value is representable with a single 16-bit code unit. [ Note: That is, provided the code point value is in the range [0,FFFF] (hexadecimal). — end note ] If the value is not representable with a single 16-bit code unit, the program is ill-formed. A UTF-16 character literal containing multiple c-chars is ill-formed.

Delete 5.13.3 [lex.ccon] paragraph 4:
Drafting Note: The contents of paragraphs 1-5 were incorporated into new paragraphs.

A character-literal that begins with U, such as U'y', is a character-literal of type char32_t, known as a UTF-32 character literal. The value of a UTF-32 character literal containing a single c-char is equal to its ISO/IEC 10646 code point value. A UTF-32 character literal containing multiple c-chars is ill-formed.

Delete 5.13.3 [lex.ccon] paragraph 5:
Drafting Note: The contents of paragraphs 1-5 were incorporated into new paragraphs. The note regarding the ability for wchar_t to store all values of the execution wide-character set is intentionally removed as it conflicts with long standing existing practice for some implementations.

A character-literal that begins with the letter L, such as L'z', is a wide-character literal. A wide-character literal has type wchar_t. ^{[ Footnote: They are intended for character sets where a character does not fit into a single byte. ]} The value of a wide-character literal containing a single c-char has value equal to the numerical value of the encoding of the c-char in the execution wide-character set, unless the c-char has no representation in the execution wide-character set, in which case the value is implementation-defined. [ Note: The type wchar_t is able to represent all members of the execution wide-character set (see [basic.fundamental]). — end note ] The value of a wide-character literal containing multiple c-chars is implementation-defined.

Add a new paragraph (W) before 5.13.3 [lex.ccon] paragraph 6:

A non-encodable character literal is a character-literal whose c-char-sequence consists of a single c-char that is not a numeric-escape-sequence and that specifies a character that either lacks representation in the literal's associated character encoding or that cannot be encoded as a single code unit. A multicharacter literal is a character-literal whose c-char-sequence consists of more than one c-char. The encoding-prefix of a non-encodable character literal or a multicharacter literal shall be absent or L. Such character-literals are conditionally-supported.

Add another new paragraph (X) and table (Y) before 5.13.3 [lex.ccon] paragraph 6:

The kind of a character-literal, its type, and its associated character encoding is determined by its encoding-prefix and its c-char-sequence as defined by table Y. The special cases for non-encodable character literals and multicharacter literals take precedence over their respective base kinds.

Table Y: Character literals

[tab:lex.ccon.literals]

Encoding prefix	Kind	Type	Associated character encoding	Example
none	ordinary character literal	char	encoding of the execution character set	'v'
	— non-encodable ordinary character literal	int		'\U0001F525'[ Footnote Y1: see below ]
	— ordinary multicharacter literal			'abcd'
L	wide character literal	wchar_t	encoding of the execution wide-character set	L'w'
	— non-encodable wide character literal			L'\U0001F32A'[ Footnote Y2: see below ]
	— wide multicharacter literal			L'abcd'
u8	UTF-8 character literal	char8_t	UTF-8	u8'x'
u	UTF-16 character literal	char16_t	UTF-16	u'y'
U	UTF-32 character literal	char32_t	UTF-32	U'z'

Y1) The example assumes that the specified character lacks representation in the execution character set, or that encoding it would require more than one code unit.
Y2) The example assumes that the specified character lacks representation in the execution wide-character set, or that encoding it would require more than one code unit.

Add a new paragraph (Z):

The value of a character-literal is determined as follows:

(Z.1) —	A non-encodable character literal or a multicharacter literal has an implementation-defined value.
(Z.2) —	A character-literal with a c-char-sequence consisting of a single numeric-escape-sequence that specifies an integer value v has a value as follows: — If v does not exceed the range of the character-literal's type, then the value is v. — Otherwise, if the character-literal's encoding-prefix is absent or L, then the value is implementation-defined. — Otherwise, the program is ill-formed.
(Z.3) —	A character-literal with a c-char-sequence consisting of a single conditional-escape-sequence is conditionally-supported and has an implementation-defined value.
(Z.4) —	A character-literal with a c-char-sequence consisting of a single basic-c-char, simple-escape-sequence, or universal-character-name is the code unit value of the specified character as encoded in the literal's associated character encoding. [ Note: If the specified character lacks representation in the literal's associated character encoding or if it cannot be encoded as a single code unit, then the literal is a non-encodable character literal. — end note ]

Change in 5.13.3 [lex.ccon] paragraph 6:
Drafting Note: The deleted text has been removed as redundant since it repeats information implicit in the grammar. The added note is content moved from the deleted footnote.

The character specified by a simple-escape-sequence is specified in table 8. [ Note: Using an escape sequence for a question mark is supported for compatibility with ISO C++ 2014 and ISO C. — end note ] Certain non-graphic characters, the single quote ', the double quote ", the question mark ?, ^{[ Footnote: Using an escape sequence for a question mark is supported for compatibility with ISO C++ 2014 and ISO C. ]} and the backslash \, can be represented according to Table 9. The double quote " and the question mark ?, can be represented as themselves or by the escape sequences \" and \? respectively, but the single quote ' and the backslash \ shall be represented by the escape sequences \' and \\ respectively. Escape sequences in which the character following the backslash is not listed in Table 9 are conditionally-supported, with implementation-defined semantics. An escape sequence specifies a single character.

Table 9: ESimple escape sequences

[tab:lex.ccon.esc]

new-line	NL(LF)	\n
horizontal tab	HT	\t
vertical tab	VT	\v
backspace	BS	\b
carriage return	CR	\r
form feed	FF	\f
alert	BEL	\a
backslash	\	\\
question mark	?	\?
single quote	'	\'
double quote	"	\"
octal number	ooo	\ooo
hex number	hhh	\xhhh

Delete 5.13.3 [lex.ccon] paragraph 7:
Drafting Note: Wording describing the form of octal and hexadecimal escape sequences has been removed as redundant; the form is implicit in the grammar.

The escape \ooo consists of the backslash followed by one, two, or three octal digits that are taken to specify the value of the desired character. The escape \xhhh consists of the backslash followed by x followed by one or more hexadecimal digits that are taken to specify the value of the desired character. There is no limit to the number of digits in a hexadecimal sequence. A sequence of octal or hexadecimal digits is terminated by the first character that is not an octal digit or a hexadecimal digit, respectively. The value of a character-literal is implementation-defined if it falls outside of the implementation-defined range defined for char (for character-literals with no prefix) or wchar_t (for character-literals prefixed by L). [ Note: If the value of a character-literal prefixed by u, u8, or U is outside the range defined for its type, the program is ill-formed. — end note ]

Delete 5.13.3 [lex.ccon] paragraph 8:
Drafting Note: The normative text was combined with wording for basic-c-char and simple-escape-sequence above. The deleted note duplicates normative text in 5.2 [lex.phases] paragraph 1.1.

A universal-character-name is translated to the encoding, in the appropriate execution character set, of the character named. If there is no such encoding, the universal-character-name is translated to an implementation-defined encoding. [ Note: In translation phase 1, a universal-character-name is introduced whenever an actual extended character is encountered in the source text. Therefore, all extended characters are described in terms of universal-character-names However, the actual compiler implementation may use its own native character set, so long as the same results are obtained. — end note ]

[lex.string]

Change in 5.13.5 [lex.string]:

string-literal:

encoding-prefix_opt " s-char-sequence_opt "

encoding-prefix_opt R raw-string

s-char-sequence:

s-char

s-char-sequence s-char

s-char:

any member of the basic source character set except the double-quote ", backslash \, or new-line character

basic-s-char

escape-sequence

universal-character-name

basic-s-char:

any member of the basic source character set except the double-quote ", backslash \, or new-line character

raw-string:

" d-char-sequence_opt ( r-char-sequence_opt ) d-char-sequence_opt "

r-char-sequence:

r-char

r-char-sequence r-char

r-char:

any member of the source character set, except a right parenthesis ) followed by

the initial d-char-sequence (which may be empty) followed by a double quote ".

d-char-sequence:

d-char

d-char-sequence d-char

d-char:

any member of the source character set except:

space, the left parenthesis (, the right parenthesis ), the backslash \, and the control characters
representing horizontal tab, vertical tab, form feed, and newline.

Add a new paragraph (X) and table (Y) before 5.13.5 [lex.string] paragraph 1:

The kind of a string-literal, its type, and its associated character encoding is determined by its encoding prefix as defined by table Y.

Table Y: String literals

[tab:lex.string.literals]

Encoding prefix	Kind	Type	Associated character encoding	Examples
none	ordinary string literal	array of n const char	encoding of the execution character set	"an ordinary string" R"(an ordinary raw string)"
L	wide string literal	array of n const wchar_t	encoding of the execution wide-character set	L"a wide string" LR"w(a wide raw string)w"
u8	UTF-8 string literal	array of n const char8_t	UTF-8	u8"a UTF-8 string" u8R"x(a UTF-8 raw string)x"
u	UTF-16 string literal	array of n const char16_t	UTF-16	u"a UTF-16 string" uR"y(a UTF-16 raw string)y"
U	UTF-32 string literal	array of n const char32_t	UTF-32	U"A UTF-32 string" UR"z(a UTF-32 raw string)z"

No changes to 5.13.5 [lex.string] paragraph 1:

A string-literal that has an R in the prefix is a raw string literal. The d-char-sequence serves as a delimiter. The terminating d-char-sequence of a raw-string is the same sequence of characters as the initial d-char-sequence. A d-char-sequence shall consist of at most 16 characters.

No changes to 5.13.5 [lex.string] paragraph 2:

[ Note: The characters '(' and ')' are permitted in a raw-string. Thus, R"delimiter((a|b))delimiter" is equivalent to "(a|b)". — end note ]

No changes to 5.13.5 [lex.string] paragraph 3:

[ Note: A source-file new-line in a raw string literal results in a new-line in the resulting execution string literal. Assuming no whitespace at the beginning of lines in the following example, the assert will succeed:

const char* p = R"(a\
b
c)";
assert(std::strcmp(p, "a\\\nb\nc") == 0);

— end note ]

No changes to 5.13.5 [lex.string] paragraph 4:

[ Example: The raw string

R"a(
)\
a"
)a"

is equivalent to "\n)\\\na\"\n". The raw string

R"(x = "\"y\"")"

is equivalent to "x = \"\\\"y\\\"\"". — end example ]

Delete 5.13.5 [lex.string] paragraph 5:
Drafting Note: The contents of paragraphs 5, 6, 8, 9, and 10 were incorporated into new paragraphs.

After translation phase 6, a string-literal that does not begin with an encoding-prefix is an ordinary string literal. An ordinary string literal has type "array of n const char" where n is the size of the string as defined below, has static storage duration ([basic.stc]), and is initialized with the given characters.

Delete 5.13.5 [lex.string] paragraph 6:
Drafting Note: The contents of paragraphs 5, 6, 8, 9, and 10 were incorporated into new paragraphs.

A string-literal that begins with u8, such as u8"asdf", is a UTF-8 string literal. A UTF-8 string literal has type "array of n const char8_t", where n is the size of the string as defined below; each successive element of the object representation ([basic.types]) has the value of the corresponding code unit of the UTF-8 encoding of the string.

No changes to 5.13.5 [lex.string] paragraph 7:

Ordinary string literals and UTF-8 string literals are also referred to as narrow string literals.

Delete 5.13.5 [lex.string] paragraph 8:
Drafting Note: The contents of paragraphs 5, 6, 8, 9, and 10 were incorporated into new paragraphs. The note has been deleted as redundant; the use of surrogate pairs is explicit in the UTF-16 encoding.

A string-literal that begins with u, such as u"asdf", is a UTF-16 string literal. A UTF-16 string literal has type "array of n const char16_t", where n is the size of the string as defined below; each successive element of the array has the value of the corresponding code unit of the UTF-16 encoding of the string. [ Note: A single c-char may produce more than one char16_t character in the form of surrogate pairs. A surrogate pair is a representation for a single code point as a sequence of two 16-bit code units. — end note ]

Delete 5.13.5 [lex.string] paragraph 9:
Drafting Note: The contents of paragraphs 5, 6, 8, 9, and 10 were incorporated into new paragraphs.

A string-literal that begins with U, such as U"asdf", is a UTF-32 string literal. A UTF-32 string literal has type "array of n const char32_t", where n is the size of the string as defined below; each successive element of the array has the value of the corresponding code unit of the UTF-32 encoding of the string.

Delete 5.13.5 [lex.string] paragraph 10:
Drafting Note: The contents of paragraphs 5, 6, 8, 9, and 10 were incorporated into new paragraphs.

A string-literal that begins with L, such as L"asdf", is a wide string literal. A wide string literal has type "array of n const wchar_t", where n is the size of the string as defined below; it is initialized with the given characters.

Change in 5.13.5 [lex.string] paragraph 11:

In translation phase 6 ([lex.phases]), adjacent string-literals are concatenated. If both string-literals have the same encoding-prefix, the resulting concatenated string-literal has 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 other operand. If a UTF-8 string literal token is adjacent to a wide string literal token, the program is ill-formed. Any other concatenations 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 setafter the string literal contents have been encoded in the string-literal's associated character encoding), a string-literal's initial rawness has no effect on the interpretation or well-formedness of the concatenation. — end note ] Table 11 has some examples of valid concatenations.

Table 11: String literal concatenations

[tab:lex.string.concat]

Source Means u"a" u"b" u"a" "b" "a" u"b" u"ab" u"ab" u"ab"

Source Means U"a" U"b" U"a" "b" "a" U"b" U"ab" U"ab" U"ab"

Source Means L"a" L"b" L"a" "b" "a" L"b" L"ab" L"ab" L"ab"

Characters in concatenated strings are kept distinct.

[ Example:

"\xA" "B"

contains the two characters '\xA' and 'B' after concatenation (and not the single hexadecimal character '\xAB'). — end example ]

Change in 5.13.5 [lex.string] paragraph 12:
Drafting note: The literal '\0' was replaced with "a null character" to avoid an otherwise needed correction to qualify the literal to be appended with an encoding prefix matching the kind of the string literal. The rationale regarding why a null character is appended was removed as unnecessary normative text. If desired, it could be restored as a note.

After any necessary concatenation, iIn translation phase 76 ([lex.phases]), after adjacent string-literals are concatenated, '\0'a null character is appended to every string-literal so that programs that scan a string can find its end.

Delete 5.13.5 [lex.string] paragraph 13:
Drafting note: This wording has been removed as misleading, incomplete, or redundant. String literal contents do not always have the same meaning as in character literals. The wording regarding single and double quotes is redundant with the grammar. The discussion of string length is unnecessary as string length is determined by encoding.

Escape sequences and universal-character-names in non-raw string literals have the same meaning as in character literals ([lex.ccon]), except that the single quote ' is representable either by itself or by the escape sequence \', and the double quote " shall be preceded by a \, and except that a universal-character-name in a UTF-16 string literal may yield a surrogate pair. In a narrow string literal, a universal-character-name may map to more than one char or char8_t element due to multibyte encoding. The size of a char32_t or wide string literal is the total number of escape sequences, universal-character-names, and other characters, plus one for the terminating U'\0' or L'\0'. The size of a UTF-16 string literal is the total number of escape sequences, universal-character-names, and other characters, plus one for each character requiring a surrogate pair, plus one for the terminating u'\0'. [ Note: The size of a char16_t string literal is the number of code units, not the number of characters. — end note ] [ Note: Any universal-character-names are required to correspond to a code point in the range [0,D800) or [E000,10FFFF] (hexadecimal) ([lex.charset]). — end note ] The size of a narrow string literal is the total number of escape sequences and other characters, plus at least one for the multibyte encoding of each universal-character-name, plus one for the terminating '\0'.

Change in 5.13.5 [lex.string] paragraph 14:
Drafting note: Wording for string literal object initialization has been moved to a new paragraph.

Evaluating a string-literal results in a string literal object with static storage duration ([basic.stc]), initialized from the given characters as specified above. Whether all string-literals are distinct (that is, are stored in nonoverlapping objects) and whether successive evaluations of a string-literal yield the same or a different object is unspecified. [ Note: The effect of attempting to modify a string-literal is undefined. — end note ]

Add a new paragraph (Z) after 5.13.5 [lex.string] paragraph 14:

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) in order as follows:

(Z.1) —	The sequence of characters denoted by each contiguous sequence of basic-s-chars, r-chars, simple-escape-sequences ([lex.ccon]), and universal-character-names ([lex.charset]) is encoded to a code unit sequence using the string-literal's associated character encoding. If a character lacks representation in the associated character encoding, then, for an ordinary string literal or a wide string literal, the string-literal is conditionally-supported and an implementation defined code unit sequence is encoded; otherwise, the string-literal is ill-formed. [ Note: The encoded code unit sequence may differ from the sequence of code units that would be obtained by encoding each character independently. — end note ] [ Note: For stateful character encodings, encoding of the first such sequence is assumed to begin with the initial encoding state; encoding of subsequent sequences is then assumed to begin with the final encoding state of the prior sequence. — end note ]
(Z.2) —	Each numeric-escape-sequence ([lex.ccon]) contributes a single code unit value with the numeric value of the octal or hexadecimal number. If the numeric value exceeds the range of the string-literal's code unit type, then, for an ordinary string literal or a wide string literal, an implementation-defined code unit value is encoded; otherwise, the string-literal is ill-formed. [ Note: These sequences are assumed to have no effect on encoding state for stateful character encodings. — end note ]
(Z.3) —	Each conditional-escape-sequence ([lex.ccon]) contributes an implementation-defined code unit sequence. [ Note: Whether these sequences have an effect on encoding state for stateful character encodings is implementation-defined. — end note ]