Doc. no. N2384=07-0244
Date:
2007-08-03
Project: Programming Language C++
Reply to: Lawrence Crowl <lawrence at
crowl.org>
Beman Dawes <bdawes at acm.org>
Two papers, N2209, UTF-8 String Literals, by Lawrence Crowl, and N2146, Raw String Literals (Revision 1), by Beman Dawes, propose additional forms of string literals for C++. Both have been approved by the Evolution Working Group and are ready for processing by the Core Working Group. Both papers make changes to the same text in the Working Paper. This proposal unifies the changed wording to avoid race conditions in editing the text.
The motivation, discussion, and other details from the original proposals remains unchanged.
Changes from Rev. 1:
"
d-char-sequence opt
[
r-char-sequenceopt
]
d-char-sequenceopt
"
was factored into a new non-terminal named
raw-string, at the request of Jens Maurer. Clark Nelson suggested the
name.The proposed text is the same as in the original papers (N2209, N2146), except:
Proposed text changes from rev. 1 of this document are high-lighted in pink.
Change 1.7 The C++ memory model [intro.memory] as indicated:
The fundamental storage unit in the C++ memory model is the
byte. A byte is at least large enough to contain
any member of the basic execution
character set the
eight-bit code units of the Unicode UTF-8 encoding
form and is composed of a contiguous sequence of
bits, the number of which is implementation-defined. The least
significant bit is called the low-order bit; the most
significant bit is called the high-order bit. The
memory available to a C++ program consists of one or more
sequences of contiguous bytes. Every byte has a unique
address.
Change 2.1 [lex.phases], paragraph 1 as indicated. (Note to reviewers: the ISO/IEC short name wording is the same as used in 2.2 Character sets [lex.charset] paragraph two.)
1. Physical source file characters are mapped, in an implementation-defined manner, to the basic source character set (introducing new-line characters for end-of-line indicators) if necessary. The set of physical source file characters accepted is implementation-defined. Trigraph sequences (2.3) are replaced by corresponding single-character internal representations. Any source file character not in the basic source character set (2.2) 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 (i.e. using the \uXXXX notation), are handled equivalently.)
Change 2.1 [lex.phases], paragraph 1 as indicated:
5. Each source character set member, escape sequence, or
universal-character-name in character literals and string
literals, and escape sequence in
character literals and regular string literals, is
converted to the corresponding member of the execution character
set (2.13.2, 2.13.4); if there is no corresponding member, it is
converted to an implementation-defined member other than the null
(wide) character.17)
Change 2.13.4 String literals [lex.string] as indicated:
string-literal:
"
s-char-sequenceopt"
u8"
s-char-sequenceopt"
u"
s-char-sequenceopt"
U"
s-char-sequenceopt"
L"
s-char-sequenceopt"
R
raw-string
u8R
raw-string
uR
raw-string
U
R
raw-string
LR
raw-string
s-char-sequence:
s-char
s-char-sequence s-chars-char:
any member of the source character set except the double-quote ", backslash \, or new-line character
escape-sequence
universal-character-nameraw-string:
"
d-char-sequence opt[
r-char-sequenceopt]
d-char-sequenceopt"
r-char-sequence:
r-char
r-char-sequence r-charr-char:
any member of the source character set, except a backslash \ followed by a u, or a right square bracket ]
followed by the initial d-char-sequence (which may be empty) followed by a double quote ".
universal-character-named-char-sequence:
d-char
d-char-sequence d-chard-char:
any member of the basic source character set, except space, the left square bracket [, the right square bracket ],
or the control characters representing horizontal tab, vertical tab, form feed, or new-line.
A string literal is a sequence of characters (as defined in
2.13.2) surrounded by double quotes, optionally
beginning with one of the
letters prefixed by
R
, u8
, u8R
, u
,
uR
,
U
, UR
, L
, or
LR
, as in
"..."
, R"[...]"
, u8"..."
, u8R"**[...]**"
,u"..."
,
uR"*@[...]*@"
,
U"..."
,UR"zzz[...]zzz"
, L"..."
, or LR"[...]"
, respectively.
A string literal that does not have
an R
in the prefix is a regular string literal. A
string literal that has an R
in the prefix is a raw
string literal. The terminating d-char-sequence of a
raw-string shall be the same sequence of characters as the
initial d-char-sequence, The maximum length of
d-char-sequence shall be 16 characters.
[Note: A source-file new-line in a raw string-literal results in a new-line in the resulting execution string-literal, unless preceded by a backslash. Assuming no whitespace at the beginning of lines in the following example, the assert will succeed:
const char * p =
R"[a\
--
end note]
b
c]";
assert(std::strcmp(p, "ab\nc") ==
0);
A string literal that does not begin with u8
, u
,
U
, or L
is an ordinary string literal,
and is initialized with the given
characters.
A string literal that begins with
u8
, such as u8
"asdf", is a
UTF-8 string literal and is initialized with the given characters
as encoded in UTF-8.
Ordinary string literals and UTF-8
string literals are also referred to as
a narrow string
literals.
An ordinary
narrow string literal has
type “array of n const char
”,
where n is the size of the string as defined below,
it and has static storage duration (3.7)
and is initialized with the given
characters.
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 has static storage
duration and is initialized with the given characters. A single
c-char may produce more than one char16_t
character in the form of surrogate pairs.
A string literal that begins with U
, such as
U"asdf"
, is a char32_t
string literal.
A char32_t
string literal has type “array of
n const char32_t
”, where n is
the size of the string as defined below; it has static storage
duration and is initialized with the given characters.
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 has static storage duration and is
initialized with the given characters.
Whether all string literals are distinct (that is, are stored in nonoverlapping objects) is implementation-defined. The effect of attempting to modify a string literal is undefined.
In translation phase 6 (2.1), adjacent string literals are concatenated. If both string literals have the same prefix, the resulting concatenated string literal has that prefix. If one string literal has no prefix, it is treated as a string literal of the same 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. —end note ] [ Example: Here are some examples of valid concatenations:
Table unchanged
—end example ] 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 ]
After any necessary concatenation, in translation phase 7
(2.1), ’\0’
is appended to every string
literal so that programs that scan a string can find its end.
Escape sequences in regular string
literals and universal-character-names in string
literals have the same meaning as in character literals (2.13.2),
except that the single quote ’
is
representable either by itself or by the escape sequence
\’
, and the double quote " shall be preceded
by a \
. In a narrow string literal, a
universal-character-name may map to more than one char 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
char16_t
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 ] Within
char32_t
and char16_t
literals, any
universal-character-names must be within the range 0x0 to
0x10FFFF. 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’
.