2 Lexical conventions [lex]

A string literal is a sequence of characters (as defined in [lex.ccon]) surrounded by double quotes, optionally 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 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.

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

[ 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 ]

[ Example: The raw string

R"a(
)\
a"
)a"

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

R"(??)"

is equivalent to "\?\?". The raw string

R"#(
)??="
)#"

is equivalent to "\n)\?\?=\"\n".  — end example ]

After translation phase 6, a string literal that does not begin with an encoding-prefix 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 narrow string literals. A narrow string literal has type “array of n const char”, where n is the size of the string as defined below, and has static storage duration ([basic.stc]).

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 ([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 literal has been translated into a value from the appropriate character set), a string literal's initial rawness has no effect on the interpretation or well-formedness of the concatenation.  — end note ] Table [tab:lex.string.concat] has some examples of valid concatenations.

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 ([lex.phases]), '\0' is appended to every string literal so that programs that scan a string can find its end.

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 \. 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 shall 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'.