String literal - cppreference.com (original) (raw)

Contents

[edit] Syntax
"s-char-seq (optional)" (1)
R"d-char-seq (optional)(r-char-seq (optional))d-char-seq (optional)" (2) (since C++11)
L"s-char-seq (optional)" (3)
LR"d-char-seq (optional)(r-char-seq (optional))d-char-seq (optional)" (4) (since C++11)
u8"s-char-seq (optional)" (5) (since C++11)
u8R"d-char-seq (optional)(r-char-seq (optional))d-char-seq (optional)" (6) (since C++11)
u"s-char-seq (optional)" (7) (since C++11)
uR"d-char-seq (optional)(r-char-seq (optional))d-char-seq (optional)" (8) (since C++11)
U"s-char-seq (optional)" (9) (since C++11)
UR"d-char-seq (optional)(r-char-seq (optional))d-char-seq (optional)" (10) (since C++11)

[edit] Explanation

s-char-seq - A sequence of one or more s-char s
s-char - One of a basic-s-char an escape sequence, as defined in escape sequences a universal character name, as defined in escape sequences
basic-s-char - A character from the basic source character set(until C++23)translation character set(since C++23), except the double-quote ", backslash \, or new-line character
d-char-seq - A sequence of one or more d-char s, at most 16 characters long
d-char - A character from the basic source character set(until C++23)basic character set(since C++23), except parentheses, backslash and spaces
r-char-seq - A sequence of one or more r-char s, except that it must not contain the closing sequence )d-char-seq"
r-char - A character from the basic source character set(until C++23)translation character set(since C++23)
Syntax Kind Type Encoding
(1,2) ordinary string literal const char[N] ordinary literal encoding
(3,4) wide string literal const wchar_t[N] wide literal encoding
(5,6) UTF-8 string literal const char[N] (until C++20) const char8_t[N] (since C++20) UTF-8
(7,8) UTF-16 string literal const char16_t[N] UTF-16
(9,10) UTF-32 string literal const char32_t[N] UTF-32

In the types listed in the table above, N is the number of encoded code units, which is determined below.

Ordinary and UTF-8(since C++11) string literals are collectively referred to as narrow string literals.

Evaluating a string literal results in a string literal object with static storage duration. Whether all string literals are stored in nonoverlapping objects and whether successive evaluations of a string literal yield the same or a different object is unspecified.

The effect of attempting to modify a string literal object is undefined.

bool b = "bar" == 3 + "foobar"; // can be true or false, unspecified   const char* pc = "Hello"; char* p = const_cast<char*>(pc); p[0] = 'M'; // undefined behavior

Raw string literals Raw string literals are string literals with a prefix containing R (syntaxes (2,4,6,8,10)). They do not escape any character, which means anything between the delimiters d-char-seq ( and **)**d-char-seq becomes part of the string. The terminating d-char-seq is the same sequence of characters as the initial d-char-seq. // OK: contains one backslash, // equivalent to "\\" R"(\)";   // OK: contains four \n pairs, // equivalent to "\\n\\n\\n\\n" R"(\n\n\n\n)";   // OK: contains one close-parenthesis, two double-quotes and one open-parenthesis, // equivalent to ")\"\"(" R"-()""()-";   // OK: equivalent to "\n)\\\na\"\"\n" R"a( )\ a"" )a";   // OK: equivalent to "x = \"\"\\y\"\"" R"(x = ""\y"")";   // R"<<(-_-)>>"; // Error: begin and end delimiters do not match // R"-()-"-()-"; // Error: )-" appears in the middle and terminates the literal (since C++11)

[edit] Initialization

String literal objects are initialized with the sequence of code unit values corresponding to the string literal’s sequence of s-char s and r-char s(since C++11), plus a terminating null character (U+0000), in order as follows:

  1. For each contiguous sequence of basic-s-char s, r-char s,(since C++11) simple escape sequences and universal character names, the sequence of character it denotes 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 the program is ill-formed.

If the associated character encoding is stateful, the first such sequence is encoded beginning with the initial encoding state and each subsequent sequence is encoded beginning with the final encoding state of the prior sequence.

  1. For each numeric escape sequence, given v as the integer value represented by the octal or hexadecimal number comprising the sequence of digits in the escape sequence, and T as the string literal’s array element type (see the table above):

If the associated character encoding is stateful, all such sequences have no effect on encoding state.

If the associated character encoding is stateful, it is implementation-defined what effect these sequences have on encoding state.

[edit] Concatenation

Adjacent string literals are concatenated at translation phase 6 (after preprocessing):

If an ordinary string literal is adjacent to a wide string literal, the behavior is undefined. (until C++11)
If an ordinary string literal is adjacent to a non-ordinary string literal, the concatenated string literal is of the kind of the latter. If a UTF-8 string literal is adjacent to a wide string literal, the program is ill-formed. Any other combination is conditionally supported with implementation-defined semantics.[1] (until C++23) Any other combination is ill-formed. (since C++23) (since C++11)

"Hello, " "world!" // at phase 6, the 2 string literals form "Hello, world!"   L"Δx = %" PRId16 // at phase 4, PRId16 expands to "d" // at phase 6, L"Δx = %" and "d" form L"Δx = %d"

  1. ↑ No known implementation supports such concatenation.

[edit] Unevaluated strings

The following contexts expect a string literal, but do not evaluate it:

It is unspecified whether non-ordinary string literals are allowed in these contexts, except that a literal operator name must use an ordinary string literal(since C++11). (until C++26)
Only ordinary string literals are allowed in these contexts.Each universal character name and each simple escape sequence in an unevaluated string is replaced by the member of the translation character set it denotes. An unevaluated string that contains a numeric escape sequence or a conditional escape sequence is ill-formed. (since C++26)

[edit] Notes

String literals can be used to initialize character arrays. If an array is initialized like char str[] = "foo";, str will contain a copy of the string "foo".

String literals are convertible and assignable to non-const char* or wchar_t* in order to be compatible with C, where string literals are of types char[N] and wchar_t[N]. Such implicit conversion is deprecated. (until C++11)
String literals are not convertible or assignable to non-const CharT*. An explicit cast (e.g. const_cast) must be used if such conversion is wanted. (since C++11)

A string literal is not necessarily a null-terminated character sequence: if a string literal has embedded null characters, it represents an array which contains more than one string.

const char* p = "abc\0def"; // std::strlen(p) == 3, but the array has size 8

If a valid hexadecimal digit follows a hexadecimal escape sequence in a string literal, it would fail to compile as an invalid escape sequence. String concatenation can be used as a workaround:

//const char* p = "\xfff"; // error: hexadecimal escape sequence out of range const char* p = "\xff""f"; // OK: the literal is const char[3] holding {'\xff','f','\0'}

Feature-test macro Value Std Feature
__cpp_char8_t 202207L (C++23)(DR20) char8_t compatibility and portability fix (allow initialization of (unsigned) char arrays from UTF-8 string literals)
__cpp_raw_strings 200710L (C++11) Raw string literals
__cpp_unicode_literals 200710L (C++11) Unicode string literals

[edit] Example

#include   // array1 and array2 contains the same values: char array1[] = "Foo" "bar"; char array2[] = {'F', 'o', 'o', 'b', 'a', 'r', '\0'};   const char* s1 = R"foo( Hello World )foo"; // same as const char* s2 = "\nHello\n World\n"; // same as const char* s3 = "\n" "Hello\n" " World\n";   const wchar_t* s4 = L"ABC" L"DEF"; // OK, same as const wchar_t* s5 = L"ABCDEF"; const char32_t* s6 = U"GHI" "JKL"; // OK, same as const char32_t* s7 = U"GHIJKL"; const char16_t* s9 = "MN" u"OP" "QR"; // OK, same as const char16_t* sA = u"MNOPQR";   // const auto* sB = u"Mixed" U"Types"; // before C++23 may or may not be supported by // the implementation; ill-formed since C++23   const wchar_t* sC = LR"--(STUV)--"; // OK, raw string literal   int main() { std::cout << array1 << ' ' << array2 << '\n' << s1 << s2 << s3 << std::endl; std::wcout << s4 << ' ' << s5 << ' ' << sC << std::endl; }

Output:

Foobar Foobar   Hello World   Hello World   Hello World   ABCDEF ABCDEF STUV

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
CWG 411(P2029R4) C++98 escape sequences in string literals werenot allowed to map to multiple code units allowed
CWG 1656(P2029R4) C++98 the characters denoted by numeric escapesequences in string literals were unclear made clear
CWG 1759 C++11 a UTF-8 string literal might have codeunits that are not representable in char char can represent all UTF-8 code units
CWG 1823 C++98 whether string literals are distinctwas implementation-defined distinctness is unspecified, and samestring literal can yield different object
CWG 2333(P2029R4) C++11 it was unclear whether numeric escape sequenceswere allowed in UTF-8/16/32 string literals made clear
CWG 2870 C++11 the concatenation result of twoordinary string literals was unclear made clear
P1854R4 C++98 ordinary and wide string literals with non-encodablecharacters were conditionally-supported programs with such literals are ill-formed
P2029R4 C++98 1. it was unclear whether string literals could contain non-encodable characters2. it was unclear whether string literals could contain numeric escape sequences such that the code units they represent are not representable in the literals' array element type 1. made conditionally-supported for ordinary and wide string literals[1]2. ill-formed if the code units are neither representable in the unsigned integer type corresponding to the underlying type
  1. ↑ P1854R4 was accepted as a DR later, overriding this resolution.

[edit] References

[edit] See also