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'c-char ' | (1) | |
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u8'c-char ' | (2) | (since C++17) |
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u'c-char ' | (3) | (since C++11) |
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U'c-char ' | (4) | (since C++11) |
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L'c-char ' | (5) | |
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'c-char-sequence ' | (6) | |
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L'c-char-sequence ' | (7) | (until C++23) |
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[edit]Explanation
1) Ordinary character literal, e.g.
'a' or
'\n' or
'\13'. Such literal has type
char and the value equal to
the representation ofc-char in theexecution character set(until C++23)the corresponding code point fromordinary literal encoding(since C++23).
2) UTF-8 character literal, e.g.
u8'a'. Such literal has type
char(until C++20)char8_t(since C++20) and the value equal to
ISO/IEC 10646 code point value of
c-char, provided that the code point value is representable with a single UTF-8 code unit (that is,
c-char is in the range 0x0-0x7F, inclusive).
3) UTF-16 character literal, e.g.
u'猫', but not
u'🍌' (
u'\U0001f34c'). Such literal has type
char16_t and the value equal to
ISO/IEC 10646 code point value of
c-char, provided that the code point value is representable with a single UTF-16 code unit (that is,
c-char is in the range 0x0-0xFFFF, inclusive).
4) UTF-32 character literal, e.g.
U'猫' or
U'🍌'. Such literal has type
char32_t and the value equal to
ISO/IEC 10646 code point value of
c-char.
5) Wide character literal, e.g.L'β' orL'猫'. Such literal has typewchar_t and the value equal tothe value ofc-char in the execution wide character set(until C++23)the corresponding code point from wide literal encoding(since C++23).
6)Ordinary multicharacter literal(until C++23)Multicharacter literal(since C++23), e.g.'AB', is conditionally-supported, has typeint and implementation-defined value.
7) Wide multicharacter literal, e.g.L'AB', is conditionally-supported, has typewchar_t and implementation-defined value.
[edit]Non-encodable characters
1-5) Given thatc-char is not a numeric escape sequence (see below), ifc-char is not representable in the literal’s associated character encoding or cannot be encoded as a single code unit in that encoding (e.g. a non-BMP value on Windows wherewchar_t is 16-bit), the program is ill-formed.
6) If any
c-char in
c-char-sequence cannot be encoded as a single code unit in
ordinary literal encoding, the program is ill-formed.
7) If any c-char in c-char-sequence cannot be encoded as a single code unit in wide literal encoding, the program is ill-formed. | (until C++23) |
[edit]Numeric escape sequences
Numeric (octal and hexadecimal) escape sequences can be used for specifying the value of the character.
If the character literal contains only one numeric escape sequence, and the value specified by the escape sequence is representable by the unsigned version of its type, the character literal has the same value as the specified value (possibly after conversion to the character type). A UTF-N character literal can have any value representable by its type. If the value does not correspond to a valid Unicode code point, or if the its corresponding code point is not representable as single code unit in UTF-N, it can still be specified by a numeric escape sequence with the value. E.g.u8'\xff' is well-formed and equal tochar8_t(0xFF). | (since C++23) |
If the value specified by a numeric escape sequence used in an ordinary or wide character literal is not representable bychar orwchar_t respectively, the value of the character literal is implementation-defined. | (until C++23) |
If the value specified by a numeric escape sequence used in an ordinary or wide character literal with onec-char is representable by the unsigned version of the underlying type ofchar orwchar_t respectively, the value of the literal is the integer value of that unsigned integer type and the specified value converted to the type of the literal. Otherwise, the program is ill-formed. | (since C++23) |
If the value specified by a numeric escape sequence used in a UTF-N character literal is not representable by the correspondingcharN_t,the value of the character literal is implementation-defined(until C++17)the program is ill-formed(since C++17). | (since C++11) |
Multicharacter literals were inherited by C from the B programming language. Although not specified by the C or C++ standard, most compilers (MSVC is a notable exception) implement multicharacter literals as specified in B: the values of each char in the literal initialize successive bytes of the resulting integer, in big-endian zero-padded right-adjusted order, e.g. the value of'\1' is0x00000001 and the value of'\1\2\3\4' is0x01020304.
In C, character constants such as'a' or'\n' have typeint, rather thanchar.
[edit]Example
#include <cstdint>#include <iomanip>#include <iostream>#include <string_view> template<typename CharT>void dump(std::string_view s,const CharT c){const uint8_t* data{reinterpret_cast<const uint8_t*>(&c)}; std::cout<< s<<"\t"<<std::hex<<std::uppercase<<std::setfill('0'); for(auto i{0U}; i!= sizeof(CharT);++i)std::cout<<std::setw(2)<<static_cast<unsigned>(data[i])<<' '; std::cout<<'\n';} void print(std::string_view str=""){std::cout<< str<<'\n';} int main(){ print("Ordinary character literals:");char c1='a'; dump("'a'", c1);char c2='\x2a'; dump("'*'", c2); print("\n""Ordinary multi-character literals:");int mc1='ab'; dump("'ab'", mc1);// implementation-definedint mc2='abc'; dump("'abc'", mc2);// implementation-defined print("\n""UTF-8 character literals:"); char8_t C1= u8'a'; dump("u8'a'", C1);// char8_t C2 = u8'¢'; dump("u8'¢'", C2); // error: ¢ maps to two UTF-8 code units// char8_t C3 = u8'猫'; dump("u8'猫'", C3); // error: 猫 maps to three UTF-8 code units// char8_t C4 = u8'🍌'; dump("u8'🍌'", C4); // error: 🍌 maps to four UTF-8 code units print("\n""UTF-16 character literals:");char16_t uc1= u'a'; dump("u'a'", uc1);char16_t uc2= u'¢'; dump("u'¢'", uc2);char16_t uc3= u'猫'; dump("u'猫'", uc3);// char16_t uc4 = u'🍌'; dump("u'🍌'", uc4); // error: 🍌 maps to two UTF-16 code units print("\n""UTF-32 character literals:");char32_t Uc1= U'a'; dump("U'a'", Uc1);char32_t Uc2= U'¢'; dump("U'¢'", Uc2);char32_t Uc3= U'猫'; dump("U'猫'", Uc3);char32_t Uc4= U'🍌'; dump("U'🍌'", Uc4); print("\n""Wide character literals:");wchar_t wc1= L'a'; dump("L'a'", wc1);wchar_t wc2= L'¢'; dump("L'¢'", wc2);wchar_t wc3= L'猫'; dump("L'猫'", wc3);wchar_t wc4= L'🍌'; dump("L'🍌'", wc4);// unsupported on Windows since C++23}Possible output:
Ordinary character literals:'a' 61 '*' 2A Ordinary multi-character literals:'ab' 62 61 00 00 'abc' 63 62 61 00 UTF-8 character literals:u8'a' 61 UTF-16 character literals:u'a' 61 00 u'¢' A2 00 u'猫' 2B 73 UTF-32 character literals:U'a' 61 00 00 00 U'¢' A2 00 00 00 U'猫' 2B 73 00 00 U'🍌' 4C F3 01 00 Wide character literals:L'a' 61 00 00 00 L'¢' A2 00 00 00 L'猫' 2B 73 00 00 L'🍌' 4C F3 01 00
[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 912 | C++98 | non-encodable ordinary character literal was unspecified | specified as conditionally-supported |
| CWG 1024 | C++98 | multicharacter literal was required to be supported | made conditionally-supported |
| CWG 1656 | C++98 | the meaning of numeric escape sequence
in a character literal was unclear | specified |
| P1854R4 | C++98 | non-encodable character literals were conditionally-supported | the program is ill-formed |
[edit]References
- C++23 standard (ISO/IEC 14882:2024):
- 5.13.3 Character literals [lex.ccon]
- C++20 standard (ISO/IEC 14882:2020):
- 5.13.3 Character literals [lex.ccon]
- C++17 standard (ISO/IEC 14882:2017):
- 5.13.3 Character literals [lex.ccon]
- C++14 standard (ISO/IEC 14882:2014):
- 2.14.3 Character literals [lex.ccon]
- C++11 standard (ISO/IEC 14882:2011):
- 2.14.3 Character literals [lex.ccon]
- C++03 standard (ISO/IEC 14882:2003):
- 2.13.2 Character literals [lex.ccon]
- C++98 standard (ISO/IEC 14882:1998):
- 2.13.2 Character literals [lex.ccon]
[edit]See also
