character constant
From cppreference.com
Syntax
' c-char '
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(1) | ||||||||
u ' c-char ' (since C11)
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(2) | ||||||||
U ' c-char ' (since C11)
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(3) | ||||||||
L ' c-char '
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(4) | ||||||||
' c-char-sequence '
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(5) | ||||||||
where
- c-char is either
- a character from the basic source character set minus single-quote (
'), backslash (\), or the newline character. - escape sequence: one of special character escapes \' \" \? \\ \a \b \f \n \r \t \v, hex escapes \x... or octal escapes \... as defined in escape sequences.
- a character from the basic source character set minus single-quote (
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(since C99) |
- c-char-sequence is a sequence of two or more c-chars.
1) single-byte integer character constant, e.g. 'a' or '\n' or '\13'. Such constant has type int and a value equal to the representation of c-char in the execution character set as a value of type char mapped to int. If c-char is not representable as a single byte in the execution character set, the value is implementation-defined.
2) 16-bit wide character constant, e.g. u'貓', but not u'🍌' (u'\U0001f34c'). Such constant has type char16_t and a value equal to the value of c-char in the 16-bit encoding produced by mbrtoc16 (normally UTF-16). If c-char is not representable or maps to more than one 16-bit character, the behavior is implementation-defined.
3) 32-bit wide character constant, e.g. U'貓' or U'🍌'. Such constant has type char32_t and a value equal to the value of c-char in in the 32-bit encoding produced by mbrtoc32 (normally UTF-32). If c-char is not representable or maps to more than one 32-bit character, the behavior is implementation-defined.
4) wide character constant, e.g. L'β' or L'貓. Such constant has type wchar_t and a value equal to the value of c-char in the execution wide character set (that is, the value that would be produced by mbtowc). If c-char is not representable or maps to more than one wide character (e.g. a non-BMP value on Windows where wchar_t is 16-bit), the behavior is implementation-defined .
5) multicharacter constant, e.g. 'AB', has type int and implementation-defined value.
Notes
Multicharacter constants were inherited by C from the B programming language. Although not specified by the C standard, compilers implement multicharacter constants as specified in B: the values of each char in the constant initialize successive bytes of the resulting integer, in big-endian zero-padded right-adjusted order, e.g. the value of '\1' is 0x00000001 and the value of '\1\2\3\4' is 0x01020304.
In C++, ordinary character constants have type char, rather than int.
Unlike integer constants, a character constant may have a negative value if char is signed: on such implementations '\xFF' is an int with the value -1.
When used in a controlling expression of #if or #elif, character constants may be interpreted in terms of the source character set, the execution character set, or some other implementation-defined character set.
Example
Run this code
#include <stddef.h> #include <stdio.h> #include <uchar.h> int main (void) { printf("constant value \n"); printf("-------- ----------\n"); // integer character constants, int c1='a'; printf("'a': %#010x\n", c1); int c2='🍌'; printf("'🍌': %#010x\n\n", c2); // implementation-defined // multicharacter constant int c3='ab'; printf("'ab': %#010x\n\n", c3); // implementation-defined // 16-bit wide character constants char16_t uc1 = u'a'; printf("'a': %#010x\n", (int)uc1); char16_t uc2 = u'¢'; printf("'¢': %#010x\n", (int)uc2); char16_t uc3 = u'猫'; printf("'猫': %#010x\n", (int)uc3); // implementation-defined (🍌 maps to two 16-bit characters) char16_t uc4 = u'🍌'; printf("'🍌': %#010x\n\n", (int)uc4); // 32-bit wide character constants char32_t Uc1 = U'a'; printf("'a': %#010x\n", (int)Uc1); char32_t Uc2 = U'¢'; printf("'¢': %#010x\n", (int)Uc2); char32_t Uc3 = U'猫'; printf("'猫': %#010x\n", (int)Uc3); char32_t Uc4 = U'🍌'; printf("'🍌': %#010x\n\n", (int)Uc4); // wide character constants wchar_t wc1 = L'a'; printf("'a': %#010x\n", (int)wc1); wchar_t wc2 = L'¢'; printf("'¢': %#010x\n", (int)wc2); wchar_t wc3 = L'猫'; printf("'猫': %#010x\n", (int)wc3); wchar_t wc4 = L'🍌'; printf("'🍌': %#010x\n\n", (int)wc4); }
Possible output:
constant value -------- ---------- 'a': 0x00000061 '🍌': 0xf09f8d8c 'ab': 0x00006162 'a': 0x00000061 '¢': 0x000000a2 '猫': 0x0000732b '🍌': 0x0000df4c 'a': 0x00000061 '¢': 0x000000a2 '猫': 0x0000732b '🍌': 0x0001f34c 'a': 0x00000061 '¢': 0x000000a2 '猫': 0x0000732b '🍌': 0x0001f34c
References
- C11 standard (ISO/IEC 9899:2011):
- 6.4.4.4 Character constants (p: 67-70)
- C99 standard (ISO/IEC 9899:1999):
- 6.4.4.4 Character constants (p: 59-61)
- C89/C90 standard (ISO/IEC 9899:1990):
- 3.1.3.4 Character constants