Escape sequences
Escape sequences are used to represent certain special characters within string literals and character literals.
The following escape sequences are available:
Escape sequence |
Description | Representation |
---|---|---|
Simple escape sequences | ||
\'
|
single quote | byte 0x27 in ASCII encoding
|
\"
|
double quote | byte 0x22 in ASCII encoding
|
\?
|
question mark | byte 0x3f in ASCII encoding
|
\\
|
backslash | byte 0x5c in ASCII encoding
|
\a
|
audible bell | byte 0x07 in ASCII encoding
|
\b
|
backspace | byte 0x08 in ASCII encoding
|
\f
|
form feed - new page | byte 0x0c in ASCII encoding
|
\n
|
line feed - new line | byte 0x0a in ASCII encoding
|
\r
|
carriage return | byte 0x0d in ASCII encoding
|
\t
|
horizontal tab | byte 0x09 in ASCII encoding
|
\v
|
vertical tab | byte 0x0b in ASCII encoding
|
Numeric escape sequences | ||
\nnn
|
arbitrary octal value | code unit nnn (1~3 octal digits)
|
\o{n...} (since C++23)
|
code unit n... (arbitrary number of octal digits)
| |
\xn...
|
arbitrary hexadecimal value | code unit n... (arbitrary number of hexadecimal digits)
|
\x{n...} (since C++23)
| ||
Conditional escape sequences[1] | ||
\c
|
Implementation-defined | Implementation-defined |
Universal character names | ||
\unnnn
|
arbitrary Unicode value; may result in several code units |
code point U+nnnn (4 hexadecimal digits)
|
\u{n...} (since C++23)
|
code point U+n... (arbitrary number of hexadecimal digits)
| |
\Unnnnnnnn
|
code point U+nnnnnnnn (8 hexadecimal digits)
| |
\N{NAME} (since C++23)
|
arbitrary Unicode character | character named by NAME (see below)
|
- ↑ Conditional escape sequences are conditionally-supported. The character
c
in each conditional escape sequence is a member of basic source character set(until C++23)basic character set(since C++23) that is not the character following the\
in any other escape sequence.
Contents |
[edit] Range of universal character names
If a universal character name corresponds to a code point that is not 0x24 ( |
(until C++11) |
If a universal character name corresponding to a code point of a member of basic source character set or control characters appear outside a character or string literal, the program is ill-formed. If a universal character name corresponds surrogate code point (the range 0xD800-0xDFFF, inclusive), the program is ill-formed. If a universal character name used in a UTF-16/32 string literal does not correspond to a code point in ISO/IEC 10646 (the range 0x0-0x10FFFF, inclusive), the program is ill-formed. |
(since C++11) (until C++20) |
If a universal character name corresponding to a code point of a member of basic source character set or control characters appear outside a character or string literal, the program is ill-formed. If a universal character name does not correspond to a code point in ISO/IEC 10646 (the range 0x0-0x10FFFF, inclusive) or corresponds to a surrogate code point (the range 0xD800-0xDFFF, inclusive), the program is ill-formed. |
(since C++20) (until C++23) |
If a universal character name corresponding to a scalar value of a character in the basic character set or a control character appear outside a character or string literal, the program is ill-formed. If a universal character name does not correspond to a scalar value of a character in the translation character set, the program is ill-formed. |
(since C++23) |
Named universal character escapes
A universal character name of the syntax above is a named universal character. It designates the corresponding character in the Unicode Standard (chapter 4.8 Name) if the n-char-sequence is equal to its character name or to one of its character name aliases of type “control”, “correction”, or “alternate”; otherwise, the program is ill-formed. These aliases are listed in the Unicode Character Database’s NameAliases.txt. None of these names or aliases have leading or trailing spaces. A valid n-char-sequence must contain only uppercase Latin letters A through Z, digits, space, and hyphen-minus. Other characters never occur in a Unicode character name, and thus their appearance in a n-char-sequence always renders the program ill-formed. |
(since C++23) |
[edit] Notes
\0 is the most commonly used octal escape sequence, because it represents the terminating null character in null-terminated strings.
The new-line character \n has special meaning when used in text mode I/O: it is converted to the OS-specific newline representation, usually a byte or byte sequence. Some systems mark their lines with length fields instead.
Octal escape sequences have a limit of three octal digits, but terminate at the first character that is not a valid octal digit if encountered sooner.
Hexadecimal escape sequences have no length limit and terminate at the first character that is not a valid hexadecimal digit. If the value represented by a single hexadecimal escape sequence does not fit the range of values represented by the character type used in this string literal (char, char8_t, (since C++20)char16_t, char32_t, (since C++11)or wchar_t), the result is unspecified.
A universal character name in a narrow string literal or a 16-bit string literal may map to more than one code unit, e.g. \U0001f34c is 4 char code units in UTF-8 (\xF0\x9F\x8D\x8C) and 2 char16_t code units in UTF-16 (\xD83C\xDF4C). |
(since C++11) |
The question mark escape sequence \? is used to prevent trigraphs from being interpreted inside string literals: a string such as "??/" is compiled as "\", but if the second question mark is escaped, as in "?\?/", it becomes "??/". As trigraphs have been removed from C++, the question mark escape sequence is no longer necessary. It is preserved for compatibility with C++14 (and former revisions) and C.(since C++17)
Feature-test macro | Value | Std | Feature |
---|---|---|---|
__cpp_named_character_escapes |
202207L | (C++23) | Named universal character escapes |
[edit] Example
Output:
This is a test She said, "Sells she seashells on the seashore?"
[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 505 | C++98 | the behavior was undefined if the character following a backslash was not one of those specified in the table |
made conditionally supported (semantic is implementation-defined) |
[edit] See also
C documentation for Escape sequence
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