5 Lexical conventions [lex]

5.1 Separate translation [lex.separate]

1
#
The text of the program is kept in units called source files in this document.
A source file together with all the headers and source files included via the preprocessing directive #include, less any source lines skipped by any of the conditional inclusion preprocessing directives, is called a preprocessing translation unit.
[Note 1: 
A C++ program need not all be translated at the same time.
— end note]
2
#
[Note 2: 
Previously translated translation units and instantiation units can be preserved individually or in libraries.
The separate translation units of a program communicate ([basic.link]) by (for example) calls to functions whose identifiers have external or module linkage, manipulation of objects whose identifiers have external or module linkage, or manipulation of data files.
Translation units can be separately translated and then later linked to produce an executable program.
— end note]

5.2 Phases of translation [lex.phases]

1
#
The precedence among the syntax rules of translation is specified by the following phases.7
1.
An implementation shall support input files that are a sequence of UTF-8 code units (UTF-8 files).
It may also support an implementation-defined set of other kinds of input files, and, if so, the kind of an input file is determined in an implementation-defined manner that includes a means of designating input files as UTF-8 files, independent of their content.
[Note 1: 
In other words, recognizing the U+feff byte order mark is not sufficient.
— end note]
If an input file is determined to be a UTF-8 file, then it shall be a well-formed UTF-8 code unit sequence and it is decoded to produce a sequence of Unicode scalar values.
A sequence of translation character set elements is then formed by mapping each Unicode scalar value to the corresponding translation character set element.
In the resulting sequence, each pair of characters in the input sequence consisting of U+000d carriage return followed by U+000a line feed, as well as each U+000d carriage return not immediately followed by a U+000a line feed, is replaced by a single new-line character.
For any other kind of input file supported by the implementation, characters are mapped, in an implementation-defined manner, to a sequence of translation character set elements ([lex.charset]), representing end-of-line indicators as new-line characters.
2.
If the first translation character is U+feff byte order mark, it is deleted.
Each sequence of a backslash character (\) immediately followed by zero or more whitespace characters other than new-line followed by a new-line character is deleted, splicing physical source lines to form logical source lines.
Only the last backslash on any physical source line shall be eligible for being part of such a splice.
[Note 2: 
Line splicing can form a universal-character-name ([lex.charset]).
— end note]
A source file that is not empty and that does not end in a new-line character, or that ends in a splice, shall be processed as if an additional new-line character were appended to the file.
3.
The source file is decomposed into preprocessing tokens ([lex.pptoken]) and sequences of whitespace characters (including comments).
A source file shall not end in a partial preprocessing token or in a partial comment.8
Each comment is replaced by one space character.
New-line characters are retained.
Whether each nonempty sequence of whitespace characters other than new-line is retained or replaced by one space character is unspecified.
As characters from the source file are consumed to form the next preprocessing token (i.e., not being consumed as part of a comment or other forms of whitespace), except when matching a c-char-sequence, s-char-sequence, r-char-sequence, h-char-sequence, or q-char-sequence, universal-character-names are recognized and replaced by the designated element of the translation character set.
The process of dividing a source file's characters into preprocessing tokens is context-dependent.
[Example 1: 
See the handling of < within a #include preprocessing directive ([cpp.include]).
— end example]
4.
Preprocessing directives are executed, macro invocations are expanded, and _Pragma unary operator expressions are executed.
A #include preprocessing directive causes the named header or source file to be processed from phase 1 through phase 4, recursively.
All preprocessing directives are then deleted.
5.
For a sequence of two or more adjacent string-literal tokens, a common encoding-prefix is determined as specified in [lex.string].
Each such string-literal token is then considered to have that common encoding-prefix.
6.
Adjacent string-literal tokens are concatenated ([lex.string]).
7.
Whitespace characters separating tokens are no longer significant.
Each preprocessing token is converted into a token ([lex.token]).
The resulting tokens constitute a translation unit and are syntactically and semantically analyzed and translated.
[Note 3: 
The process of analyzing and translating the tokens can occasionally result in one token being replaced by a sequence of other tokens ([temp.names]).
— end note]
It is implementation-defined whether the sources for module units and header units on which the current translation unit has an interface dependency ([module.unit], [module.import]) are required to be available.
[Note 4: 
Source files, translation units and translated translation units need not necessarily be stored as files, nor need there be any one-to-one correspondence between these entities and any external representation.
The description is conceptual only, and does not specify any particular implementation.
— end note]
8.
Translated translation units and instantiation units are combined as follows:
[Note 5: 
Some or all of these can be supplied from a library.
— end note]
Each translated translation unit is examined to produce a list of required instantiations.
[Note 6: 
This can include instantiations which have been explicitly requested ([temp.explicit]).
— end note]
The definitions of the required templates are located.
It is implementation-defined whether the source of the translation units containing these definitions is required to be available.
[Note 7: 
An implementation can choose to encode sufficient information into the translated translation unit so as to ensure the source is not required here.
— end note]
All the required instantiations are performed to produce instantiation units.
[Note 8: 
These are similar to translated translation units, but contain no references to uninstantiated templates and no template definitions.
— end note]
The program is ill-formed if any instantiation fails.
9.
All external entity references are resolved.
Library components are linked to satisfy external references to entities not defined in the current translation.
All such translator output is collected into a program image which contains information needed for execution in its execution environment.
7)7)
Implementations behave as if these separate phases occur, although in practice different phases can be folded together.
8)8)
A partial preprocessing token would arise from a source file ending in the first portion of a multi-character token that requires a terminating sequence of characters, such as a header-name that is missing the closing " or >.
A partial comment would arise from a source file ending with an unclosed /* comment.

5.3 Character sets [lex.charset]

1
#
The translation character set consists of the following elements:
  • (1.1)
    each abstract character assigned a code point in the Unicode codespace as specified in the Unicode Standard, and
  • (1.2)
    a distinct character for each Unicode scalar value not assigned to an abstract character.
[Note 1: 
Unicode code points are integers in the range [0, 10FFFF] (hexadecimal).
A surrogate code point is a value in the range [D800, DFFF] (hexadecimal).
A Unicode scalar value is any code point that is not a surrogate code point.
— end note]
2
#
The basic character set is a subset of the translation character set, consisting of 99 characters as specified in Table 1.
[Note 2: 
Unicode short names are given only as a means to identifying the character; the numerical value has no other meaning in this context.
— end note]
Table 1: Basic character set [tab:lex.charset.basic]
character
glyph
U+0009
character tabulation
U+000b
line tabulation
U+000c
form feed
U+0020
space
U+000a
line feed
new-line
U+0021
exclamation mark
!
U+0022
quotation mark
"
U+0023
number sign
#
U+0024
dollar sign
$
U+0025
percent sign
%
U+0026
ampersand
&
U+0027
apostrophe
'
U+0028
left parenthesis
(
U+0029
right parenthesis
)
U+002a
asterisk
*
U+002b
plus sign
+
U+002c
comma
,
U+002d
hyphen-minus
-
U+002e
full stop
.
U+002f
solidus
/
U+0030 ..
U+0039
digit zero .. nine
0 1 2 3 4 5 6 7 8 9
U+003a
colon
:
U+003b
semicolon
;
U+003c
less-than sign
<
U+003d
equals sign
=
U+003e
greater-than sign
>
U+003f
question mark
?
U+0040
commercial at
@
U+0041 ..
U+005a
latin capital letter a .. z
A B C D E F G H I J K L M
N O P Q R S T U V W X Y Z
U+005b
left square bracket
[
U+005c
reverse solidus
\
U+005d
right square bracket
]
U+005e
circumflex accent
^
U+005f
low line
_
U+0060
grave accent
`
U+0061 ..
U+007a
latin small letter a .. z
a b c d e f g h i j k l m
n o p q r s t u v w x y z
U+007b
left curly bracket
{
U+007c
vertical line
|
U+007d
right curly bracket
}
U+007e
tilde
~
4
#
A universal-character-name of the form \u hex-quad, \U hex-quad hex-quad, or \u{simple-hexadecimal-digit-sequence} designates the character in the translation character set whose Unicode scalar value is the hexadecimal number represented by the sequence of hexadecimal-digits in the universal-character-name.
The program is ill-formed if that number is not a Unicode scalar value.
5
#
A universal-character-name that is a named-universal-character 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.
[Note 3: 
These aliases are listed in the Unicode Character Database's NameAliases.txt.
None of these names or aliases have leading or trailing spaces.
— end note]
6
#
If a universal-character-name outside the c-char-sequence, s-char-sequence, or r-char-sequence of a character-literal or string-literal (in either case, including within a user-defined-literal) corresponds to a control character or to a character in the basic character set, the program is ill-formed.
[Note 4: 
A sequence of characters resembling a universal-character-name in an r-char-sequence ([lex.string]) does not form a universal-character-name.
— end note]
7
#
The basic literal character set consists of all characters of the basic character set, plus the control characters specified in Table 2.
Table 2: Additional control characters in the basic literal character set [tab:lex.charset.literal]
character
U+0000
null
U+0007
alert
U+0008
backspace
U+000d
carriage return
8
#
A code unit is an integer value of character type ([basic.fundamental]).
Characters in a character-literal other than a multicharacter or non-encodable character literal or in a string-literal are encoded as a sequence of one or more code units, as determined by the encoding-prefix ([lex.ccon], [lex.string]); this is termed the respective literal encoding.
The ordinary literal encoding is the encoding applied to an ordinary character or string literal.
The wide literal encoding is the encoding applied to a wide character or string literal.
9
#
A literal encoding or a locale-specific encoding of one of the execution character sets ([character.seq]) encodes each element of the basic literal character set as a single code unit with non-negative value, distinct from the code unit for any other such element.
[Note 5: 
A character not in the basic literal character set can be encoded with more than one code unit; the value of such a code unit can be the same as that of a code unit for an element of the basic literal character set.
— end note]
The U+0000 null character is encoded as the value 0.
No other element of the translation character set is encoded with a code unit of value 0.
The code unit value of each decimal digit character after the digit 0 (U+0030) shall be one greater than the value of the previous.
The ordinary and wide literal encodings are otherwise implementation-defined.
For a UTF-8, UTF-16, or UTF-32 literal, the implementation shall encode the Unicode scalar value corresponding to each character of the translation character set as specified in the Unicode Standard for the respective Unicode encoding form.

5.4 Preprocessing tokens [lex.pptoken]

1
#
Each preprocessing token that is converted to a token shall have the lexical form of a keyword, an identifier, a literal, or an operator or punctuator.
2
#
A preprocessing token is the minimal lexical element of the language in translation phases 3 through 6.
In this document, glyphs are used to identify elements of the basic character set ([lex.charset]).
The categories of preprocessing token are: header names, placeholder tokens produced by preprocessing import and module directives (import-keyword, module-keyword, and export-keyword), identifiers, preprocessing numbers, character literals (including user-defined character literals), string literals (including user-defined string literals), preprocessing operators and punctuators, and single non-whitespace characters that do not lexically match the other preprocessing token categories.
If a U+0027 apostrophe or a U+0022 quotation mark character matches the last category, the program is ill-formed.
If any character not in the basic character set matches the last category, the program is ill-formed.
Preprocessing tokens can be separated by whitespace; this consists of comments ([lex.comment]), or whitespace characters (U+0020 space, U+0009 character tabulation, new-line, U+000b line tabulation, and U+000c form feed), or both.
As described in [cpp], in certain circumstances during translation phase 4, whitespace (or the absence thereof) serves as more than preprocessing token separation.
Whitespace can appear within a preprocessing token only as part of a header name or between the quotation characters in a character literal or string literal.
3
#
If the input stream has been parsed into preprocessing tokens up to a given character:
  • (3.1)
    If the next character begins a sequence of characters that could be the prefix and initial double quote of a raw string literal, such as R", the next preprocessing token shall be a raw string literal.
    Between the initial and final double quote characters of the raw string, any transformations performed in phase 2 (line splicing) are reverted; this reversion shall apply before any d-char, r-char, or delimiting parenthesis is identified.
    The raw string literal is defined as the shortest sequence of characters that matches the raw-string pattern
    encoding-prefix R raw-string
  • (3.2)
    Otherwise, if the next three characters are <​::​ and the subsequent character is neither : nor >, the < is treated as a preprocessing token by itself and not as the first character of the alternative token <:.
  • (3.3)
    Otherwise, the next preprocessing token is the longest sequence of characters that could constitute a preprocessing token, even if that would cause further lexical analysis to fail, except that a header-name ([lex.header]) is only formed
[Example 1: #define R "x" const char* s = R"y"; // ill-formed raw string, not "x" "y" — end example]
4
#
The import-keyword is produced by processing an import directive ([cpp.import]), the module-keyword is produced by preprocessing a module directive ([cpp.module]), and the export-keyword is produced by preprocessing either of the previous two directives.
[Note 1: 
None has any observable spelling.
— end note]
5
#
[Example 2: 
The program fragment 0xe+foo is parsed as a preprocessing number token (one that is not a valid integer-literal or floating-point-literal token), even though a parse as three preprocessing tokens 0xe, +, and foo can produce a valid expression (for example, if foo is a macro defined as 1).
Similarly, the program fragment 1E1 is parsed as a preprocessing number (one that is a valid floating-point-literal token), whether or not E is a macro name.
— end example]
6
#
[Example 3: 
The program fragment x+++++y is parsed as x ++ ++ + y, which, if x and y have integral types, violates a constraint on increment operators, even though the parse x ++ + ++ y can yield a correct expression.
— end example]

5.5 Alternative tokens [lex.digraph]

1
#
Alternative token representations are provided for some operators and punctuators.9
2
#
In all respects of the language, each alternative token behaves the same, respectively, as its primary token, except for its spelling.10
The set of alternative tokens is defined in Table 3.
Table 3: Alternative tokens [tab:lex.digraph]
Alternative
Primary
Alternative
Primary
Alternative
Primary
<%
{
and
&&
and_eq
&=
%>
}
bitor
|
or_eq
|=
<:
[
or
||
xor_eq
^=
:>
]
xor
^
not
!
%:
#
compl
~
not_eq
!=
%:%:
##
bitand
&
9)9)
These include “digraphs” and additional reserved words.
The term “digraph” (token consisting of two characters) is not perfectly descriptive, since one of the alternative preprocessing-tokens is %:%: and of course several primary tokens contain two characters.
Nonetheless, those alternative tokens that aren't lexical keywords are colloquially known as “digraphs”.
10)10)
Thus the “stringized” values ([cpp.stringize]) of [ and <: will be different, maintaining the source spelling, but the tokens can otherwise be freely interchanged.

5.6 Tokens [lex.token]

1
#
There are five kinds of tokens: identifiers, keywords, literals,11 operators, and other separators.
Blanks, horizontal and vertical tabs, newlines, formfeeds, and comments (collectively, “whitespace”), as described below, are ignored except as they serve to separate tokens.
[Note 1: 
Some whitespace is required to separate otherwise adjacent identifiers, keywords, numeric literals, and alternative tokens containing alphabetic characters.
— end note]
11)11)
Literals include strings and character and numeric literals.

5.7 Comments [lex.comment]

1
#
The characters /* start a comment, which terminates with the characters */.
These comments do not nest.
The characters // start a comment, which terminates immediately before the next new-line character.
If there is a form-feed or a vertical-tab character in such a comment, only whitespace characters shall appear between it and the new-line that terminates the comment; no diagnostic is required.
[Note 1: 
The comment characters //, /*, and */ have no special meaning within a // comment and are treated just like other characters.
Similarly, the comment characters // and /* have no special meaning within a /* comment.
— end note]

5.9 Preprocessing numbers [lex.ppnumber]

1
#
Preprocessing number tokens lexically include all integer-literal tokens ([lex.icon]) and all floating-point-literal tokens ([lex.fcon]).
2
#
A preprocessing number does not have a type or a value; it acquires both after a successful conversion to an integer-literal token or a floating-point-literal token.

5.10 Identifiers [lex.name]

identifier-start:
nondigit
an element of the translation character set with the Unicode property XID_Start
identifier-continue:
digit
nondigit
an element of the translation character set with the Unicode property XID_Continue
nondigit: one of
a b c d e f g h i j k l m
n o p q r s t u v w x y z
A B C D E F G H I J K L M
N O P Q R S T U V W X Y Z _
digit: one of
0 1 2 3 4 5 6 7 8 9
1
#
[Note 1: 
The character properties XID_Start and XID_Continue are Derived Core Properties as described by UAX #44 of the Unicode Standard.13
— end note]
The program is ill-formed if an identifier does not conform to Normalization Form C as specified in the Unicode Standard.
[Note 2: 
Identifiers are case-sensitive.
— end note]
[Note 3: 
[uaxid] compares the requirements of UAX #31 of the Unicode Standard with the C++ rules for identifiers.
— end note]
[Note 4: 
In translation phase 4, identifier also includes those preprocessing-tokens ([lex.pptoken]) differentiated as keywords ([lex.key]) in the later translation phase 7 ([lex.token]).
— end note]
2
#
The identifiers in Table 4 have a special meaning when appearing in a certain context.
When referred to in the grammar, these identifiers are used explicitly rather than using the identifier grammar production.
Unless otherwise specified, any ambiguity as to whether a given identifier has a special meaning is resolved to interpret the token as a regular identifier.
Table 4: Identifiers with special meaning [tab:lex.name.special]
final
import
module
override
3
#
In addition, some identifiers appearing as a token or preprocessing-token are reserved for use by C++ implementations and shall not be used otherwise; no diagnostic is required.
  • (3.1)
    Each identifier that contains a double underscore __ or begins with an underscore followed by an uppercase letter is reserved to the implementation for any use.
  • (3.2)
    Each identifier that begins with an underscore is reserved to the implementation for use as a name in the global namespace.
13)13)
On systems in which linkers cannot accept extended characters, an encoding of the universal-character-name can be used in forming valid external identifiers.
For example, some otherwise unused character or sequence of characters can be used to encode the \u in a universal-character-name.
Extended characters can produce a long external identifier, but C++ does not place a translation limit on significant characters for external identifiers.

5.11 Keywords [lex.key]

keyword:
any identifier listed in Table 5
import-keyword
module-keyword
export-keyword
1
#
The identifiers shown in Table 5 are reserved for use as keywords (that is, they are unconditionally treated as keywords in phase 7) except in an attribute-token ([dcl.attr.grammar]).
[Note 1: 
The register keyword is unused but is reserved for future use.
— end note]
Table 5: Keywords [tab:lex.key]
alignas
constinit
false
public
true
alignof
const_cast
float
register
try
asm
continue
for
reinterpret_cast
typedef
auto
co_await
friend
requires
typeid
bool
co_return
goto
return
typename
break
co_yield
if
short
union
case
decltype
inline
signed
unsigned
catch
default
int
sizeof
using
char
delete
long
static
virtual
char8_t
do
mutable
static_assert
void
char16_t
double
namespace
static_cast
volatile
char32_t
dynamic_cast
new
struct
wchar_t
class
else
noexcept
switch
while
concept
enum
nullptr
template
const
explicit
operator
this
consteval
export
private
thread_local
constexpr
extern
protected
throw
2
#
Furthermore, the alternative representations shown in Table 6 for certain operators and punctuators ([lex.digraph]) are reserved and shall not be used otherwise.
Table 6: Alternative representations [tab:lex.key.digraph]
and
and_eq
bitand
bitor
compl
not
not_eq
or
or_eq
xor
xor_eq

5.12 Operators and punctuators [lex.operators]

1
#
The lexical representation of C++ programs includes a number of preprocessing tokens that are used in the syntax of the preprocessor or are converted into tokens for operators and punctuators:
preprocessing-operator: one of
# ## %: %:%:
operator-or-punctuator: one of
{ } [ ] ( )
<: :> <% %> ; : ...
? :: . .* -> ->* ~
! + - * / % ^ & |
= += -= *= /= %= ^= &= |=
== != < > <= >= <=> && ||
<< >> <<= >>= ++ -- ,
and or xor not bitand bitor compl
and_eq or_eq xor_eq not_eq
Each operator-or-punctuator is converted to a single token in translation phase 7.

5.13 Literals [lex.literal]

5.13.1 Kinds of literals [lex.literal.kinds]

1
#
There are several kinds of literals.14
[Note 1: 
When appearing as an expression, a literal has a type and a value category ([expr.prim.literal]).
— end note]
14)14)
The term “literal” generally designates, in this document, those tokens that are called “constants” in ISO C.

5.13.2 Integer literals [lex.icon]

binary-digit: one of
0 1
octal-digit: one of
0 1 2 3 4 5 6 7
nonzero-digit: one of
1 2 3 4 5 6 7 8 9
hexadecimal-prefix: one of
0x 0X
hexadecimal-digit: one of
0 1 2 3 4 5 6 7 8 9
a b c d e f
A B C D E F
unsigned-suffix: one of
u U
long-suffix: one of
l L
long-long-suffix: one of
ll LL
size-suffix: one of
z Z
1
#
In an integer-literal, the sequence of binary-digits, octal-digits, digits, or hexadecimal-digits is interpreted as a base N integer as shown in table Table 7; the lexically first digit of the sequence of digits is the most significant.
[Note 1: 
The prefix and any optional separating single quotes are ignored when determining the value.
— end note]
2
#
The hexadecimal-digits a through f and A through F have decimal values ten through fifteen.
[Example 1: 
The number twelve can be written 12, 014, 0XC, or 0b1100.
The integer-literals 1048576, 1'048'576, 0X100000, 0x10'0000, and 0'004'000'000 all have the same value.
— end example]
3
#
The type of an integer-literal is the first type in the list in Table 8 corresponding to its optional integer-suffix in which its value can be represented.
Table 8: Types of integer-literals [tab:lex.icon.type]
none
int
int
long int
unsigned int
long long int
long int
unsigned long int
long long int
unsigned long long int
u or U
unsigned int
unsigned int
unsigned long int
unsigned long int
unsigned long long int
unsigned long long int
l or L
long int
long int
long long int
unsigned long int
long long int
unsigned long long int
Both u or U
unsigned long int
unsigned long int
and l or L
unsigned long long int
unsigned long long int
ll or LL
long long int
long long int
unsigned long long int
Both u or U
unsigned long long int
unsigned long long int
and ll or LL
z or Z
the signed integer type corresponding
the signed integer type
  to std​::​size_t ([support.types.layout])
  corresponding to std​::​size_t
std​::​size_t
Both u or U
std​::​size_t
std​::​size_t
and z or Z
4
#
Except for integer-literals containing a size-suffix, if the value of an integer-literal cannot be represented by any type in its list and an extended integer type ([basic.fundamental]) can represent its value, it may have that extended integer type.
If all of the types in the list for the integer-literal are signed, the extended integer type is signed.
If all of the types in the list for the integer-literal are unsigned, the extended integer type is unsigned.
If the list contains both signed and unsigned types, the extended integer type may be signed or unsigned.
If an integer-literal cannot be represented by any of the allowed types, the program is ill-formed.
[Note 2: 
An integer-literal with a z or Z suffix is ill-formed if it cannot be represented by std​::​size_t.
— end note]

5.13.3 Character literals [lex.ccon]

encoding-prefix: one of
u8  u  U  L
basic-c-char:
any member of the translation character set except the U+0027 apostrophe,
   U+005c reverse solidus, or new-line character
simple-escape-sequence-char: one of
' " ? \ a b f n r t v
conditional-escape-sequence-char:
any member of the basic character set that is not an octal-digit, a simple-escape-sequence-char, or the characters N, o, u, U, or x
1
#
A multicharacter literal is a character-literal whose c-char-sequence consists of more than one c-char.
A multicharacter literal shall not have an encoding-prefix.
If a multicharacter literal contains a c-char that is not encodable as a single code unit in the ordinary literal encoding, the program is ill-formed.
Multicharacter literals are conditionally-supported.
2
#
The kind of a character-literal, its type, and its associated character encoding ([lex.charset]) are determined by its encoding-prefix and its c-char-sequence as defined by Table 9.
Table 9: Character literals [tab:lex.ccon.literal]
Encoding
Kind
Type
Associated char-
Example
prefix
acter encoding
none
char
ordinary literal
'v'
multicharacter literal
int
encoding
'abcd'
L
wchar_t
wide literal
L'w'
encoding
u8
char8_t
UTF-8
u8'x'
u
char16_t
UTF-16
u'y'
U
char32_t
UTF-32
U'z'
3
#
In translation phase 4, the value of a character-literal is determined using the range of representable values of the character-literal's type in translation phase 7.
A multicharacter literal has an implementation-defined value.
The value of any other kind of character-literal is determined as follows:
4
#
The character specified by a simple-escape-sequence is specified in Table 10.
[Note 1: 
Using an escape sequence for a question mark is supported for compatibility with ISO C++ 2014 and ISO C.
— end note]
Table 10: Simple escape sequences [tab:lex.ccon.esc]
character
U+000a
line feed
\n
U+0009
character tabulation
\t
U+000b
line tabulation
\v
U+0008
backspace
\b
U+000d
carriage return
\r
U+000c
form feed
\f
U+0007
alert
\a
U+005c
reverse solidus
\\
U+003f
question mark
\?
U+0027
apostrophe
\'
U+0022
quotation mark
\"

5.13.4 Floating-point literals [lex.fcon]

sign: one of
+ -
floating-point-suffix: one of
f l f16 f32 f64 f128 bf16 F L F16 F32 F64 F128 BF16
1
#
The type of a floating-point-literal ([basic.fundamental], [basic.extended.fp]) is determined by its floating-point-suffix as specified in Table 11.
[Note 1: 
The floating-point suffixes f16, f32, f64, f128, bf16, F16, F32, F64, F128, and BF16 are conditionally-supported.
— end note]
Table 11: Types of floating-point-literals [tab:lex.fcon.type]
type
none
double
f or F
float
l or L
long double
f16 or F16
std::float16_t
f32 or F32
std::float32_t
f64 or F64
std::float64_t
f128 or F128
std::float128_t
bf16 or BF16
std::bfloat16_t
2
#
In the significand, the sequence of digits or hexadecimal-digits and optional period are interpreted as a base N real number s, where N is 10 for a decimal-floating-point-literal and 16 for a hexadecimal-floating-point-literal.
[Note 2: 
Any optional separating single quotes are ignored when determining the value.
— end note]
If an exponent-part or binary-exponent-part is present, the exponent e of the floating-point-literal is the result of interpreting the sequence of an optional sign and the digits as a base 10 integer.
Otherwise, the exponent e is 0.
The scaled value of the literal is for a decimal-floating-point-literal and for a hexadecimal-floating-point-literal.
[Example 1: 
The floating-point-literals 49.625 and 0xC.68p+2 have the same value.
The floating-point-literals 1.602'176'565e-19 and 1.602176565e-19 have the same value.
— end example]
3
#
If the scaled value is not in the range of representable values for its type, the program is ill-formed.
Otherwise, the value of a floating-point-literal is the scaled value if representable, else the larger or smaller representable value nearest the scaled value, chosen in an implementation-defined manner.

5.13.5 String literals [lex.string]

basic-s-char:
any member of the translation character set except the U+0022 quotation mark,
   U+005c reverse solidus, or new-line character
r-char:
any member of the translation character set, except a U+0029 right parenthesis followed by
   the initial d-char-sequence (which may be empty) followed by a U+0022 quotation mark
d-char:
any member of the basic character set except:
   U+0020 space, U+0028 left parenthesis, U+0029 right parenthesis, U+005c reverse solidus,
   U+0009 character tabulation, U+000b line tabulation, U+000c form feed, and new-line
1
#
The kind of a string-literal, its type, and its associated character encoding ([lex.charset]) are determined by its encoding prefix and sequence of s-chars or r-chars as defined by Table 12 where n is the number of encoded code units as described below.
Table 12: String literals [tab:lex.string.literal]
Encoding
Kind
Type
Associated
Examples
prefix
character
encoding
none
array of n
const char
ordinary literal encoding
"ordinary string"
R"(ordinary raw string)"
L
array of n
const wchar_t
wide literal
encoding
L"wide string"
LR"w(wide raw string)w"
u8
array of n
const char8_t
UTF-8
u8"UTF-8 string"
u8R"x(UTF-8 raw string)x"
u
array of n
const char16_t
UTF-16
u"UTF-16 string"
uR"y(UTF-16 raw string)y"
U
array of n
const char32_t
UTF-32
U"UTF-32 string"
UR"z(UTF-32 raw string)z"
2
#
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.
3
#
[Note 1: 
The characters '(' and ')' are permitted in a raw-string.
Thus, R"delimiter((a|b))delimiter" is equivalent to "(a|b)".
— end note]
4
#
[Note 2: 
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]
5
#
[Example 1: 
The raw string R"a( )\ a" )a" is equivalent to "\n)\\\na\"\n".
The raw string R"(x = "\"y\"")" is equivalent to "x = \"\\\"y\\\"\"".
— end example]
6
#
Ordinary string literals and UTF-8 string literals are also referred to as narrow string literals.
7
#
The common encoding-prefix for a sequence of adjacent string-literals is determined pairwise as follows: If two string-literals have the same encoding-prefix, the common encoding-prefix is that encoding-prefix.
If one string-literal has no encoding-prefix, the common encoding-prefix is that of the other string-literal.
Any other combinations are ill-formed.
[Note 3: 
A string-literal's rawness has no effect on the determination of the common encoding-prefix.
— end note]
8
#
In translation phase 6 ([lex.phases]), adjacent string-literals are concatenated.
The lexical structure and grouping of the contents of the individual string-literals is retained.
[Example 2: 
"\xA" "B" represents the code unit '\xA' and the character 'B' after concatenation (and not the single code unit '\xAB').
Similarly, R"(\u00)" "41" represents six characters, starting with a backslash and ending with the digit 1 (and not the single character 'A' specified by a universal-character-name).
Table 13 has some examples of valid concatenations.
— end example]
Table 13: String literal concatenations [tab:lex.string.concat]
Source
Means
Source
Means
Source
Means
u"a"
u"b"
u"ab"
U"a"
U"b"
U"ab"
L"a"
L"b"
L"ab"
u"a"
"b"
u"ab"
U"a"
"b"
U"ab"
L"a"
"b"
L"ab"
"a"
u"b"
u"ab"
"a"
U"b"
U"ab"
"a"
L"b"
L"ab"
9
#
Evaluating a string-literal results in a string literal object with static storage duration ([basic.stc]).
Whether all string-literals are distinct (that is, are stored in nonoverlapping objects) and whether successive evaluations of a string-literal yield the same or a different object is unspecified.
[Note 4: 
The effect of attempting to modify a string literal object is undefined.
— end note]
10
#
String literal objects are initialized with the sequence of code unit values corresponding to the string-literal's sequence of s-chars (originally from non-raw string literals) and r-chars (originally from raw string literals), plus a terminating U+0000 null character, in order as follows:
  • (10.1)
    The sequence of characters denoted by each contiguous sequence of basic-s-chars, r-chars, simple-escape-sequences ([lex.ccon]), and universal-character-names ([lex.charset]) 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.
    [Note 5: 
    No character lacks representation in any Unicode encoding form.
    — end note]
    When encoding a stateful character encoding, implementations should encode the first such sequence beginning with the initial encoding state and encode subsequent sequences beginning with the final encoding state of the prior sequence.
    [Note 6: 
    The encoded code unit sequence can differ from the sequence of code units that would be obtained by encoding each character independently.
    — end note]
  • (10.2)
    Each numeric-escape-sequence ([lex.ccon]) contributes a single code unit with a value as follows:
    When encoding a stateful character encoding, these sequences should have no effect on encoding state.
  • (10.3)
    Each conditional-escape-sequence ([lex.ccon]) contributes an implementation-defined code unit sequence.
    When encoding a stateful character encoding, it is implementation-defined what effect these sequences have on encoding state.

5.13.6 Unevaluated strings [lex.string.uneval]

2
#
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.
3
#
An unevaluated-string is never evaluated and its interpretation depends on the context in which it appears.

5.13.7 Boolean literals [lex.bool]

boolean-literal:
false
true
1
#
The Boolean literals are the keywords false and true.
Such literals have type bool.

5.13.8 Pointer literals [lex.nullptr]

1
#
The pointer literal is the keyword nullptr.
It has type std​::​nullptr_t.
[Note 1: 
std​::​nullptr_t is a distinct type that is neither a pointer type nor a pointer-to-member type; rather, a prvalue of this type is a null pointer constant and can be converted to a null pointer value or null member pointer value.
— end note]

5.13.9 User-defined literals [lex.ext]

1
#
If a token matches both user-defined-literal and another literal kind, it is treated as the latter.
[Example 1: 
123_km is a user-defined-literal, but 12LL is an integer-literal.
— end example]
The syntactic non-terminal preceding the ud-suffix in a user-defined-literal is taken to be the longest sequence of characters that could match that non-terminal.
2
#
A user-defined-literal is treated as a call to a literal operator or literal operator template ([over.literal]).
To determine the form of this call for a given user-defined-literal L with ud-suffix X, first let S be the set of declarations found by unqualified lookup for the literal-operator-id whose literal suffix identifier is X ([basic.lookup.unqual]).
S shall not be empty.
3
#
If L is a user-defined-integer-literal, let n be the literal without its ud-suffix.
If S contains a literal operator with parameter type unsigned long long, the literal L is treated as a call of the form operator ""X(nULL)
Otherwise, S shall contain a raw literal operator or a numeric literal operator template ([over.literal]) but not both.
If S contains a raw literal operator, the literal L is treated as a call of the form operator ""X("n")
Otherwise (S contains a numeric literal operator template), L is treated as a call of the form operator ""X<'', '', ... ''>() where n is the source character sequence .
[Note 1: 
The sequence can only contain characters from the basic character set.
— end note]
4
#
If L is a user-defined-floating-point-literal, let f be the literal without its ud-suffix.
If S contains a literal operator with parameter type long double, the literal L is treated as a call of the form operator ""X(fL)
Otherwise, S shall contain a raw literal operator or a numeric literal operator template ([over.literal]) but not both.
If S contains a raw literal operator, the literal L is treated as a call of the form operator ""X("f")
Otherwise (S contains a numeric literal operator template), L is treated as a call of the form operator ""X<'', '', ... ''>() where f is the source character sequence .
[Note 2: 
The sequence can only contain characters from the basic character set.
— end note]
5
#
If L is a user-defined-string-literal, let str be the literal without its ud-suffix and let len be the number of code units in str (i.e., its length excluding the terminating null character).
If S contains a literal operator template with a non-type template parameter for which str is a well-formed template-argument, the literal L is treated as a call of the form operator ""X<str>()
Otherwise, the literal L is treated as a call of the form operator ""X(str, len)
6
#
If L is a user-defined-character-literal, let ch be the literal without its ud-suffix.
S shall contain a literal operator whose only parameter has the type of ch and the literal L is treated as a call of the form operator ""X(ch)
7
#
[Example 2: long double operator ""_w(long double); std::string operator ""_w(const char16_t*, std::size_t); unsigned operator ""_w(const char*); int main() { 1.2_w; // calls operator ""_w(1.2L) u"one"_w; // calls operator ""_w(u"one", 3) 12_w; // calls operator ""_w("12") "two"_w; // error: no applicable literal operator } — end example]
8
#
In translation phase 6 ([lex.phases]), adjacent string-literals are concatenated and user-defined-string-literals are considered string-literals for that purpose.
During concatenation, ud-suffixes are removed and ignored and the concatenation process occurs as described in [lex.string].
At the end of phase 6, if a string-literal is the result of a concatenation involving at least one user-defined-string-literal, all the participating user-defined-string-literals shall have the same ud-suffix and that suffix is applied to the result of the concatenation.
9
#
[Example 3: int main() { L"A" "B" "C"_x; // OK, same as L"ABC"_x "P"_x "Q" "R"_y; // error: two different ud-suffixes } — end example]