5 Lexical conventions [lex]

5.10 Identifiers [lex.name]

identifier:
	identifier-nondigit
	identifier identifier-nondigit
	identifier digit

identifier-nondigit:
	nondigit
	universal-character-name

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

An identifier is an arbitrarily long sequence of letters and digits. Each universal-character-name in an identifier shall designate a character whose encoding in ISO 10646 falls into one of the ranges specified in Table 2. The initial element shall not be a universal-character-name designating a character whose encoding falls into one of the ranges specified in Table 3. Upper- and lower-case letters are different. All characters are significant.21

Table 2 — Ranges of characters allowed

00A8	00AA	00AD	00AF	00B2-00B5
00B7-00BA	00BC-00BE	00C0-00D6	00D8-00F6	00F8-00FF
0100-167F	1681-180D	180F-1FFF
200B-200D	202A-202E	203F-2040	2054	2060-206F
2070-218F	2460-24FF	2776-2793	2C00-2DFF	2E80-2FFF
3004-3007	3021-302F	3031-D7FF
F900-FD3D	FD40-FDCF	FDF0-FE44	FE47-FFFD
10000-1FFFD	20000-2FFFD	30000-3FFFD	40000-4FFFD	50000-5FFFD
60000-6FFFD	70000-7FFFD	80000-8FFFD	90000-9FFFD	A0000-AFFFD
B0000-BFFFD	C0000-CFFFD	D0000-DFFFD	E0000-EFFFD

Table 3 — Ranges of characters disallowed initially (combining characters)

0300-036F

1DC0-1DFF

20D0-20FF

FE20-FE2F

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

override

final

In addition, some identifiers 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.

21)

On systems in which linkers cannot accept extended characters, an encoding of the universal-character-name may be used in forming valid external identifiers. For example, some otherwise unused character or sequence of characters may be used to encode the \u in a universal-character-name. Extended characters may produce a long external identifier, but C++ does not place a translation limit on significant characters for external identifiers. In C++, upper- and lower-case letters are considered different for all identifiers, including external identifiers.