2 Lexical conventions [lex]

2.10 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
1

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 [tab:charname.allowed]. The initial element shall not be a universal-character-name designating a character whose encoding falls into one of the ranges specified in Table [tab:charname.disallowed]. 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
2

The identifiers in Table [tab:identifiers.special] 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
3

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.