[lex.name] # 5 Lexical conventions [[lex]](./#lex) ## 5.11 Identifiers [lex.name] [identifier:](#nt:identifier "5.11 Identifiers [lex.name]") [*identifier-start*](#nt:identifier-start "5.11 Identifiers [lex.name]") [*identifier*](#nt:identifier "5.11 Identifiers [lex.name]") [*identifier-continue*](#nt:identifier-continue "5.11 Identifiers [lex.name]") [identifier-start:](#nt:identifier-start "5.11 Identifiers [lex.name]") [*nondigit*](#nt:nondigit "5.11 Identifiers [lex.name]") an element of the translation character set with the Unicode property XID_Start [identifier-continue:](#nt:identifier-continue "5.11 Identifiers [lex.name]") [*digit*](#nt:digit "5.11 Identifiers [lex.name]") [*nondigit*](#nt:nondigit "5.11 Identifiers [lex.name]") an element of the translation character set with the Unicode property XID_Continue [nondigit:](#nt:nondigit "5.11 Identifiers [lex.name]") 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:](#nt:digit "5.11 Identifiers [lex.name]") one of 0 1 2 3 4 5 6 7 8 9 [1](#1) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/lex.tex#L928) [*Note [1](#note-1)*: The character properties XID_Start and XID_Continue are described by UAX #44 of the Unicode Standard[.](#1.sentence-1)[13](#footnote-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.") — *end note*] The program is ill-formed if an [*identifier*](#nt:identifier "5.11 Identifiers [lex.name]") does not conform to Normalization Form C as specified in the Unicode Standard[.](#1.sentence-2) [*Note [2](#note-2)*: Identifiers are case-sensitive[.](#1.sentence-3) — *end note*] [*Note [3](#note-3)*: [[uaxid]](uaxid "Annex E (informative) Conformance with UAX #31") compares the requirements of UAX #31 of the Unicode Standard with the C++ rules for identifiers[.](#1.sentence-4) — *end note*] [*Note [4](#note-4)*: In translation phase 4,[*identifier*](#nt:identifier "5.11 Identifiers [lex.name]") also includes those [*preprocessing-token*](lex.pptoken#nt:preprocessing-token "5.5 Preprocessing tokens [lex.pptoken]")*s* ([[lex.pptoken]](lex.pptoken "5.5 Preprocessing tokens")) differentiated as keywords ([[lex.key]](lex.key "5.12 Keywords")) in the later translation phase 7 ([[lex.token]](lex.token "5.10 Tokens"))[.](#1.sentence-5) — *end note*] [2](#2) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/lex.tex#L962) The identifiers in Table [4](#tab:lex.name.special "Table 4: Identifiers with special meaning") have a special meaning when appearing in a certain context[.](#2.sentence-1) When referred to in the grammar, these identifiers are used explicitly rather than using the [*identifier*](#nt:identifier "5.11 Identifiers [lex.name]") grammar production[.](#2.sentence-2) Unless otherwise specified, any ambiguity as to whether a given[*identifier*](#nt:identifier "5.11 Identifiers [lex.name]") has a special meaning is resolved to interpret the token as a regular [*identifier*](#nt:identifier "5.11 Identifiers [lex.name]")[.](#2.sentence-3) Table [4](#tab:lex.name.special) — Identifiers with special meaning [[tab:lex.name.special]](./tab:lex.name.special) | [🔗](#tab:lex.name.special-row-1)
final | import | post | replaceable_if_eligible | | --- | --- | --- | --- | | [🔗](#tab:lex.name.special-row-2)
override | module | pre | trivially_relocatable_if_eligible | [3](#3) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/lex.tex#L988) In addition, some identifiers appearing as a [*token*](lex.token#nt:token "5.10 Tokens [lex.token]") or [*preprocessing-token*](lex.pptoken#nt:preprocessing-token "5.5 Preprocessing tokens [lex.pptoken]") are reserved for use by C++ implementations and shall not be used otherwise; no diagnostic is required[.](#3.sentence-1) - [(3.1)](#3.1) Each identifier that contains a double underscore__or begins with an underscore followed by an uppercase letter, other than those specified in this document (for example, __cplusplus ([[cpp.predefined]](cpp.predefined "15.12 Predefined macro names"))),is reserved to the implementation for any use[.](#3.1.sentence-1) - [(3.2)](#3.2) Each identifier that begins with an underscore isreserved to the implementation for use as a name in the global namespace[.](#3.2.sentence-1) [13)](#footnote-13)[13)](#footnoteref-13) On systems in which linkers cannot accept extended characters, an encoding of the [*universal-character-name*](lex.universal.char#nt:universal-character-name "5.3.2 Universal character names [lex.universal.char]") can be used in forming valid external identifiers[.](#footnote-13.sentence-1) For example, some otherwise unused character or sequence of characters can be used to encode the\u in a [*universal-character-name*](lex.universal.char#nt:universal-character-name "5.3.2 Universal character names [lex.universal.char]")[.](#footnote-13.sentence-2) Extended characters can produce a long external identifier, but C++ does not place a translation limit on significant characters for external identifiers[.](#footnote-13.sentence-3)