[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.")

â *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)

â *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)

â *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)

â *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)  

| [ð](#tab:lex.name.special-row-1)<br>final | import | post | replaceable_if_eligible |
| --- | --- | --- | --- |
| [ð](#tab:lex.name.special-row-2)<br>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)
