Annex E (informative) Conformance with UAX #31 [uaxid]

E.1 General [uaxid.general]

E.2 R1 Default identifiers [uaxid.def]

E.2.1 General [uaxid.def.general]

E.2.2 R1b Stable identifiers [uaxid.def.stable]

E.3 R2 Immutable identifiers [uaxid.immutable]

E.4 R3 Pattern_White_Space and Pattern_Syntax characters [uaxid.pattern]

E.5 R4 Equivalent normalized identifiers [uaxid.eqn]

E.6 R5 Equivalent case-insensitive identifiers [uaxid.eqci]

E.7 R6 Filtered normalized identifiers [uaxid.filter]

E.8 R7 Filtered case-insensitive identifiers [uaxid.filterci]

E.9 R8 Hashtag identifiers [uaxid.hashtag]

E.1 General [uaxid.general]

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This Annex describes the choices made in application of UAX #31 (“Unicode Identifier and Pattern Syntax”) to C++ in terms of the requirements from UAX #31 and how they do or do not apply to this document.
In terms of UAX #31, this document conforms by meeting the requirements R1 “Default Identifiers” and R4 “Equivalent Normalized Identifiers” from UAX #31.
The other requirements from UAX #31, also listed below, are either alternatives not taken or do not apply to this document.

E.2 R1 Default identifiers [uaxid.def]

E.2.1 General [uaxid.def.general]

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UAX #31 specifies a default syntax for identifiers based on properties from the Unicode Character Database, UAX #44.
The general syntax is 
<Identifier> := <Start> <Continue>* (<Medial> <Continue>+)*
where <Start> has the XID_Start property, <Continue> has the XID_Continue property, and <Medial> is a list of characters permitted between continue characters.
UAX #31 also specifies some standard profiles, including the Mathematical Compatability Notation profile.
For C++ we apply the Mathematical Compatability Notation profile, and we add the character U+005f low line, or _, to the set of permitted <Start> characters.
The <Medial> set is empty.
In the grammar used in UAX #31, this is 
<Identifier> := <Start> <Continue>*
<Start> := XID_Start + ID_Compat_Math_Start + U+005f
<Continue> := <Start> + XID_Continue + ID_Compat_Math_Continue
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This is described in the C++ grammar in [lex.name], where identifier is formed from identifier-start or identifier followed by identifier-continue.

E.2.2 R1b Stable identifiers [uaxid.def.stable]

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An implementation of UAX #31 may choose to guarantee that identifiers are stable across versions of the Unicode Standard.
Once a string qualifies as an identifier it does so in all future versions.
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C++ does not make this guarantee, except to the extent that UAX #31 guarantees the stability of the XID_Start and XID_Continue properties.

E.3 R2 Immutable identifiers [uaxid.immutable]

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An implementation may choose to guarantee that the set of identifiers will never change by fixing the set of code points allowed in identifiers forever.
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C++ does not choose to make this guarantee.
As scripts are added to Unicode, additional characters in those scripts may become available for use in identifiers.

E.4 R3 Pattern_White_Space and Pattern_Syntax characters [uaxid.pattern]

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UAX #31 describes how formal languages such as computer languages should describe and implement their use of whitespace and syntactically significant characters during the processes of lexing and parsing.
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This document does not claim conformance with this requirement from UAX #31.

E.5 R4 Equivalent normalized identifiers [uaxid.eqn]

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UAX #31 requires that implementations describe how identifiers are compared and considered equivalent.
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This document requires that identifiers be in Normalization Form C and therefore identifiers that compare the same under NFC are equivalent.
This is described in [lex.name].

E.6 R5 Equivalent case-insensitive identifiers [uaxid.eqci]

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This document considers case to be significant in identifier comparison, and does not do any case folding.
This requirement from UAX #31 does not apply to this document.

E.7 R6 Filtered normalized identifiers [uaxid.filter]

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If any characters are excluded from normalization, UAX #31 requires a precise specification of those exclusions.
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This document does not make any such exclusions.

E.8 R7 Filtered case-insensitive identifiers [uaxid.filterci]

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C++ identifiers are case sensitive, and therefore this requirement from UAX #31 does not apply.

E.9 R8 Hashtag identifiers [uaxid.hashtag]

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There are no hashtags in C++, so this requirement from UAX #31 does not apply.