\infannex{uaxid}{Conformance with \UAX{31}}

\rSec1[uaxid.general]{General}

This Annex describes the choices made in application of

\UAX{31} (``Unicode Identifier and Pattern Syntax'')

to \Cpp{} 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.

\rSec1[uaxid.def]{R1 Default identifiers}

\rSec2[uaxid.def.general]{General}

\indextext{XID_Start}%

\indextext{XID_Continue}%

\UAX{31} specifies a default syntax for identifiers

based on properties from the Unicode Character Database, \UAX{44}.

The general syntax is

\begin{outputblock}

<Identifier> := <Start> <Continue>* (<Medial> <Continue>+)*

\end{outputblock}

where \tcode{<Start>} has the XID_Start property,

\tcode{<Continue>} has the XID_Continue property, and

\tcode{<Medial>} is a list of characters permitted between continue characters.

For \Cpp{} we add the character \unicode{005f}{low line}, or \tcode{_},

to the set of permitted \tcode{<Start>} characters,

the \tcode{<Medial>} set is empty, and

the \tcode{<Continue>} characters are unmodified.

In the grammar used in \UAX{31}, this is

\begin{outputblock}

<Identifier> := <Start> <Continue>*

<Start> := XID_Start + @\textrm{\ucode{005f}}@

<Continue> := <Start> + XID_Continue

\end{outputblock}

This is described in the \Cpp{} grammar in \ref{lex.name},

where \grammarterm{identifier} is formed from

\grammarterm{identifier-start} or

\grammarterm{identifier} followed by \grammarterm{identifier-continue}.

\rSec2[uaxid.def.stable]{R1b Stable identifiers}

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.

\Cpp{} does not make this guarantee,

except to the extent that \UAX{31} guarantees

the stability of the XID_Start and XID_Continue properties.

\rSec1[uaxid.immutable]{R2 Immutable identifiers}

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.

\Cpp{} 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.

\rSec1[uaxid.pattern]{R3 Pattern_White_Space and Pattern_Syntax characters}

\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.

This document does not claim conformance with this requirement from \UAX{31}.

\rSec1[uaxid.eqn]{R4 Equivalent normalized identifiers}

\UAX{31} requires that implementations describe

how identifiers are compared and considered equivalent.

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 \ref{lex.name}.

\rSec1[uaxid.eqci]{R5 Equivalent case-insensitive identifiers}

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.

\rSec1[uaxid.filter]{R6 Filtered normalized identifiers}

If any characters are excluded from normalization,

\UAX{31} requires a precise specification of those exclusions.

This document does not make any such exclusions.

\rSec1[uaxid.filterci]{R7 Filtered case-insensitive identifiers}

\Cpp{} identifiers are case sensitive, and

therefore this requirement from \UAX{31} does not apply.

\rSec1[uaxid.hashtag]{R8 Hashtag identifiers}

There are no hashtags in \Cpp{}, so this requirement from \UAX{31} does not apply.