Improve readability of the C++ grammar by adding a syntax for groups and repetitions

1 In the syntax notation used in this document, syntactic categories non-terminal symbols are indicated by italic, sans-serif type, and literal words and characters terminal symbols in constant width type. A syntactic element is a terminal symbol, non-terminal symbol, or a group of syntactic elements. A group of syntactic elements is delimited by ⟪ and ⟫. Consecutive syntactic elements are listed from left to right. Alternatives are listed on separate lines except in a few cases where a long set of alternatives is marked by the phrase one of. If the text of an alternative is too long to fit on a line, the text is continued on subsequent lines indented from the first one.

2 An optional terminal or non-terminal symbol syntactic element is indicated by the postfix subscript _opt, so .

{ expression_opt }

indicates an optional expression enclosed in braces.

One or more repetitions of a syntactic element are indicated by the postfix subscript _seq.

[Example:

initializer-list:

initializer-clause ..._opt ⟪ , initializer-clause ..._opt ⟫_{seq opt}

This notation means that the non-terminal symbol initializer-list is matched by an initializer-clause, optionally followed by ..., followed by zero or more repetitions of ,, initializer-clause, and optionally .... — end example]

2 3 [Note: Names for syntactic categories have generally been chosen according to the following rules:

• X-name is a use of an identifier in a context that determines its meaning (e.g., class-name, typedef-name).
• X-id is an identifier with no context-dependent meaning (e.g., qualified-id).
• X-seq is one or more Xs without intervening delimiters (e.g., declaration-seq is a sequence of declarations).
• X-list is one or more Xs separated by intervening commas (e.g., identifier-list is a sequence of identifiers separated by commas).

— end note]

n-char-sequence n-char_seq
simple-hexadecimal-digit-sequence hexadecimal-digit_seq
h-char-sequence h-char_seq
q-char-sequence q-char_seq
c-char-sequence c-char_seq
simple-octal-digit-sequence octal-digit_seq
s-char-sequence s-char_seq
r-char-sequence r-char_seq
d-char-sequence d-char_seq
declaration-seq declaration_seq
attribute-specifier-seq attribute-specifier_seq
function-contract-specifier-seq function-contract-specifier_seq
lambda-specifier-seq lambda-specifier_seq
requirement-seq requirement_seq
statement-seq statement_seq
label-seq label_seq
cv-qualifier-seq cv-qualifier_seq
virt-specifier-seq virt-specifier_seq
balanced-token-seq balanced-token_seq
class-property-specifier-seq class-property-specifier_seq
handler-seq handler_seq
embed-parameter-seq embed-parameter_seq
pp-balanced-token-seq pp-balanced-token_seq

identifier:

identifier-start identifier-continue_{seq opt}

identifier identifier-continue

binary-literal:

⟪ one of 0b 0B ⟫ binary-digit ⟪ '_opt binary-digit ⟫_{seq opt}

0B binary-digit

binary-literal '_opt binary-digit

octal-literal:

0 ⟪ '_opt octal-digit ⟫_{seq opt}

octal-literal '_opt octal-digit

decimal-literal:

nonzero-digit ⟪ '_opt digit ⟫_{seq opt}

decimal-literal '_opt digit

hexadecimal-digit-sequence:

hexadecimal-digit ⟪ '_opt hexadecimal-digit ⟫_{seq opt}

hexadecimal-digit-sequence '_opt hexadecimal-digit

exponent-part:

⟪ one of e E ⟫ sign_opt digit-sequence

E sign_opt digit-sequence

binary-exponent-part:

⟪ one of p P ⟫ sign_opt digit-sequence

P sign_opt digit-sequence

digit-sequence:

digit ⟪ '_opt digit ⟫_{seq opt}

digit-sequence '_opt digit

[…]

raw-string:

" d-char-sequence _{seq opt} ( r-char-sequence _{seq opt} d-char-sequence _{seq opt} ) "

r-char-sequence:

r-char r-char-sequence_opt

r-char:

any member of the translation character set, except a U+0029 RIGHT PARENTHESIS followed by the initial d-char-sequence (which may be empty) d-char_{seq opt} followed by U+0022 QUOTATION MARK

[…]

decl-specifier-seq decl-specifiers-and-attributes:

decl-specifier_seq attribute-specifier-seq _{seq opt}

decl-specifier decl-specifier-seq

type-specifier-seq type-specifiers-and-attributes:

type-specifier_seq attribute-specifier-seq _{seq opt}

type-specifier type-specifier-seq

defining-type-specifier-seq defining-type-specifiers-and-attributes:

defining-type-specifier_seq attribute-specifier-seq _{seq opt}

defining-type-specifier type-specifier-seq

ptr-declarator:

ptr-operator_{seq opt} noptr-declarator

ptr-operator ptr-declarator

parameter-declaration-list:

parameter-declaration ⟪ , parameter-declaration ⟫_{seq opt}

parameter-declaration-list , parameter-declaration

initializer-list:

initializer-clause ..._opt ⟪ , initializer-clause ..._opt ⟫_{seq opt}

initializer-list , initializer-clause ..._opt

designated-initializer-list:

designated-initializer-clause ⟪ , designated-initializer-clause ⟫_{seq opt}

designated-initializer-list , designated-initializer-clause

enumerator-list:

enumerator-definition ⟪ , enumerator-definition ⟫_{seq opt}

enumerator-list , enumerator-definition

enumerator-definition:

enumerator

enumerator ⟪ = constant-expression ⟫_opt

using-declarator-list:

using-declarator ..._opt ⟪ , using-declarator ..._opt ⟫_{seq opt}

using-declarator-list , using-declarator ..._opt

annotation-list:

annotation ..._opt ⟪ , annotation ..._opt ⟫_{seq opt}

annotation-list , annotation ..._opt

selection-statement:

if constexpr_opt ( init-statement_opt condition ) statement ⟪ else statement ⟫_opt

if constexpr_opt ( init-statement_opt condition ) statement else statement

if !_opt consteval compound-statement ⟪ else statement ⟫_opt

if !_opt consteval compound-statement else statement

switch ( init-statement_opt condition ) statement

If the condition ([stmt.pre]) yields true, the first substatement is executed. If the else part statement of the selection statement selection-statement is present and the condition yields false, the second substatement is executed. If the first substatement is reached via a label, the condition is not evaluated and the second substatement is not executed. In the second form of if statement (the one including else), In an if statement where the else statement of the selection-statement is present, if the first substatement is also an if statement , then that inner if statement shall contain an else part statement.

sb-identifier-list:

sb-identifier ⟪ , sb-identifier ⟫_{seq opt}

sb-identifier-list , sb-identifier

static_assert-declaration:

static_assert ( constant-expression ⟪ , static_assert-message ⟫_opt )

static_assert ( constant-expression , static_assert-message )

[…] Integer values have the format:

units:

digits

digits thousands-sep units

digit_seq ⟪ thousands-sep digit_seq ⟫_{seq opt}

digits:

digit digits_opt

and floating-point values have:

floatval:

sign_opt units fractional_opt exponent_opt

sign_opt decimal-point digits digit_seq exponent_opt

fractional:

decimal-point digits digit_{seq opt}

exponent:

e ⟪ one of e E ⟫ sign_opt digits digit_seq

e:

e

E

where the number of digits between thousands-seps is as specified by do_grouping(). For parsing, if the digits digit_seq portion contains no thousands-separators, no grouping constraint is applied.

The format of the numeric monetary value is a decimal number:

value:

units fractional_opt

decimal-point digits adigit_seq

fractional:

decimal-point digits adigit_{seq opt}

if frac_digits() returns a positive value, or

value:

units

otherwise. The symbol decimal-point indicates the character returned by decimal_point(). The other symbols are defined as follows:

units:

digits

digits thousands-sep units

adigit_seq ⟪ thousands-sep adigit_seq ⟫_{seq opt}

In the syntax specification, the symbol adigit is any of the values ct.widen(c) for c in the range '0' through '9' (inclusive) and ct is a reference of type const ctype<charT>& […]

positive-integer:

nonzero-digit digit_{seq opt}

positive-integer digit

nonnegative-integer:

digit_seq

nonnegative-integer digit

[…]

relative-path:

filename

filename directory-separator relative-path

an empty path

⟪ filename directory-separator ⟫_{seq opt} filename_opt

filename:

non-empty sequence of characters other than directory-separator characters

directory-separator:

preferred-separator directory-separator_opt

fallback-separator directory-separator_opt

[…]