CODEWITHSUNDEEP
c++language-lawyer
Belloc
Belloc

Reputation: 6390

Why do we need the second definition below in a `simple-declaration`?

simple-declaration:
     decl-specifier-seq init-declarator-listopt ;
     attribute-specifier-seq decl-specifier-seq init-declarator-list ;                                          <====
     attribute-specifier-seqopt decl-specifier-seq ref-qualifieropt [identifier-list]initializer ;

Note that the attribute-specifier-seq is required in this definition. When does that happen?

Upvotes: 2

Views: 88

Answers (1)

Ben Voigt
Ben Voigt

Reputation: 283684

Given that we want our grammar to accept:

DSS;
DSS IDL;
ASS DSS IDL;

(plus accept array forms, which this answer will not deal with any further)

but not

ASS DSS;

That is, if the attribute-specifier(s) are provided, the init-declarator-list becomes required.

The grammar productions as shown in the question provide for this and importantly only parse any legal simple-declaration one way.

Every declaration which does not have an attribute-specifier-seq is parsed using the first case. Every declaration which does is parsed using the second case. There's no overlap between the two cases.

If the second case were

attribute-specifier-seqopt decl-specifier-seq init-declarator-list ;

then the same inputs would be allowed, but the rules overlap -- inputs of the form DSS IDL; would match both the first and the second, creating an ambiguous parse.

Overlapping rules are not wanted.

There's more than one way to solve this with non-overlapping rules. The following would also be ok:

decl-specifier-seq ;

attribute-specifier-seqopt decl-specifier-seq init-declarator-list ;

In effect, this comes from the Karnaugh map for the truth table for the implication operator (ASS provided implies IDL provided), which has three True cells in an L pattern. One solution arises from using a vertical circle, the other solution arises from using a horizontal circle.

In digital logic, overlap prevents glitching (good!). In language grammar, overlap produces parsing ambiguity (bad!).

Upvotes: 5

Related Questions