Submitter: Joseph Myers (UK)
Submission Date: 2007-03-24
Source: Joseph Myers <firstname.lastname@example.org>
Reference Document: ISO/IEC WG14 N1221
Subject: Composite types for variable-length arrays
The definition of composite types in 6.2.7#3 says "If one type is an array of known constant size, the composite type is an array of that size; otherwise, if one type is a variable length array, the composite type is that type." and also "These rules apply recursively to the types from which the two types are derived.". Which of these wins for variable length array types? Are the element types composed recursively, or is the element type of the variable length array type taken even though it may have less information than the other element type? (That loss of information in the composite type would mean some sequences of three or more declarations of the same function are constraint violations for some orderings of the declarations and undefined behavior for other orderings.)
See reflector messages 11145-11147 for discussion.
Suggested Technical Corrigendum
6.2.7 paragraph 3, change "is that type" to "is an array of that size; in either case, the element type of the composite type is the composite type of the two element types".
The element types are composed. Suggested TC is close, but not quite right. The example from 11145 should be included here.
Also see N1238 and WG14 e-mail SC22WG14.11145.
No consensus was reached on the words from N1238.
Proposed Technical CorrigendumIn subclause 6.2.7, paragraph 3, change the first bullet to the following.
– If both types are array types, the following rules are applied:If one type is an array of known constant size, the composite type is an array of that size.In subclause J.2, paragraph 1, insert the following bullet in order.
Otherwise, if one type is a variable length array whose size is specified by an expression that is not evaluated, the behavior is undefined.
Otherwise, if one type is a variable length array whose size is specified, the composite type is a variable length array of that size.
Otherwise, if one type is a variable length array of unspecified size, the composite type is a variable length array of unspecified size.
Otherwise, both types are arrays of unknown size, and the composite type is an array of unknown size.
The element type of the composite type is the composite type of the two element types.– A program requires the formation of a composite type from a variable length array type whose size is specified by an expression that is not evaluated (6.2.7).
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