Submitter: UK C Panel
Submission Date: 2005-03-04
Source: Joseph Myers <joseph@codesourcery.com>
Reference Document: ISO/IEC WG14
N1100
Version: 1.0
Date: 2005-03-04
Subject: Meaning of "known constant size"
Summary
Does "known constant size" mean something different from "not a VLA"? The phrase is used in the definition of composite types, 6.2.7#3:
-- 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 in an example in 6.5.6#11 (where it doesn't cause problems), and in 6.7.5.2#4 to define VLAs:
[#4] If the size is not present, the array type is an
incomplete type. If the size is * instead of being an
expression, the array type is a variable length array type
of unspecified size, which can only be used in declarations
with function prototype scope;122) such arrays are
nonetheless complete types. If the size is an integer
constant expression and the element type has a known
constant size, the array type is not a variable length array
type; otherwise, the array type is a variable length array
type.
Suppose the implementation does not accept any non-standard forms of
constant expressions under 6.6#10, so that (int)+1.0 is an arithmetic
constant expression but not an integer constant expression. Thus
int[(int)+1.0] is a VLA type. But is int[1][(int)+1.0] a VLA type?
The element type is a VLA type, but the element size is a known
constant. If "known constant size" is interpreted to include some VLA
cases, this also means further indeterminacy of composite types in
such cases; is "an array of that size" a VLA of that size, or a
non-VLA of that size, and may cases involving compatible array types
with different known constant sizes (which would yield undefined
behavior if executed) be rejected at translation time?
Suggested Technical Corrigendum