Submitter: UK C Panel
Submission Date: 2005-03-04
Source: Joseph Myers <firstname.lastname@example.org>
Reference Document: ISO/IEC WG14 N1100
Subject: Meaning of "known constant size"
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 18.104.22.168#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[(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?
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