Submitter: UK C Panel
Submission Date: 2005-03-04
Source: Joseph Myers
<joseph@codesourcery.com>
Reference Document: ISO/IEC
WG14 N1100
Version: 1.2
Date: 2006-04-04
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
Committee Discussion (for history only)
The statement, "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." , implies an interpretation of the standard that the implementation can extend the meaning of what constitutes an integer constant expression. For example, that (int)+1.0 is an integer constant expression.
The committee does not believe that it does. Even if an implementation accepts other forms of constant expressions, paragraph 6.6#10 does not change the definition of an integer constant expression given by paragraph 6.6#6, and int[(int)+1.0] is still a VLA.
Paragraph 6.6#10 cannot be used to get around issuing diagnostics for constraint violations where integer constant expressions are required. Which we believe is what the first paragraph of the introductory text is implying
Technical Corrigendum
Add to 6.2.5, after Paragraph 22
A type has known constant size if the type is not incomplete and is not a variable length array type.