Submitter: UK C Panel
Submission Date: 2005-03-04
Source: Joseph Myers
<joseph@codesourcery.com>
Reference Document: ISO/IEC
WG14 N1099
Version: 1.6
Date: 2007-09-06
Subject: Definition of variably modified types
Summary
Variably modified types are defined by 6.7.5#3:
[#3] A full declarator is a declarator that is not part of another declarator. The end of a full declarator is a sequence point. If the nested sequence of declarators in a full declarator contains a variable length array type, the type specified by the full declarator is said to be variably modified.
It is desirable for the definition to look at the declarator rather than just the resulting type, so that function parameters adjusted from array to pointer type are variably modified if the array size is variable: in
void
f (int i, int a[static ++i])
{
// ...
}
the increment of i must be evaluated for the
definition of static in this context to make
sense. However, what it means for the declarators to "contain"
a type is unclear. The natural interpretation is that they
include an array declarator with array size [*] or
an expression which is not an integer constant expression.
However, this does not cover cases such as
int x;
// ...
typedef int vla[x];
vla y[3];
where a typedef for a variably modified type is used.
y is a VLA, and clearly ought to be variably
modified, but nothing about the declarators makes it variably
modified; only the declaration specifier does so.
Suggested Technical Corrigendum
Committee Discussion (for history only)
Declarators don't contain a type, it is the sequence of declarators that contains types.
The sentence in 6.7.5;p3 that defines variably modified
types may be wrong, and that may not even be the right place
for it to be defined. The definition ties it too closely to the
declarator. In the example provided in the DR, the type of
y is variably modified. The declarator for
y does not include a variable length array
type. Para 3 needs to have it's wording adjusted in some
fashion, the text there is insufficient to provide us the
answer.
The definition in the standard for variable length array does not seem to be in italics.
2006-Mar-29:
For the reason noted in the Summary, variably modified types do need to be tied to the declarator syntax. But the current definition fails to state that a new type derived from a variably modified type is itself variably modified.
Commitee Response
Yes, this is a defect in the definition.. The
definition will be modified to state explicitly that types
derived from a variably modified type are themselves variably
modified.
Technical Corrigendum
[Note, these are relative to wg14's N1124.]
Change 3rd sentence in 6.7.5p3 from:
If the nested sequence of declarators in a full declarator contains a variable length array type, the type specified by the full declarator is said to be variably modified.
to
If in the nested sequence of declarators in a full declarator there is a declarator specifying a variable length array type, the type specified by the full declarator is said to be variably modified. Furthermore, any type derived by declarator type derivation from a variably modified type is itself variably modified.