Submitter: Joseph Myers (UK)
Submission Date: 2007-03-24
Source: Joseph Myers <joseph@codesourcery.com>
Reference Document: ISO/IEC
WG14 N1120
Version: 1.4
Date: 2008-07-21
Subject: Variably modified compound literals
Summary
Consider the code:
extern int a;
void *p = &(int (*)[a]){ 0 };
Does such a variably modified compound literal at file scope violate any constraint? 6.7.5.2#2 says:
[#2] Only an ordinary identifier (as defined in 6.2.3) with both block scope or function prototype scope and no linkage shall have a variably modified type. If an identifier is declared to be an object with static storage duration, it shall not have a variable length array type.
However, this only seems to constrain declarations of
identifiers, not any other expression with variably modified type
(such as a compound literal, inside or outside a function). If
however the above code is valid, when is a evaluated
for the purposes of the requirement in 6.7.5.2#5 that "each time
it is evaluated it shall have a value greater than zero"? Must
a have positive value throughout execution of the
program, or is it only the initial value of a which
must be positive? (I think the initializer is a
constant expression, being the address of an object of static
storage duration.)
The variably modified compound literal is an object, and I
think it should be treated like other objects outside functions
and required not to have variably modified type (even if inside
sizeof, just like the initializers of compound literals outside
functions must be constant even if inside
sizeof).
Suggested Technical Corrigendum
6.5.2.5 paragraph 3, after "shall consist of constant
expressions" add "and the type name shall not specify a variably
modified type".
Committee Discussion
Paragraph in question (paragraph 3 of 6.5.2.5) has been changed by DR 328.
The suggested Technical Corrigendum looks appropriate, however, it could be redundant.
Committee Response
This defect report is answered by DR 328.
Constraints and Semantics are the same as 6.7.8 Initialization