Re: [boost] Access to unaligned (packed) structs

From: Sebastian Redl (sebastian.redl_at_[hidden])
Date: 2007-10-12 03:54:18

Roland Schwarz wrote:
> Hmm, you may have missed the details of my example, so I repeat the 
> essentials:
>
> struct foo1
> {
>    unsigned data[4];
> };
>
> struct foo2
> {
>    unsigned data[2];
> };
>
> struct bar
> {
>    foo1 a;
>    foo2 b;
> };
>
> union { data[6]; bar b; };
>
> Are those types not "layout compatible" as the standard requires for the 
> access I intend? I mean foo1, foo2 are POD. So the question boils down 
> to whether foo1 and unsigned data[4] are layout compatible, not?
>   
The answer is still no. The standard says (9.2/16):
> If  a  POD-union  contains two or more POD-structs that share a common
>   initial sequence, and if the POD-union object currently  contains  one
>   of  these  POD-structs,  it is permitted to inspect the common initial
>   part of any of them.  Two POD-structs share a common initial  sequence
>   if  corresponding  members have layout-compatible types (and, for bit-
>   fields, the same widths) for a sequence of one or  more  initial  mem-
>   bers.
This is the only case where access through a different member is allowed.

unsigned data[6] is not a POD-struct, even though it is a POD.

Furthermore, you definitely can't access foo2's data, because there
might be padding between foo1 and foo2. (Not likely that any compiler
does that, but they're allowed to.)

Sebastian Redl