[lug] An unusual C construct

[David Morris]

On Wed, Jul 28, 2010 at 22:57, Kenneth D Weinert <kenw at quarter-flash.com> wrote:
>
> sizeof(((tCLAIM_tbl_def *)0x0)->acHealth_Service_Id)
>
> tCLAIM_tbl_def is a structure typedef.
>
> I understand the intent of this code - it's to get the size of the
> field in the tCLAIM_tbl_def structure so the equivalent field in the
> current structure (not shown) can be the same size.
>
> It's just that in all my years of coding C I've never seen this
> technique used. It does work, at least on the AIX compiler we're using
> here.
>
> I'll also note that it's hard to google - at least I've not come up
> with the correct search terms to find an explanation.
>
> It's nothing I need to change - I'd just like to understand how it works :)

The reason this works (If I recall correctly) is that "sizeof" is an
operator which is calculated at compile time.  Yes it looks weird to
be apparently dereferencing a NULL pointer, but that code is never
actually executed...when the code runs, that entire statement has
already been changed to the size of acHealth_Service_Id within that
structure.  Basically it is the (ugly) C equivalent of the C++
statement "sizeof(tCLAIM_tbl_def::acHealth_Service_Id)".

> They also have the habit of passing in a pointer and using it like:
>
> ptr[0].field

I've seen this type of construct a lot from people who have never seen
pointers before and don't understand what they are (the most common
misconception I've encountered is that a pointer is simply a form of
declaring an array).  Then once that construct exists in the code at
all, it tends to propagate, even if other coders should know better.

David