Re: Using (SCM)0

[Marius Vollmer]

Ludovic Courtès <address@hidden> writes:

> Sure.  Actually I meant the following: Suppose I have a structure with
> an SCM field and I use calloc () to allocate it; can I write in my code
> something like:
>
>   if (object->field == (SCM)0)
>     /* Unitialized, assume there is no "real" Scheme object here */
>   else
>     /* Assume object->field is a "real" Scheme object */

Yes, you can do this (tho it might be easier to initialze the field
with SCM_BOOL_F once, say).

However (and I didn't think about this before) the type SCM is not
necessarily an integer or pointer.  It should be treated as opaque.
You need to use SCM_PACK:

  if (object->field == SCM_PACK (0))

but there is no real guarantee that SCM_PACK(0) is identical to
all-bits-zero.  Hmm.  It seems cleaner to avoid testing a SCM for
zero.

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