Thu, 1 Oct 1998 13:05:12 -0500
Scott Edgerton wrote:
[quoting Michael Feldman]
> For example, if P and Q designate the same value, and you deallocate
> through P, what is the behavior if you try to dereference (access the
> designated block of) Q?
> Once P has been deallocated, it's value is NULL
> (ref RM 13.11.2 (7)). Therefore, the value at Q
> should also be NULL (provided that that storage
> hasn't been reallocated for something else)
> Unless I'm missing something?
Sorry, yes, you are.
The value of P is null, but that's the pointer, not the pointed-at
storage space. Meanwhile, Q (and any other duplicate pointers to
that space) still have their old value -- on some machines this is
now meaningless garbage, on others it still refers to the same
block of memory (which is now available for re-allocation).
> Perhaps this is why people say that "Ada is hard to learn"...
> Aliasing (on the surface) seems like a pretty
> straight forward concept.
Aliasing is a general concept, and is pretty straightforward. It
means you have two names for the same variable.
In this particular version of aliasing, using pointers, you can
you can have a name that outlives the storage allocation for the
variable. This is a problem. It's a common cause, in C, of
hard-to-trace weird behavior and "segmentation fault - core dumped"
Ada's access-value design tries to help the user keep track of
what he's doing.
Samuel Mize -- [log in to unmask] (home email) -- Team Ada
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