Re: Safety of the Booch Ada 95 Components

[herwin@gmu.edu (Harry Erwin)]

Matthew Heaney <matthew_heaney@acm.org> wrote:

> In article <1e2lds4.7trgj21rgj9i0N%herwin@gmu.edu> , herwin@gmu.edu 
> (Harry Erwin) wrote:
> 
> > Now I've been looking at the Booch components for Ada 95, and have
> > noticed that the Copy function typically starts by clearing the To
> > container.
> 
> I haven't studied the Booch components specifically, but here's how Copy
> operations should be implemented in general.
> 
> Assuming the data structure is implemented as a by-reference type (the
> type is tagged and/or limited), you can invoke RM95 13.3 (16) to legally
> compare the addresses of the subprogram parameters:
> 
>   procedure Copy
>     (From : in     Source_Type;
>      To   : in out Target_Type) is
>   begin
>     if From'Address = To'Address then
>       return;  -- do nothing, because From is same object as To
>     end if;

That's missing from the code I've been looking at.

> 
>     Clear (To);  -- safe, because we now know To isn't From
> 
>     <do rest of copy>
>   end Copy;
> 
> If the Booch Components don't first check whether aliasing has occurred,
> prior to clearing the target object, then perhaps something is wrong.
> 
> 
> > That immediately implies that they are not exception-safe,
> > but I'm concerned that they may not be safe under self-assignment as
> > well.
> 
> I don't understand your comment about not being "exception-safe."
> Perhaps you could elaborate on that point.

Strong exception-safety == supports commit/rollback semantics.
Apparently in the Booch components, if the copy goes bad, the To
container has been cleared and is partially written. Basic exception
safety (== the container is still useable) is probably supported.

> 
> 
> >  Is there something about the Ada 95 standard that guarantees that
> > there will be no aliasing of container args?
> 
> No, it's up to you the programmer to determine whether aliasing has
> occurred.

That may not be possible.

> 
> The idiom for determining whether aliasing has occurred is to compare
> object addresses, per RM95 13.3 (16).

Got it.  Thanks!

> 
> 
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-- 
Harry Erwin, PhD, <http://mason.gmu.edu/~herwin>