Re: How to print Task_ID type? (GNAT SunOS)

[kst@thomsoft.com (Keith Thompson)]

In <dewar.828713391@schonberg> dewar@cs.nyu.edu (Robert Dewar) writes:
[...]
> The method is fine, the test for null is appropriate (I missed that
> in my message before!) The caution is probably unnecessary, I think
> you will find that even in implementations where descriptors etc
> are used, that 'Address is likely to point to the data. Remember
> that X.all is the value of the type, and 'Address will normally
> point to this data.

One would think so, but I just tried an experiment with one
particular Ada 83 compiler.  For an object P declared as an access to
unconstrained String, P and P.all'Address have the same value (when
Uncheckedly_Converted to an integer type), but P(P'First)'Address has
a different value.  It turns out that P points to a descriptor, and
P(P'First)'Address is the address of the first character of the string
itself.

This should probably (almost certainly) be considered a bug in that
compiler's implementation of P.all'Address, which should be the address
of the string, not of the descriptor.

>                     You could make doubly sure in the only case
> where there even conceivably could be trouble (arrays), by using
> 
>   x.all(x'first)'address

Which will raise Constraint_Error if X.all is a null array.  For what I
suspect is the most common application for this kind of thing, interfacing
to C, this shouldn't matter, since C doesn't have null arrays (though
future versions of the C standard might).  (Incidentally, the ".all" in
the above expression is optional.)

> but at this stage, this is getting a bit junky, so even though it is
> a bit more complexity, it is probably ceaner to instantiate 
> address_to_access conversions to get the adress value.

How does Address_To_Access_Conversions help in this case?  It only
lets you convert between System.Addess and an access type declared in
Address_To_Access_Conversions; it doesn't provide operations on arbitrary
user-declared access types.

-- 
Keith Thompson (The_Other_Keith) kst@thomsoft.com
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