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From: firth@sei.cmu.edu (Robert Firth)
Newsgroups: comp.lang.ada
Subject: Re: Dynamic Address Clauses??
Message-ID: <5697@aw.sei.cmu.edu>
Date: 2 Jun 88 12:39:30 GMT
Article-I.D.: aw.5697
References: <8806011944.AA06549@ti.com>
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Reply-To: firth@bd.sei.cmu.edu.UUCP (Robert Firth)
Organization: Carnegie-Mellon University, SEI, Pgh, Pa
List-Id: <comp.lang.ada>

In article <8806011944.AA06549@ti.com> LINNIG@eg.csc.ti.COM (Mike Linnig) writes:
>Folks,
>
>The following package was compiled with two compilers (DEC Ada and
>Tartan 1750a Ada).  Both allowed the declaration of X.  DEC ada
>complained about the type conversion used for the rep clause
>in ANOTHER.

>Assuming one or both is legal, what do they mean???

Wow!  Yes, both address clauses are legal according to the RM [13.5],
which requires a "simple expression" yielding a value of type ADDRESS.
Note particularly that the RM does NOT require a static expression!

However, the second simple expression - stupid(x) - is not portable.
It assumes that INTEGER and ADDRESS are interconvertible, which will
be so only if ADDRESS is a numeric type.  Evidently it is on the one
machine and not on the other.

Finally, there is a problem here - there is a reference to the
value in X when X in fact has no value.  The program is therefore
"erroneous", by RM [3.2.1 (17,18)]

And what does it "mean"?  I've commented the code to show what I think
it means.

------------------------------------------------------------
with SYSTEM;
use SYSTEM;
package DYNAMIC_ADDR is
  subtype STUPID is SYSTEM.ADDRESS;

  function DYNAMIC return STUPID;  -- just some function
  X: INTEGER;
  for X use at DYNAMIC;  -- just what does this mean??
		-- [RF] call the function DYNAMIC.  The return value
		--      is the address of X.  The implementation will
		--	probably declare a hidden pointer and then
		--	reference X indirectly through it

  ANOTHER: INTEGER;
  for ANOTHER USE AT stupid(x); 
		-- [RF] similarly, fetch the value of X (NOT its address)
		--	convert that value to an address, and put ANOTHER
		--	there.  Again, this probably means building a
		--	hidden pointer

                -- I guess we can use x as a pointer
                -- but is it only evaluated at package elaboration?

	-- [RF] the code I'd expect to see:
	--	declare space for 2 pointers (REFX and REFANOTHER)
	--	call DYNAMIC
	--	store return value in REFX
	--	fetch value of X (ie load indirect through REFX)
	--	store that value in REFANOTHER

end DYNAMIC_ADDR;