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Date: 23 Sep 91 17:32:19 GMT
From: agate!spool.mu.edu!caen!uakari.primate.wisc.edu!aplcen!ddsdx2.jhuapl.edu!
dlc@ucbvax.Berkeley.EDU  (Dave Collard)
Subject: Re: Untyped "constant"?
Message-ID: <1991Sep23.173219.15440@aplcen.apl.jhu.edu>
List-Id: <comp.lang.ada>

In <1991Sep23.164446.13412@aplcen.apl.jhu.edu> gralia@ddsdx2.jhuapl.edu (Mars J
. Gralia) writes:


>I have a question about Ada.  It has to do with "named numbers" and 
>their subsequent use.

>a) Suppose I have a compiler/RTE in which integer'last is 32_767.
>b) And suppose the system has a long_integer type in which
>   long_integer'last = 2_147_483_647.
>c) And suppose I have the program:
>          procedure Named1 is
>             X : constant := integer'last+5;
>          begin
>              for j in 1..X loop
>                  null;
>              end loop;
>          end Named1;

>d) Finally, suppose the "loop" variables are really used. E.g., by
>   disabling optimization when I invoke the compiler.  


>Then:  is this a legal program or not?
Nope.  The declaration
  X : constant := integer'last+5;
is illegal.   The expression given for the initialization of a number 
declaration must always be an expression of universal type.  The attribute 
integer'last returns an integer, not a universal_integer.  Therefore, the
result of the addition will resolve to integer.  Universal types must be made
up entirely of expressions involving only other universal types (literals and
other named numbers).

--Thor


>DISCUSSION

>Some compilers, admittedly archaic or for very small computers, 
>will raise a Constraint_Error, thereby forcing me to say:
>             X : constant long_integer := integer'last+5;

This, being a constant declaration and not a number declaration should
be fine if you convert the result of 'last to long_integer:

  X : constant long_integer := long_integer(integer'last) + 5.

>I would prefer the program given above, because it would appear to be 
>more portable.
>     

--Thor
collard@capsrv.jhuapl.edu