Re: why can't we declare unconstrained objects ?

[Martin Krischik <martin@krischik.com>]

Michael Mounteney wrote:

> Is there a simple way in Ada of simulating C/C++ unions ?  It seems to
> me that this is gratuitously prevented, that is, it can be done with
> safety, by extending an existing run-time check, but it is in fact
> prevented by the compiler.
 
> Hopefully the following commented source will illustrate my point.
> 
> with Ada.text_IO;
> 
> procedure unconstrained is
> 
> -- Very simple discriminated type
> type thing (what : Boolean) is

You need to provide a default:

type thing (what : Boolean := true) is

to get a mutable type.

> record
> case what is
> when false =>
> letter : character;
> when true =>
> number : natural;
> end case;
> end record;

Ada ada will only allocate the minimum needed storrage for the discriminated
types. i.E. for false 8 bit and for true 32 bit and that cannot be changed
later - unless the type is mutable - then the maximum is allocated. 

> -- No problems here:  we provide a discriminant.
> sample : thing := (false, 'X');
> 
> -- This is alright as well of course.
> type thing_pointer is access all thing;
> 
> -- This is also fine:  a pointer to any `thing'.
> indirect : thing_pointer;
> 
> -- This causes a problem:  I want an unconstrained `thing', one
> -- that can be switched between holding a character and a number
> -- but the initialisation makes it constrained.
> sample2 : thing := (true, 12);
> -- Omitting the initialisation doesn't work:
> -- this just fails at compile-time.
> sample3 : thing;

As said, you need a mutable type to do that.

> begin
> -- Just reference a field in the `thing'.
> Ada.text_IO.put (sample.letter);
> 
> -- Create a new access object and access its field
> -- This requires a RUN-TIME check that indirect.what is true.
> indirect := new thing (true);
> indirect.number := 12;
> 
> -- This will generate a RUN-TIME failure of course.
> indirect.letter := 'A';
> 
> -- Warning at compile-time, failure at run-time.  I want to change
> the
> -- discriminant.  Since the compiler will insert a run-time check
> -- for field selection via an access, why not for a direct variable ?
> sample2 := thing'(false, 'Z');
> end;

With Regards

Martin
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