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From: Simon Wright <simon@pushface.org>
Newsgroups: comp.lang.ada
Subject: Re: Generic formal access types
Date: 30 Apr 2003 20:27:32 +0100
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Rodrigo Garc�a <rodrigo.garcia.ARROBA.epfl.ch> writes:

> Does anybody know why Ada does not have something like:
> 
> generic
>     type Access_Type is access (<>);  --  ?
> 
> I would like to specify in a generic package that it should be
> instantiated by an access type, no matter what type it is pointing
> to.
> 
> Ok, it is not very useful to have an access type if you do not know
> its content, but the package implements a list and I am just storing
> them. I "need" that because I want to return the value "null" from a
> function in the generic package that returns "Access_Type".
> 
> There is a possible workaround:
> 
> generic
>     type Element is (<>);
>     type Access_Type is access Element;
> 
> But I do not like it because it forces me to pass also the "Element"
> in the instantiation.

I think 'type Element is limited private' would be better (I can't
rememer if you can allow it to be unconstrained as well, 'type Element
(<>) is limited private' ... GNAT is happy).

I can understand why a formal type has to be tagged for you to declare
it abstract, you would have thought a private constraint { (<>) }
would have been a possibility here ..

There's no way I can see that you can instantiate a generic with an
access-to-subprogram, but I'm not sure that the uses would be worth
the trouble!