Re: Generic formal access types

Simon Wright wrote:
> 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).

Sorry, I meant:

generic
    type Elment (<>) is private;
    --  Yes, (<>) allows Element to be unconstrained.
    type Access_Type is access Element;

But the "problem" is still there. I have to pass an actual "Element" as 
well as an actual "Access_Type" when I instantiate the generic package. 
I would like to only pass the "Access_Type".

> 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 ..

You are probably right, but I am not dealing with tagged types here. My 
problem is much simpler.

> 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!

Hmmm... I was not thinking of that either. Just access to objects by 
now.  :^)

Rodrigo