Re: Announce: Grace project site operational

[Dmitry A. Kazakov <mailbox@dmitry-kazakov.de>]

On Wed, 15 May 2002 13:32:48 +0200 (MET DST), "Grein, Christoph"
<christoph.grein@eurocopter.com> wrote:

>From: "Dmitry A.Kazakov" <mailbox@dmitry-kazakov.de>
>> Consider:
>> 
>>    type Graph_Node is private;
>>    No_Node : constant Graph_Node; -- Deferred, but visible
>>    function Get_Next_Child
>>       (Parent : Graph_Node; Child : Graph_Node := No_Node)
>>          return Graph_Node;
>> 
>> Here I exposed No_Node, which then can be abused like:
>> 
>>    Get_Next_Child (No_Node);
>> 
>> A solution could be some sort of incomplete declaration for
>> subprograms:
>> 
>>    type Graph_Node is private;
>>    function Get_Next_Child
>>       (Parent : Graph_Node; Child : Graph_Node := <>)
>>          return Graph_Node;
>
>Your caller has to have some variable Current_Parent, which might have the value 
>No_Node, e.g. when it is undefined (private types should be defaulted to 
>something like this so that they always have a value even when never assigned a 
>value).

Fortunately, with Ada 95 we have a way to ensure that all variables
get initialized:

type Graph_Node (<>) is private; -- Prevents uninitialized variables

>So you must make sure in any case that the actual for Parent is different from 
>No_Node, so the abusive
>
>  Get_Next_Child (No_Node);
>
>is caught in any case.

That's for sure, because Graph_Node will probably be a record
containing an access type, dereferenced and thus checked at run-time.
But this is no matter, because no run-time checks may supersede
compile-time ones.

Another story is that with a more advanced ADT there might be a way to
easily create a supertype Graph_Node_With_No_Node and then leave the
work to the compiler. But this is a long way to go.

>There are cases where
>
>         .---------------------------------------.
>         | Information Hiding Considered Harmful |
>         `---------------------------------------'

Egh, where am I? Is this comp.lang.ada? (:-))

---
Regards,
Dmitry Kazakov
www.dmitry-kazakov.de