Re: Generic Embedded List Nodes

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

On 2016-06-21 20:54, Niklas Holsti wrote:
> On 16-06-21 10:15 , Dmitry A. Kazakov wrote:
>> On 21/06/2016 07:52, Niklas Holsti wrote:
>>
>>> If you don't get the address arithmetic to work,
>>
>> There is a subtle difference between access and X'Address, but otherwise
>> address arithmetic works. E.g. I can instantiate my implementation of
>> linked lists with any type including unconstrained and non-tagged ones.
>>
>>> I believe that a set of
>>> mixin-generics could be used to add as many embedded Emb_Nodes to a
>>> tagged limited record, with access discriminants pointing to the
>>> containing record.
>>
>> Adding links afterwards is even worse than deriving objects from a
>> abstract list node.
>
> Why do you think so? For philosophical or practical reasons?

Practical reasons. If you derive from the list element base you cannot 
have scalar or other non-tagged objects as elements, but you still can 
have a class of elements. When you add links last, you cannot have a 
class, that in effect finalizes the list item.

> But your pool-based design restricts objects to be linked into at most
> one list. The OP said that an object can be placed in two or more lists
> at the same time, and apparently the number of such lists varies
> according to the type of the object, depending on the number of embedded
> list nodes within the object.
>
> The pool-based design can no doubt be extended to allow multiple lists,
> but it then becomes more complex.

It is no more complex. In fact my design is multiple lists from the 
start. Doubly-linked list is a specialization of.

BTW, a design based on inheritance requires multiple additive 
inheritance for lists. Ada does not have this even for interfaces. E.g.

    type Abstract_List_Item is abstract tagged record
       Next : not null access Abstract_List_Item'Class :=
                  Abstract_List_Item'Access;
       Previous : not null access Abstract_List_Item'Class :=
                  Abstract_List_Item'Access;
    end record;

    type My_List_Item is -- This is not Ada!
       new Abstract_List_Item as List_One
       or  Abstract_List_Item as List_Two with
    record
       X : Integer;
    end record;

Members and primitive operations are renamed to resolve name clashes.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de