Re: Generic Embedded List Nodes

[Niklas Holsti <niklas.holsti@tidorum.invalid>]

On 16-06-21 05:20 , Warren wrote:
> On Monday, 20 June 2016 15:33:16 UTC-4, Niklas Holsti  wrote:
>> On 16-06-20 15:26 , Warren wrote:
>>
>>> Anyway folks- thanks for your help but I now have a working solution. I'm signing off this thread.
>>
>> Before signing off, do please describe your solution.
>
> I thought I had the problem licked using the following generic
> Object_Of function, but when I whipped up an example, the compile
> problem returned (or there was pilot error):
>
>    function Object_Of(
>       Node: access Emb_Node;
>       Member: Natural
>    ) return Object_Type is
>       use System.Storage_Elements;
>
>       A:    constant System.Address := Node.all'Address;
>       B:    constant System.Address := A - Storage_Offset(Member);
>       R:    Object_Type;
>       for R'Address use B;
>       pragma Import(Convention => Ada, Entity => R);
>    begin
>       return R;
>    end Object_Of;
>
> The compiler is complaining with:
>
> warning: controlled object "R" must not be overlaid.

The component My_Recd.Name, of the controlled type Unbounded_String, is 
making Object_Type also controlled. I well understand that the compiler 
does not want controlled objects to be overlaid with address clauses.

Instead of a function returning Object_Type, you could try returning an 
access to Object_Type, as produced by an instance of 
System.Address_To_Access_Conversions. In fact, if I understand your 
goals, you do not want Object_Of to return a _copy_ of the object 
containing the Emb_Node, the need is to _find_ that very object. 
Returning an access value is closer to what you want, I believe.

> The test record looks like this:
>
>    type My_Recd is
>       record
>          ID:         Natural;
>          Node:       aliased Emb_Node;
>          Name:       Unbounded_String;
>       end record;

If you don't get the address arithmetic to work, 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. This would eliminate the address arithmetic, at the 
cost of increasing the size of the record by the sizes of the 
discriminants (not a great deal).

-- 
Niklas Holsti
Tidorum Ltd
niklas holsti tidorum fi
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