Re: Generic Embedded List Nodes

[Warren <ve3wwg@gmail.com>]

On Tuesday, 21 June 2016 01:52:51 UTC-4, Niklas Holsti  wrote:
> 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.

That is correct (no copy). The suggestion for System.Address_To_Access_Conversions seems like a potential solution. The only other thing I can do is try to cheat with a C routine returning an address, but GNAT is just as likely to complain.

Warren