Re: Help on arrays of tagged types

[mheaney@ni.net (Matthew Heaney)]

In article <34BC054C.5DB2@peak.org>, jamesjb@peak.org wrote:

>Hello.  This problem is quite similar to one I have been having also.
>I am trying to create a linked list type for objects derived from a
>root tagged type.  
>
>The goal is to have a homogenous Linked_List object, that stores objects
>of a tagged type derived from "Root_Data".  So you could set up as
>follows:
>
>        Socket_List : Linked_List;
>        Descriptor_List : Linked_List;
>        etc.
>
>The list is set up analogously to the example below, but the problem
>arises 
>when I try to access the elements of the list. I think the problem is
>that 
>I try to convert from "access Root_Data'Class" to "access <type derived
>from Root_Data>",
>which cannot work because it would entail extending the structure of the
>object to the
>derived type.
>
>> 
>> type Root_Data_Class_Access_All is access all Root_Data'Class;
>> 
>> type Root_Data_Array is
>>    array (Positive range <>) of Root_Data_Class_Access_All;

You want a "typeless" data structure.  You'll have to downcast in that case:

declare
   Descriptor_As_Data : Root_Data'Class 
      renames Heading (Descriptor_List).all;

   File : File_Descriptor 
      renames File_Descriptor (Descriptor_As_Data);
begin

It's perfectly legal to downcast a class-wide object into the type you
want, because there'll be a tag check to make sure the object really is (or
is descended from) File_Descriptor.

That being said, yes, you really do want to use a generic, if the data
structure is homogeneous.

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Matthew Heaney
Software Development Consultant
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