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From: "Adam Beneschan" <adam@irvine.com>
Newsgroups: comp.lang.ada
Subject: Re: generic question
Date: 20 Nov 2006 09:31:17 -0800
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Date: 2006-11-20T09:31:17-08:00
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markww wrote:
> Hi,
>
> I've been going through ada tutorials, and am looking at the generics
> support. I can't quite understand how to accomplish having a record
> which has a generic data member. For example, something like:
>
>     type NODE_REC is
>         record
>             unknown_data_type : generic;  -- not sure how to express
> this in ada
>             Next_Node : NODE_REC_PTR;
>             Prev_Node : NODE_REC_PTR;
>       end record;
>
> I'd like to be able to store either an integer, a float, or some other
> user defined record type in there so that this node record becomes a
> container. Is this possible?

Before we possibly get too far off track, I think we need to know: Are
all of the "unknown_data_type" fields in the same container going to
have the *same* type?  Or are you planning on mixing types---e.g.
creating a linked list where some elements in the list have an integer
for the unknown_data_type field and some elements in the same list
store a float or a user-defined record or something.

If it's the second, the generic solutions others have posted won't
work.  When you declare a generic, you can instantiate it with, say,
"integer", "float", or any other type; but then each generic instance
will define its own version of NODE_REC and NODE_REC_PTR, and those are
*different* types even though they have the same name.  Therefore, a
NODE_REC_PTR from one generic instance cannot point to a NODE_REC in a
different generic instance.

If this is what you're trying to do, we'll need a different approach,
probably one that will involve tagged types and classwide types.

                             -- Adam