Generic units and child units

Generic units and child units

[Steve Folly]

Hi,

I'm having a bit of bother combining generic units and child units...

Originally I had a List [of integers] package which dynamically added
integers into a List.Object type
using the standard Node with a Previous and Next pointer, dynamically
allocating each node as required.
Fine so far.

Next, I implemented a List.Iterator child package to enable iteration
through a List.Object
backwards or forwards, starting at the head or tail of the list as required.
Fine again.

Now, what I really wanted was to make List a generic package so it could
store any type rather than
just integers. OK, that works.

The problem I have is how to define the (now) generic List.Iterator package.
The only way I
could get it to compile was to include the keyword 'generic' ie. I now
have...

generic
   type Item is private;
package List is
    ... blah blah
end List;

generic
package List.Iterator is
   ... blah blah
end List.Iterator;


In my test program I can instantiate a List for integers:

with List;
procedure Test is
    package Integer_List is new List (Item => Integer);
begin
    ... blah blah
end Test;

BUT, how do I get access to List.Iterator for the instantiated integer list.
What is the correct syntax?

Am I even going about this the right way? If possible, I would like to keep
the Iterator child package separate.

I couldn't find anything in the LRM about this.

Thanks to anyone who can help.

--
Regards,
Steve Folly -  Y2K compliant since 32nd Februark 1998
http://www.follysplace.demon.co.uk
donationsto:myaccount@mybank.co.uk

Re: Generic units and child units

[Brian Rogoff]

On Wed, 12 May 1999, Steve Folly wrote:

> Hi,
> 
> I'm having a bit of bother combining generic units and child units...
> 
> Originally I had a List [of integers] package which dynamically added
> integers into a List.Object type
> using the standard Node with a Previous and Next pointer, dynamically
> allocating each node as required.
> Fine so far.
> 
> Next, I implemented a List.Iterator child package to enable iteration
> through a List.Object
> backwards or forwards, starting at the head or tail of the list as required.
> Fine again.
> 
> Now, what I really wanted was to make List a generic package so it could
> store any type rather than
> just integers. OK, that works.
> 
> The problem I have is how to define the (now) generic List.Iterator package.
> The only way I
> could get it to compile was to include the keyword 'generic' ie. I now
> have...
> 
> generic
>    type Item is private;
> package List is
>     ... blah blah
> end List;
> 
> generic
> package List.Iterator is
>    ... blah blah
> end List.Iterator;

That's right, all children of a generic package must be generic. 
> 
> 
> In my test program I can instantiate a List for integers:
> 
> with List;
> procedure Test is
>     package Integer_List is new List (Item => Integer);

      package Integer_List_Iterator is new Integer_List.Iterator;

is the correct syntax.

> begin
>     ... blah blah
> end Test;
> 
> BUT, how do I get access to List.Iterator for the instantiated integer list.
> What is the correct syntax?
> 
> Am I even going about this the right way? If possible, I would like to keep
> the Iterator child package separate.

I'm still wondering the same thing. In similar libraries, I've used child
packages as you have used them. 

-- Brian

Re: Generic units and child units

[Robert A Duff]

Brian Rogoff <bpr@shell5.ba.best.com> writes:

> > generic
> >    type Item is private;
> > package List is
> >     ... blah blah
> > end List;
> > 
> > generic
> > package List.Iterator is
> >    ... blah blah
> > end List.Iterator;
> 
> That's right, all children of a generic package must be generic. 
> > 
> > 
> > In my test program I can instantiate a List for integers:
> > 
> > with List;
> > procedure Test is
> >     package Integer_List is new List (Item => Integer);
> 
>       package Integer_List_Iterator is new Integer_List.Iterator;
> 
> is the correct syntax.

Right, but don't forget to say "with List.Iterator;".  When you
instantiate List to create Integer_List, it's as if you were
instantiating the whole hierarchy of children, except that the children
of Integer_List are not visible unless the corresponding children of
List are with'ed.  That is, "with List.Iterator;" causes
Integer_List.Iterator to be visible, so you can then instantiate it.

This is all sort of theoretical -- in practise, the compiler will
typically just instantiate the children it needs to; it typically
doesn't even know about the whole hierarchy, because new children can be
added later.

See RM-10.1.1 and 8.3(20).

- Bob
-- 
Change robert to bob to get my real email address.  Sorry.

Re: Generic units and child units

[Matthew Heaney]

"Steve Folly" <steve@follysplace.demon.co.uk> writes:

> Am I even going about this the right way? If possible, I would like to keep
> the Iterator child package separate.

Why?  The list and its iterator are a highly cohesive pair of types, and
therefore get declared together, in the same package.