Re: Container reqs

[Stephen Leake <stephen.a.leake.1@gsfc.nasa.gov>]

Jeffrey Carter <jrcarter@acm.org> writes:

> Stephen Leake wrote:
> > 
> > Jeffrey Carter <jeffrey.carter@boeing.com> writes:
> > 
> > > Ehud Lamm wrote:
> > > > <snip>
> > > > What I dodn't like is the need for several, possibly nested, instantiations
> > > > simply to get a simple queue or hash table.
> > >
> > > I wholeheartedly agree. Wright's posting here a few days ago of the
> > > sequence of instantiations needed to obtain a simple structure with the
> > > Booch components clearly indicates that they fail this requirement.
> > 
> > This is easy to say. But can you provide an alternative, that still
> > allows the container to handle all reasonable Ada types?
> 
> I refer you to the PragmAda Reusable Components as an example of a
> component library that provides single instantiations for simple
> structures that handle all reasonable Ada types.
> 
> http://home.earthlink.net/~jrcarter010/pragmarc.htm

Well, I've looked at it (briefly). Here's the relevant part of the
unbounded list package:

generic -- PragmARC.List_Unbounded
   type Element is limited private; -- Do not instantiate with a scalar

   with procedure Assign (To : in out Element; From : in Element) is <>;
package PragmARC.List_Unbounded is

And the equivalent from my library:
generic
   type Item_Type (<>) is limited private;
   type Item_Node_Type is private;
   with function To_Item_Node (Item : in Item_Type) return Item_Node_Type;
   with procedure Free_Item (Item : in out Item_Node_Type);
   -- If Item_Type is definite non-limited, Item_Node_Type should just be
   -- Item_Type. Then To_Item_Node should just return Item, and Free_Item
   -- should be null (and both should be inlined). See
   -- SAL.Aux.Definite_Private_Items.
   --
   -- If Item_Type is indefinite, Item_Node_Type should be 'access
   -- Item_Type'. Then To_Item_Node should allocate Item and return a
   -- pointer to it, and Free_Item should be Unchecked_Deallocation. See
   -- SAL.Aux.Indefinite_Private_Items.
   --
   -- To create a list of limited objects (say of type Limited_Type),
   -- Item_Type can be a non-limited type holding the parameters needed to
   -- create an object (non-limited to allow the user to create aggregates
   -- of creation parameters), and Item_Node_Type can be 'access
   -- Limited_Type'. Then To_Item_Node must allocate an object of type
   -- Limited_Type and initialize it using the parameters in Item_Type. See
   -- SAL.Aux.Indefinite_Limited_Items.
   --
   -- Other usages may be possible.
   --

   with function Copy (Source : in Item_Node_Type) return Item_Node_Type is <>;
   -- Deep copy of Source, for use in allocators in Copy (List).

   Node_Storage_Pool : in out System.Storage_Pools.Root_Storage_Pool'Class;
   -- Root_Storage_Pool is limited, which does not allow defaults. Default
   -- for a global list type should be <some_global_access_type>'Storage_Pool.
   -- Default for a local list type, which should be reclaimed when the
   -- list type goes out of scope, is implementation defined (sigh).
package SAL.Gen.Lists.Double is

PragmARC does _not_ support indefinite types, which are a large part
of Ada (strings, unconstrained variant records, tagged types with
unconstrained discriminants). In addition, the comment says "no
scalars"; I'm not clear why.

So I don't think PragmARC meets the criteria of "simple instantiation
for all reasonable Ada types".

Perhaps we disagree on the meaning of "reasonable" here.

-- 
-- Stephe