Re: Full view of a private partial view cannot be a subtype

[Jere <jhb.chat@gmail.com>]

On Tuesday, December 19, 2017 at 4:08:46 AM UTC-5, Dmitry A. Kazakov wrote:
> On 2017-12-19 02:01, Jere wrote:
> > On Sunday, December 17, 2017 at 10:39:17 AM UTC-5, Dmitry A. Kazakov wrote:
> >> On 2017-12-17 16:26, Jere wrote:
> >>
> > Something like:
> > 
> > generic
> >     type Item_Type(<>);
> >     type Item_Access is access Item_type;
> >     with procedure Deallocation(Ref : in out Item_Access);
> > package Sledgehammer is
> >     <types>
> >     <operations>
> > private
> >     <implementation>
> > end Sledgehammer;
> > 
> > This would be used in maybe 5% or less of the code base and only
> > when absolutely necessary.  I want to convert it to:
> > 
> > generic
> >     type Item_Type(<>) is limited private;
> > package Nicer_Package is
> >     type Item_Access is access Item_Type;
> >     <same types>
> >     <same operations>
> > private
> >     procedure Deallocate is new Ada.Unchecked_Deallocation
> >        (Item_Type,Item_Access);
> > 
> >     package Implementation is new Sledgehammer
> >        (Item_Type => Item_Type,
> >         Item_Access => Item_Access,
> >         Deallocation => Deallocate);
> > 
> >     <implementation>
> > end Nicer_Package;
> > 
> > Since all I am doing is automating the access type and
> > the deallocation operation, I don't think a new type
> > is really the right design. Additionally, I don't want
> > to expose the deallocation operation as it should never
> > be called directly.
> 
> How do you create objects?

Explicit.  The client does.  It's mostly due to the need
for of the incomplete type.  I can't make or manage objects
of type Item_Type, so the client has to allocate.

> 
> There are two approaches:
> 
> 1. Explicit new, implicit deallocate. This works only with containers, 
> which should know the access type and the deallocator.
> 
> 2. Factory (a Create call), implicit deallocate. This requires reference 
> counting or custom GC pool.
> 
> P.S., declaring an access type in the public part of a generic is 
> suspicious for bad design.
> 
Again, it is only for basically to keep the client from having to
specify in 90-95% of the situations where they don't really need it.
The type is never used to create variables.  In the more complex
generic, I make the client specify instead, but the simpler one is
just for quick utility and hopefully lack of exposure to the deallocation
routine.