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Date: Mon, 18 Dec 2017 12:45:28 -0800 (PST)
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Subject: Re: Full view of a private partial view cannot be a subtype
From: Stephen Leake <stephen_leake@stephe-leake.org>
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Xref: reader02.eternal-september.org comp.lang.ada:49519
Date: 2017-12-18T12:45:28-08:00
List-Id: <comp.lang.ada>

On Saturday, December 2, 2017 at 8:14:50 PM UTC-6, Jere wrote:
> There are times where in another package I want to
> subtype Base.Instance and do renames of the operations in order
> to bring them all into scope (or whatever the correct term is):
> 
>    package New_Type1 is
>       subtype Instance is Base.Instance;
>       procedure Operation(Object : in out Instance);
>       
>    private
>       
>       procedure Operation(Object : in out Instance) renames Base.Operation;
>    end New_Type1;

Why subtype? why not a derived type? why do you need a new name with the same operations?

> (NOTE:  is there a better way to do this?)

Yes, use a derived type:

type Instance is new Base.Instance;

Since Base.Instance is not abstract, you don't need to do anything else; all the operations are inherited.

Optionally, you can override some operations to change what they do.

> So I guess my question is two part:
> 
> 1.  Is there a way to make the full view of a private type
>     a subtype when the partial view is not a subtype? Or am
>     I forced to derive a new type?
> 
> 2.  If it isn't possible, are there any language reasons for
>     why that must be so?  Would making a full view of a type
>     actually a subtype under the hood break something?

subtypes just declare a new name with new constraints; derived types declare a new type. So you'd be lying to the clients of the package; that always causes problems.

-- Stephe