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From: "G.B." <bauhaus@futureapps.invalid>
Newsgroups: comp.lang.ada
Subject: Re: operation can be dispatching in only one type
Date: Wed, 25 Nov 2015 13:27:08 +0100
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Xref: news.eternal-september.org comp.lang.ada:28537
Date: 2015-11-25T13:27:08+01:00
List-Id: <comp.lang.ada>

On 25.11.15 09:24, Dmitry A. Kazakov wrote:
> On Tue, 24 Nov 2015 14:48:24 -0700, Jeffrey R. Carter wrote:
>
>> I mean that a protected type defines a set of values and operations on those
>> values,
>
> This is a type, not yet a class. BTW, it is not even a proper class-type (a
> root type in Ada terms) because you could not derive anything from it.
>
>> encapsulates the values and operations, and hides the implementation of
>> the values.
>
> In fact it does not hide the implementation of values.

I understand that for abstraction, you'd use abstraction features
of the language.

    interface_type_definition ::=
      [limited | task | protected | synchronized] interface
         [and interface_list]

That is,

     type Abstraction is protected interface;

     ... operations ...

And then derive a concrete protected type that implements
the interface:

     protected type Concrete is new Abstraction with ...

> That is one of its
> design flaws. The implementation of protected components is mixed with the
> interface.

Every Ada package tends to have "implementation of components"
after "private",  such as a full type definition? That's a long
standing flaw then, justified by separate compilation needing
full types.

> It should have had public components with read access acting as
> a protected function and write access doing as a protected procedure. The
> private components should have been declared in the package's private
> section or the package body.

That seems approximately possible, I think, even when it is not
formally possible:


package Approx is

    type Opaque is private;
    Initial_Value : constant Opaque;

    protected type P is
       entry E1;
       function F1 return Boolean;
    private
       Data : Opaque := Initial_Value;
    end P;

private
    type Index is range 1 .. 10;
    type Opaque is array (Index) of Boolean;
    Initial_Value : constant Opaque := (others => False);
end Approx;