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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: Protected types and visibility of their internals
Date: Tue, 8 Jul 2008 16:03:04 -0500
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Date: 2008-07-08T16:03:04-05:00
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"Maciej Sobczak" <see.my.homepage@gmail.com> wrote in message 
news:b1f2203b-ed02-4031-bcba-40c0e1b1be07@j22g2000hsf.googlegroups.com...
> Consider a protected type in some package:
>
> package P is
>   protected type PT is
>      procedure Foo;
>   private
>      X : Some_Type;
>   end PT;
> end P;
>
> The protected type has a component X of Some_Type, which is entirely
> the private business of the protected type.
>
> Where this type should be declared?

I think you are asking the wrong question. The question is "Where should the 
protected type be declared?". Since there are no operations that are 
specific to protected types, there is very little reason to make those a 
visible type.

Thus, I would generally structure the package with a private type and the 
protected type (if any) declared in the private part. Something like:

 package P2 is
     type Priv is tagged limited private;
     procedure Foo (Obj : in out Priv);
 private
    type Some_Type is ...
    protected type PT is
        procedure Foo;
   private
      X : Some_Type;
   end PT;
   type Priv is tagged limited record
        Lock : PT;
   end record;
end P2;

This structure works well because it is always important to make the 
protected portion of the operation as short as possible, and this layout 
allows a place to do things that don't need mutual exclusion.

There is an alternative if you really need access to the protected object 
directly, which is to declare a protected interface in the visible part of 
the package and then derive the protected type from it in the private part.

Either way, you will find that you put most of your declarations in the 
private part. It's not unusual for the majority of the code of a package 
specification to be in the private part.

                                Randy.