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From: Stephen Leake <Stephen.Leake@gsfc.nasa.gov>
Subject: Re: Can't export object of private type
Date: 1999/03/01
Message-ID: <ubtid83dq.fsf@gsfc.nasa.gov>#1/1
X-Deja-AN: 450047992
References: <7b3f6v$lc1$1@nnrp1.dejanews.com> <F7r2DF.23y@syd.csa.com.au>
 <7b5u87$s88$1@nnrp1.dejanews.com>
Organization: NASA Goddard Space Flight Center -- Greenbelt, Maryland USA
Newsgroups: comp.lang.ada
Date: 1999-03-01T00:00:00+00:00
List-Id: <comp.lang.ada>

robert_dewar@my-dejanews.com writes:

> The advantage of private parts is of course the nice
> compartmentalization. Indeed we have on the drawing boards
> for GNAT a feature to have the private parts in separate
> files which will be very handy for target specialization
> and indeed for general abstraction.

This would help in a situation I just encountered. I'm interfacing to
a hardware device, so I've written a thin Ada binding package to the device
driver. I'd like to make that thin binding private, so normal users
can't use it. But I'd also like to use it in the private part of the
public, thick binding:

package SBS_Device is
    pragma Pure;
    -- just a root package
end SBS_Device;

private package SBS_Device.Thin is

    type Low_Level_Device_Type is record
	...
    end record;

end SBS_Device.Thin;


with SBS_Device.Thin; -- illegal!
package SBS_Device.Thick is

    type Device_Type is private;

    ...

private

    type Device_Type is record
	Low_Level : SBS_Device.Thin.Low_Level_Device_Type;
	...
    end record;
end SBS_Device.Thick;

"with SBS_Device.Thin" is illegal, because it's a private package.

If the private part of SBS_Device.Thick were in a separate file, I
could put the with clause there, and everybody would be happy.

In my code, I made Low_Level a pointer, and deferred the full type to
the body, where I can with SBS_Device.Thin.

Perhaps an alternate solution would be to allow the keyword "private"
in context clauses:

with private SBS_Device.Thin;
package SBS_Device.Thick is ...

The semantics being that the private package is only allowed to be
referenced in the private part.

-- Stephe