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From: Robert A Duff <bobduff@world.std.com>
Subject: Re: "out" or "access"
Date: 1998/10/22
Message-ID: <wccr9w1bfav.fsf@world.std.com>#1/1
X-Deja-AN: 403752649
Sender: bobduff@world.std.com (Robert A Duff)
References: <908956499.754394@dedale.pandemonium.fr>
 <362DF0D3.BC101364@spam.innocon.com> <70kvnv$gvo$1@supernews.com>
Organization: The World Public Access UNIX, Brookline, MA
Newsgroups: comp.lang.ada
Date: 1998-10-22T00:00:00+00:00
List-Id: <comp.lang.ada>

"Pat Rogers" <progers@NOclasswideSPAM.com> writes:

> A bit too strong there, IMHO.  Access parameters are a very handy
> alternative when you want to derive from a type that has primitive
> subprograms with a formal parameter of a named access type.  Why?
> Because access parameters are not named access types, so the
> derivation provides primitives with the desired signatures.  For
> example:
> 
>   type T1 is tagged ...
> 
>   type T1_Pointer is access T1;
> 
>   procedure Foo( This : in T1;   That : in T1_Pointer );

But in *most* cases, "That" should be either "in out T1" or "in out
T1'Class".  You only need it to be a pointer in certain special cases
(eg you want Foo to save the pointer into some global data structure).
I agree that if you need a pointer, "access T1" is often what you want.

I agree with the other posters, who said to use "out" or "in out"
usually, and only use "access" if there's some particular reason for it.

I must admit, thought that it's annoying that T2_Pointer is not
implicitly convertible to T1_Pointer.  That conversion is perfectly safe
(whereas the other direction involves a run-time check, and should be
explicit).

> Then, in a derivation:
> 
>   type T2 is new T1 with ...
> 
> This gives us:
> 
> 
>   procedure Foo( This : in T2;  That : in T1_Pointer );
> 
> Note that the type of the formal 'That' is still T1_Pointer,
> probably not what we wanted.
> 
> On the other hand, had we defined things as:
> 
>   type T1 is tagged ...
> 
>   -- don't need this now...  type T1_Pointer is access T1;
> 
>   procedure Foo( This : in T1;   That : access T1 );
> 
> The derivation would provide
> 
>   procedure Foo( This : in T2;   That : access T2 );
> 
> which is probably the desired result in this case.

- Bob
-- 
Change robert to bob to get my real email address.  Sorry.