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From: "Pat Rogers" <progers@NOclasswideSPAM.com>
Subject: Re: "out" or "access"
Date: 1998/10/21
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 <362DF0D3.BC101364@spam.innocon.com>
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Date: 1998-10-21T00:00:00+00:00
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Jeff Carter wrote in message <362DF0D3.BC101364@spam.innocon.com>...
>=:-) Vincent wrote:
>>

>> <snipped question of access parameters versus mode out
parameters>
>
> <snipped good answer by Jeff>
> Access parameters exist primarily to interfaces with subprograms
from other
>languages.

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 );

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.

---
Pat Rogers                            Training, Development and
Consulting in:
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