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From: "David Botton" <David@Botton.com>
Subject: Re: friend classes in ada95
Date: 2000/04/17
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Newsgroups: comp.lang.ada
Date: 2000-04-17T00:00:00+00:00
List-Id: <comp.lang.ada>

The format / approach of the tagged type doesn't matter to me. It is the
interfaces format that matters.

What I would like to do would look something like this in today's Ada with a
twist:

-- Interface IUnknown
type IUnknown is abstract tagged null record;
--  I like the prefix idea so I would use:  interface type IUnknown;

type PUnknown is access all IUnknown;

function QueryInterface (This : IUnknown, etc ...) return Interfaces.C.long
is abstract;
pragma Convention (C, QueryInterface);

function AddRef (This : IUnknown) return Interfaces.C.unsigned is abstract;
pragma Convention (C, AddRef);

function Release (This : IUnknown) return Interfaces.C.unsigned  is
abstract;
pragma Convention (C, Release);

-- Interface IBeep
type IBeep is new abstract IUnknown with null record;
--  I want to be able to extend interface definitions so I need something
that does this
--  I would put here:  interface type IBeep is new IUnknown;
type PBeep is access all IBeep;

function Beep (This : IBeep) return Interfaces.C.unsigned is abstract;
pragma Convention (C, Beep);

-- Interface IMessage
type IMessage  is new abstract IUnknown with null record;
type PMessage is access all IMessage;

function Message (This : IMessage) return Interfaces.C.unsigned is abstract;
pragma Convention (C, Beep);

type Beep_Class is tagged

-- class type Beep_Class is  would be so much nicer here.

-- Some how designate that Beep_Class implements IBeep and IMessage say with
the interface section I mentioned like:
--  Interface
--      IBeep, IMessage

    record
        Ref_Count : Interfaces.C.long;
    end record;

-- Implements  IUnknown for both IBeep and IMessage
function QueryInterface (This : Beep_Class, etc ...) return
Interfaces.C.long;
function AddRef (This : Beep_Class) return Interfaces.C.unsigned;
function Release (This : Beep_Class) return Interfaces.C.unsigned;

-- Implements IBeep
function Beep (This : Beep_Class) return Interfaces.C.unsigned;

-- Implements IMessage
function Message (This : IMessage) return Interfaces.C.unsigned;


The I could do:

P1 : PUnknown := new Beep_Class;
P2 : PMessage := PMessage (P1);
P3 : PBeep := PBeep (P1);

hr := AddRef (P1);
hr := Beep (P2);

etc.

I would then make P1 guts look like a pointer to:

    record
        QueryInterface : System.Address;
        AddRef            : System.Address;
        Release            : System.Address;
        Object             : System.Address;
   end record;

P2 like

    record
        QueryInterface : System.Address;
        AddRef            : System.Address;
        Release            : System.Address;
        Beep                : System.Address;
        Object              : System.Address;
    end record;

etc.

Or comething like that where I could pass a pointer to a clean table of
functions. and still get back to my object when needed. The layout of
Beep_Class doesn't matter to me.

David Botton


Robert Dewar <robert_dewar@my-deja.com> wrote in message
news:8df6fb$mm3$1@nnrp1.deja.com...
> > On Win32 I would implement the interfaces using the same
> > "vtbl" style used by MSVC++ and therefore COM.
>
> Well tagged types use this *identical* implementation approach
> so I really can't understand the distinction you are trying
> to make here.