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From: Robert A Duff <bobduff@world.std.com>
Subject: Re: Inheritance and Polymorphism in Ada !!
Date: 1999/10/18
Message-ID: <wccvh841u7i.fsf@world.std.com>#1/1
X-Deja-AN: 537563581
Sender: bobduff@world.std.com (Robert A Duff)
References: <7u64k3$l1d$1@hiline.shinbiro.com> <3806DC34.1513E8B1@frqnet.de>
Organization: The World Public Access UNIX, Brookline, MA
Newsgroups: comp.lang.ada
Date: 1999-10-18T00:00:00+00:00
List-Id: <comp.lang.ada>

Andreas Winckler <andreas.winckler@frqnet.de> writes:

> Yes, but in my opinion polymorphism is kind of restricted in Ada.

Compared to which language?  Polmorphism in Ada works pretty much the
same as in Java, C++, Eiffel, &c, roughly speaking -- although the
syntax is rather different.  I suppose you could argue that Ada's
polymorphism is more restrictive (for good reason!) than Smalltalk's,
but your example wouldn't be allowed in Smalltalk anyway, so I don't
know what you're comparing Ada against.

> See this example:
> 
> 1  procedure inheritance_polymorphism is
> 2
> 3  type type_a is tagged record
> 4           first_field	: integer;
> 5       end record;
> 6
> 7  type type_b is new type_a with record
> 8           second_field : integer;
> 9       end record;
> 10
> 11 type pointer_type is access all type_a'class;
> 12 pointer	: pointer_type;
> 13
> 14 begin
> 15     pointer := new type_b;
> 16     type_b(pointer.all).second_field := 1;
> 17 end inheritance_polymorphism;
> 
> See line 16, the type of the referenced object must be known in advance.
> With "pointer.second_field :=1" the compiler fails. It seems that the
> strong typing restricts the features of polyormism in Ada. Any comments?
> Am I wrong?

Hard to tell -- you just said it's "restrictive", but you didn't say
compared to *what*.  It seems clear to me that you shouldn't *want* the
above example to be legal.

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