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From: eachus@spectre.mitre.org (Robert I. Eachus)
Subject: Re: Help - Constructors - ASAP.
Date: 1998/07/30
Message-ID: <EACHUS.98Jul30114523@spectre.mitre.org>#1/1
X-Deja-AN: 376364941
References: <6p75qi$rcj@news.latnet.lv> <m3lnpg7j5s.fsf@mheaney.ni.net>
 <6pi4jq$j73$1@nnrp1.dejanews.com>
Organization: The Mitre Corp., Bedford, MA.
Newsgroups: comp.lang.ada
Date: 1998-07-30T00:00:00+00:00
List-Id: <comp.lang.ada>

In article <6pi4jq$j73$1@nnrp1.dejanews.com> dennison@telepath.com writes:

  > Hmm. You have probably been through this more than I, but it would
  > think it would be useful in some circumstances to have the
  > function be inheritable. If you need a new version of the
  > "constructor" for a new derived type, you can override the old one
  > and the client won't be able to call the old one anymore.  If you
  > make the original "constructor" class-wide instead, clients will
  > always be able to call it with any derived type.

    There are two reasons for following the advice.  First is that Ada
has a rule that inherited functions with a controlling result are
abstract, so they must be overridden.  Yes, there are cases where
constructor functions can be safely inherited from the parent, but
this rule makes you think it through. Second is that when you do want
a constructor which can return any type in the class, the right thing
to do usually is to make it a classwide type.

     So you normally finding yourself either defining a classwide
constructor or making the constructors non-primitive so they are not
derived.  For example:

    package Mixed_Lists is

      type Element_Kind is (Integer_Item, Float_Item, String_Item);
      type List is private;
      type List_Element is tagged private;
      type Integer_Element is new List_Element with private;
      type Float_Element is new List_Element with private;
      type String_Element is new List_Element with private;

      function New_Element(Kind: in Element_Kind) 
                                     return List_Element'Class;
      ...
    private
      ...
    end Mixed_Lists;

    package Mixed_List.Constructors is

      function New_Element(Value: Integer := 0) return Integer_Element;
      function New_Element(Value: Float := 0.0) return Float_Element;
      function New_Element(Value: String := "") return String_Element;

    end Mixed_List.Constructors;

    This example is contrived, becuase it uses both models and you
usually want one or the other.  (And for a type like this, you would
probably want direct append operations rather than explicit
constructors for the list elements.)



--

					Robert I. Eachus

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