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From: Maxim Senin <maks@tech.swh.lv>
Subject: Help - Constructors - ASAP.
Date: 1998/07/15
Message-ID: <35ACBAD6.CC060CEF@tech.swh.lv>#1/1
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Organization: SWH Technology
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Reply-To: maxim_senin@bigfoot.com
Newsgroups: comp.lang.ada
Date: 1998-07-15T00:00:00+00:00
List-Id: <comp.lang.ada>

How to write constructor?

I tried to define constraints in type def, but if  have derived type, this will
not work:

type    A (param1, param2 : TypeOfParam) is new BaseType with record
        fld1 : TypeOfParam := param1;
        fld2 : TypeOfParam := param2;
    end record;

type    B (param : AnotherType) is new A with record    -- how can I set fld1
and fld2 using constraints?
        fld3 : AnotherType := param;
    end record;

Furthermore, if A and B are private types, then following code will also not
compile:

private
    type A.......;
    type B... is new A.....;

instanceOfA := new B' (fld1 => ..., fld2 => ..., fld3 => ...);
-- error: expected private type "B"
-- found a composite type

So, I think, constraints will not work. I see the following way to handle this:
types can defined public or setters must be provided. 1st approach is not good -
for example, if I want the object to be constucted once, and then be read-only.
For example, an event object is constucted once, and then clients can read data
from it:

    event := new Event (eventSource, eventType, eventData);
    ...
    if ((getSource (event) = this'Access) and (getType (event) = KEY_PRESSED))
then
        ...

Will anybody tell me the way to provide constuctors for objects in this manner?

I thought about defining functions:

function newEvent (eventSource : Object, eventType : EventTypes, eventData :
EventDataType) return Event is
ret    : Event;
begin
    ret := new Event;
    ret.source := eventSource;
    return  (eventSource, eventType, eventData);
end;

Hope to hear from you...

Maxim