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From: dmurdoch@watstat.waterloo.edu (Duncan Murdoch)
Newsgroups: comp.lang.pascal
Subject: Re: TP6.0 OOP Quiery
Message-ID: <1991Jun17.005251.1184@maytag.waterloo.edu>
Date: 17 Jun 91 00:52:51 GMT
References: <1991Jun16.075044.18283@cs.mcgill.ca>
Sender: news@maytag.waterloo.edu (News Owner)
Organization: University of Waterloo
Lines: 27

In article <1991Jun16.075044.18283@cs.mcgill.ca> storm@cs.mcgill.ca (Marc WANDSCHNEIDER) writes:
>
>In the TP 6.0 Users manual, they start building up OOPs with objects typed
>Location and Point.
...
>
>MY problem is: Why have they declared an INIT procedure for type Point, when 
>Location already has this procedure declared.  If objects have inheritance,
>then there should already be an INIT procedure built into the Point type.

In the example you gave, you are correct:  though declared as 
constructors, the two Init routines are just procedures.  However, in more
complicated cases, it's essential that an object have its own constructor,
because it's the constructor that "stamps" the object with its dynamic
type.  If you had virtual methods in your Point, but called the Location
constructor to initialize it, you'd get the Location virtual methods, not
the Point ones.  

This causes problems sometimes.  If in your Point constructor, you make a
call to an ancestor's constructor without giving the type (impossible here,
because they both have the same name) it'll "transmute"  your object into
the ancestor type, and you'll end up very confused later.

I hope this helps a bit.

Duncan Murdoch
dmurdoch@watstat.waterloo.edu