Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!utgpu!water!watmath!clyde!rutgers!seismo!mcvax!ukc!eagle!icdoc!dcw
From: dcw@icdoc.UUCP
Newsgroups: comp.lang.c++
Subject: Re: Questions About C++
Message-ID: <456@ivax.doc.ic.ac.uk>
Date: Fri, 12-Jun-87 12:37:26 EDT
Article-I.D.: ivax.456
Posted: Fri Jun 12 12:37:26 1987
Date-Received: Thu, 18-Jun-87 01:00:28 EDT
Sender: dcw@doc.ic.ac.uk
Reply-To: dcw@doc.ic.ac.uk (Duncan C White)
Organization: Dept. of Computing, Imperial College, London, UK.
Lines: 56


Summary, part 2/3:


Our 'Basic Difference'		We thought that SIMULA allows a subclass
----------------------		instance to be assigned to a superclass
				variable, and that C++ doesn't.

Many people pointed out that THIS IS DEAD WRONG :  [ ooooppps... ]

In SIMULA, the ONLY way of declaring an instance is:

	ref( fred ) x;		// x is an instance of fred, which is a
				// class declared somewhere above

Our mistake was that we assumed that C++, like SIMULA, had ONLY one way to
define instances :

	fred x;			// instance of fred

whereas, of course, an instance can also be declared using :

	fred *xptr;		// not initialised yet: like SIMULA ref
or:
	fred& xref = x;		// must have some initialization with it,
				// except when its a function parameter

So, C++ is quite happy with :

	vehicle *v = new car;
and
	car c;
	vehicle &v = c;

This, of course, means that you can, after all:

>...declare a function which takes a parameter of type vehicle *,
>and then call it with an expression which is a car *.
>
>...define a 'list of vehicles' into which you can put any
>subclass-instances [ cars, planes, boats, pogo-sticks, minis, fords,
>aston martins etc.. ]

Several people commented that C++ would indeed be a weak form of object
orientated language if it couldn't handle this kind of thing: this is
precisely what we thought [ and had trouble believing ].

NB: Another common problem was mentioned by a few people: if any member of
the class hierarchy was not derived publicly from its immediate superclass,
even the pointer conversions would not work...

For example:

class vehicle ..
class car : vehicle ..		// if 'public' omitted, then
vehicle *v = new car;		// this declaration is rejected, apparently