Xref: utzoo comp.object:1466 comp.lang.c++:8469
Path: utzoo!utgpu!watserv1!watmath!att!rutgers!psuvax1!uwm.edu!cs.utexas.edu!uunet!microsoft!bobatk
From: bobatk@microsoft.UUCP (Bob ATKINSON)
Newsgroups: comp.object,comp.lang.c++
Subject: Re: inheritance and `type loss'
Message-ID: <55845@microsoft.UUCP>
Date: 16 Jul 90 17:52:22 GMT
References: <SRA.90Jul10155606@alonzo.ecs.soton.ac.uk> <1130013@gore.com>
Reply-To: bobatk@microsoft.UUCP (Bob ATKINSON)
Organization: Microsoft Corp., Redmond WA
Lines: 53

In article <1130013@gore.com> jacob@gore.com (Jacob Gore) writes:
>/ comp.object / sra@ecs.soton.ac.uk (Stephen Adams) / Jul 10, 1990 /
>(Of course, if this was a common enough operation, "+" would be better
>defined as
>	+: Like Self x Integer --> Like Self
>or
>	+: Like Self x Integer --> Integer.)

I have run across a similar desire in C++ for a "same type as receiver" 
type expression in constructing collection classes.  For example, I
would like to express the fact that all collections can return
a copy of themselves.  Naturally, I would like to put this declaration
in the same class in which I express the other fundamental collection
operations, such as, for example, insertion of elements or enumeration.
This is in class Collection.  Thus, I am tempted to write:

	class Collection 
	{
	public:
		virtual	Collection* 	Copy();
	};

but of course, this gives the incorrect return type, since expressions
of the form 

	class Set : public Collection { /* ... */ };
	//...
	Set* pset = pSomeOtherSet->Copy();

give a type error.


What I really want to say is something like

	class Collection
	{
	public:
		virtual	typeof(*this)*	Copy();
	};

Though I have only thought about it in briefly, I believe this interacts
cleanly with the automatic type conversions of Derived* to Base*.

It strikes me that the typeof() operator might be generally useful
in other situations as well.

One might ask if parameterized types would allow us to express this
construction.  I don't see how the current parameterized type proposal
would accomplish it.


	Bob Atkinson
	Microsoft