Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!wuarchive!uunet!microsoft!jimad
From: jimad@microsoft.UUCP (Jim ADCOCK)
Newsgroups: comp.lang.c++
Subject: Re: Smart pointers and stupid people (was: garbage collection...)
Message-ID: <70776@microsoft.UUCP>
Date: 19 Feb 91 18:03:25 GMT
References: <TOM.91Jan28082617@hcx2.ssd.csd.harris.com> <3779@lupine.NCD.COM> <70606@microsoft.UUCP> <3945@lupine.NCD.COM>
Reply-To: jimad@microsoft.UUCP (Jim ADCOCK)
Distribution: comp
Organization: Microsoft Corp., Redmond WA
Lines: 54

In article <3945@lupine.NCD.COM> rfg@NCD.COM (Ron Guilmette) writes:
|In article <70606@microsoft.UUCP> jimad@microsoft.UUCP (Jim ADCOCK) writes:
|+In article <3779@lupine.NCD.COM> rfg@NCD.COM (Ron Guilmette) writes:
|+
|+|Remember, C++ objects don't move.
|+
|+Essentially, GC proposals for C++ fall into two camps:
|+
|+1)  Those GC techniques that don't move C++ objects.
|+
|+2)  Those GC techniques that do move C++ objects.
|
|Jim Adcock (and others) have shown an inability to grasp the meaning of
|a simple sentence written in plain english.
|
|Read my lips:  C++ objects do not move.

I fully understand what you're saying Ron -- what you are saying is
not a hard concept to grasp.

It's just that I disagree with what you are saying.

|You can copy the contents of one C++ object to another C++ object, and
|you can then delete the first object, but C++ objects do not "move".
|
|Oh, you can make tricky use of pointers and maybe even memcpy() to
|make it *appear* that a C++ object has moved, but as far as the language
|itself is concerned, there is no such thing as a "move object"
|operation.

The language also doesn't have a concept of "page this region of memory
to secondary store" -- but that doesn't mean a particular implementation
can't use virtual memory.  Usually with hardware assist, some implementations
do indeed move objects, but do so in a way that is totally transparent
to the language.  Thus, the language doesn't care if objects move or not,
as long as the requirements of the language are met.

I claim implementations of the C++ language are free to move objects or
not -- as long as the implementation meets the stated requirements of
ARM.

Mind you, I think there are going to be some regions of ARM that are very
hard to meet using a garbage collected moveable objects strategy -- 
thus some implementations using GC may do so as an extension to the 
language, or alternatively as a restriction on some of the traditionally
allowed pointer hacks.

But then, *any* successful GC scheme is going to have to make some 
restrictions on the traditionally available set of pointer hacks.

In Summary:

Ron says C++ objects don't move period.
I say C++ objects can move as long as they obey ARM.