Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!csd4.csd.uwm.edu!lll-winken!uunet!mcsun!sunic!tut!tukki!sakkinen
From: sakkinen@tukki.jyu.fi (Markku Sakkinen)
Newsgroups: comp.lang.c++
Subject: Re: this=this
Message-ID: <1277@tukki.jyu.fi>
Date: 6 Sep 89 07:38:28 GMT
References: <9638@alice.UUCP> <1273@tukki.jyu.fi> <TK.89Sep5145704@bebop.tempo.nj.att.com>
Reply-To: markku@jytko.jyu.fi (Markku Sakkinen)
          SAKKINEN@FINJYU.bitnet (alternative)
Organization: University of Jyvaskyla, Finland
Lines: 41

In article <TK.89Sep5145704@bebop.tempo.nj.att.com> tk@allegra.att.com (Thomas Kirk) writes:
>In article <1273@tukki.jyu.fi> sakkinen@tukki.jyu.fi (Markku Sakkinen) writes:
> > What useful purpose can 'this=this' possibly have served even
> > in a constructor or destructor? Help against some bug in earlier
> > releases of cfront? ...
>
>no. in constructors of objects that do their own storage
>management (that is, by assignment to `this'), it is necessary
>that all execution paths through the constructor include the
>assignment. such constructors typically check to see whether
>the object was allocated on the stack or otherwise; `this=this'
>makes sense for objects allocated on the stack.

I am familiar with the additional effects of assignments to "this"
in C++ versions so far (before 2.0);
they are even described in Stroustrup's book.
In my paper at ECOOP'88, "On the darker side of C++",
I pointed out that the book's definitions about the matter
don't work correctly in general with derived classes,
and that releases 1.1 and 1.2 of "cfront" in fact did not follow
those definitions but made an equally gross error in an opposite direction.

It is the special case of 'this=this' that I still cannot see any sense in.
Namely, if a constructor or destructor contains _any_ assignment
to "this", the translator deduces that it wants to take care of its
own memory (de)allocation, and thus does not generate a call to
"new" or "delete". Why should there be, in addition to that
meaningful assignment to "this", also 'this=this'?
Even if it is on a different execution path, it does not accomplish
anything there.

(This discussion might still have some relevance:
there is a lot of old software around, and it will be a long time
before everybody gets Release 2.0 or equivalent).

Markku Sakkinen
Department of Computer Science
University of Jyvaskyla (a's with umlauts)
Seminaarinkatu 15
SF-40100 Jyvaskyla (umlauts again)
Finland