Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!bloom-beacon!mit-eddie!uw-beaver!apollo!mrst!sdti!wmm
From: wmm@sdti.SDTI.COM (William M. Miller)
Newsgroups: comp.lang.c++
Subject: Re: Constructors and new
Message-ID: <458@sdti.SDTI.COM>
Date: 2 Jun 89 12:14:00 GMT
References: <2684@ssc-vax.UUCP>
Reply-To: wmm@sdti.UUCP (0006-William M. Miller)
Organization: Software Development Technologies, Sudbury MA
Lines: 16

In article <2684@ssc-vax.UUCP> dmg@ssc-vax.UUCP (David Geary) writes:
>  According to BS, section 5.5.7, a constructor for a class can determine whether
>it was called by new or not.
That only works if the constructor assigns to `this.'  (That's the context
of the statement in 5.5.7 you mention, although the connection is not as
clear as it ought to be.)  What happens is that, if there is an assignment
to `this' in the body of the constructor, the value of `this' is left as
it is upon entry until that assignment occurs: zero if allocated via `new,'
nonzero for static and auto.  If there is no assignment to `this,' the
compiler inserts code at the beginning of the constructor, before any of
the statements in the body, to check if `this' is zero and do the allocation
via operator new() if needed.  The allocation and assignment to `this' have
already occurred before the test in your example constructor.

This question illustrates the reason assignment to `this' is replaced in
version 2.0: it's confusing, unclear, error-prone, etc.