Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!tut.cis.ohio-state.edu!unmvax!polyslo!ttwang
From: ttwang@polyslo.CalPoly.EDU (Thomas Wang)
Newsgroups: comp.lang.c++
Subject: Re: Constructors and new
Message-ID: <11629@polyslo.CalPoly.EDU>
Date: 3 Jun 89 16:15:24 GMT
References: <363@msor0.UUCP>
Reply-To: ttwang@polyslo.CalPoly.EDU (Thomas Wang)
Distribution: usa
Organization: Cal Poly State University -- San Luis Obispo
Lines: 35

In article <363@msor0.UUCP> kt@msor.UUCP (Keith Tizzard) writes:
>According to BS, section 5.5.7, a constructor for a class can determine whether
>it was called by new or not.
>"If it is called by new, the pointer this has the value zero at entry,
>otherwise this points to space already allocated for the object..."

>#include <stdio.h>
>class testclass
>{ public:
>testclass()   { if(this == 0) puts("Used new..."); else puts("Did not use new..."); }
> ~testclass()   { puts("I'm a destructor..."); }
>};
>main() {
>  testclass   var;
>  new testclass; }

Testing (this == 0) only works if you set 'this' to some value inside the
constructor.  I have a non-portable hack though.

testclass::testclass()
{
  int dummy;
  if (this < &dummy) puts("Used new\n");
  else puts("did not use new\n");
}

This hack depends on the fact that stack and heap usually grows into each
other from opposite side of memory.


 -Thomas Wang (Mak-Kuro Kurosuke, come on out!  If you don't come out,
               we'll pull your eyeballs out!
                                      - as heard in Tonari No Totoro )

                                                     ttwang@polyslo.calpoly.edu