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From: dld@F.GP.CS.CMU.EDU (David Detlefs)
Newsgroups: comp.lang.c++
Subject: Re: Constructors and new
Message-ID: <DLD.89May31104058@F.GP.CS.CMU.EDU>
Date: 31 May 89 14:40:58 GMT
References: <2684@ssc-vax.UUCP>
Organization: CMU CS Department
Lines: 45
In-reply-to: dmg@ssc-vax.UUCP's message of 30 May 89 22:10:40 GMT

Dave Geary asks why this program:

#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;
>}

produces this output:

>Did not use new...
>Did not use new...
>I'm a destructor...

I believe I can explain this.  Look at 'main.'  The first line of the
output is caused by the constructor call to initialize 'var', a
stack-allocated automatic variable.  The second line is caused by the
evaluation of the expression 'new testclass;'  This *does* use new, so
why does it say it doesn't?  Well if you look at testclass, you'll
find that sizeof(testclass) is 0.  I would guess that the 'malloc' that
operator new is calling returns 0 when it is asked to allocate a
zero-sized object.  Thus, 'this' is 0 on entry to the constructor.
Finally, the testclass destructor is called when 'var' goes out of
scope.

Hope this helps.

Dave

--
Dave Detlefs			Any correlation between my employer's opinion
Carnegie-Mellon CS		and my own is statistical rather than causal,
dld@cs.cmu.edu			except in those cases where I have helped to
				form my employer's opinion.  (Null disclaimer.)
--