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From: steve@taumet.com (Stephen Clamage)
Newsgroups: comp.lang.c++
Subject: Re: operator++() and testing 'this'
Message-ID: <482@taumet.com>
Date: 19 Oct 90 18:01:15 GMT
References: <1990Oct14.224706.1934@nowhere.uucp> <58242@microsoft.UUCP> <479@taumet.com> <58351@microsoft.UUCP>
Organization: Taumetric Corporation, San Diego
Lines: 45

jimad@microsoft.UUCP (Jim ADCOCK) writes:

>There does exist some compilers that support the (this==0) test --
>provided you also perform the assignment to this hack.  I guess this
>is a case where two wrongs *do* make a right!

>What do those compilers do for embedded objects? Presumably they always
>state (this!=0) even if the embedded object is being created on the heap?
>[IE, the (this==0) test, should your compiler decide to accept it, doesn't
>really test if an object is on the stack vs heap or not, but rather whether
>space has already been allocated for that object or not.]

You are really making this more complicated than it is.  The assign-to-this
was a hack to allow constructors to do their own allocation.  The
technique was found to be unworkable in the presence of mulitple
inheritance (and was ugly as sin anyway), so was declared obsolescent
in favor of overloading new and delete.  This nicely separates the
orthogonal concerns of object initialization and space allocation.

If a class C has one or more base classes, let's call C a complete object,
and each of the base classes a sub-object.  Data members of C are each
complete objects, and each may in turn have sub-objects.

If a constructor for C contains an assignment to "this" and is invoked
as a result of using "new" to create a complete object of type C, then
"this" will be zero on entry to C's constructor.  In all other cases,
"this" will be the address of the object (or sub-object) being
constructed.

Ordinarily, base classes are constructed prior to executing any of
the user code in the derived class constructor.  Continuing the above
example (C::C() contains an assignment to "this"), a statement like
	C *cp = new C;
means that C::C() is entered with this==0 and no base classes have
been constructed -- and of course no space for the object has been
allocated.  Immediately following each assignment to "this" in C::C(),
the compiler generates code to construct all of the base classes and
perform all of the initializations in the constructor header.  Such
construction and initialization cannot be done sooner than assignment
to "this", and must not be done later.  This is why each possible
execution path through C::C() must contain one and only one assignment
to "this".
-- 

Steve Clamage, TauMetric Corp, steve@taumet.com