Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!rutgers!att!ulysses!andante!alice!bs
From: bs@alice.UUCP (Bjarne Stroustrup)
Newsgroups: comp.lang.c++
Subject: Re: Eiffel vs. C++ -- Let's drop the garbage collection arguments
Summary: A question about C++ GC
Message-ID: <9522@alice.UUCP>
Date: 23 Jun 89 13:56:03 GMT
References: <CLINE.89Jun9152152@sun.soe.clarkson.edu> <6590166@hplsla.HP.COM>
Organization: AT&T Bell Laboratories, Murray Hill NJ
Lines: 30


There is an important question that needs to be asked in regards to GC
of C++ programs:

	``When an object is garbage collected should its destructor
	(if any) be called?''

People often simply assume that the answer is `yes.'


This is not obvious. Consider GC as simply a relatively cheap way of
achieving an infinite memory. If we had a genuine infinite memory and
used `new' and `delete' the constructors for objects allocated using
`new' and not deleted would not be called (as C++ is currently defined).
In other words, it would still be the programmer's task to call `delete'
to trigger the destructor even if memory usage was of no concern.

There is therefore two possible models for garbage collection of C++:

	(1) the `infinite memory' model where destructors are only called
		for explicitly deleted objects, and
	(2) the `automatic destruction' model where destructors are called
		where the memory used to store objects are recycled.

The former is more `manual' and more predictable in terms of when things
happen. The latter more automatic and (I think) imposes the rule that every
object on the free store must be destroyed on normal program termination.

I suspect that the programming of destructors will differ significantly
dependent on the model adopted.