Re: True faiths ( was Re: The true faith )

[Tim Bradshaw <tfb@cley.com>]

* Hyman Rosen wrote:

> It does no such thing. The original problem specified an object with
> two subobjects which were owned by the parent and with a non-empty
> base object. You must write assignment for this type so that the
> destination acquires its own private copies of the subobjects of the
> source, and such that if copying any subcomponent fails (with an
> exception) you leave the destination object undamaged.

Well, one thing here is a terminological issue - in many GCd languages
assignment would not make a copy, so assignment and copying are
conceptually different operations.

But what GC does solve is not the copying problem - which, in Lisp at
least (together with the related equality problem), is regarded as an
issue which does not have a general solution nor any hope of one: Lisp
people at least do not regard copying as trivial - but the problem of
needing to copy to resolve low-level ownership issues.  If you have to
manually manage memory then it's crucial that only one person frees
the memory associated with some object, and one (bad) way of resolving
this is, if you need two handles on something, to actually make a
copy, so each copy can then be independently freed.  In a GCd language
this is a non-problem since the reuse of memory is dealt with by the
GC, so you can happily pass around a reference to the thing instead.

--tim