Re: in defense of GC (was Re: How come Ada isn't more popular?)

[Ray Blaak <rAYblaaK@STRIPCAPStelus.net>]

Gautier <gautier@fakeaddress.nil> writes:
> Sorry for my ignorance in this field, but from a real household point of view,
> it seems to me that there is a big difference between
>   (1) "stop using an object"
> and
>   (2) "drop an object on the floor".
> 
> In case of (1), I would hate that my object is taken from the table and thrown
> away by my garbage collecting robot GeeCee. Hey, does he know I really won't
> use my object anymore ?! In the case (2), it's OK that GeeCee takes it away,
> but then there is an action (Drop_on_the_floor) and I can do it in Ada
> (procedure Drop_on_the_floor_and_even_directly_throw_away is new
> Ada.Unchecked_Deallocation).

I was not sure if this was serious or not, so if it was...

Regarding (1), if you are not sure you will use an object anymore, then the GC
will not collect it, *by definition*, since it must be still reachable in some
way in order for it to be even possible to use. GC only collects unreachable
memory.

Regarding (2), doing a matching "drop" for every "new" is the whole trouble,
of course. What if you miss one? Well, with GC, you don't (need to) do an
explicit "drop" at all. And that's the freedom.

Now nothing is completely perfect and free. You can still have memory leaks
with GC, of course, but they are of a different kind. Instead of a lost
pointer that still refers to allocated memory, one has live objects still
being referred to in some way, i.e., "not lost" pointers that one no longer
needs to refer to.

So, the most one needs to do is to clear a pointer/reference occasionally.
Note though, that this is far far simpler than, for example, carefully
deallocating each node of some long lived complex data structure, cycles and
all. Instead one simply clears the reference to the root of entire complex
structure, and it all just goes away.

In the case of stacked based references (which are the most common kind),
there is nothing do to. New objects are made, maybe returned out to the
caller, maybe not, without regard to destruction, copy construction, etc. It
just is no longer necessary.

-- 
Cheers,                                        The Rhythm is around me,
                                               The Rhythm has control.
Ray Blaak                                      The Rhythm is inside me,
rAYblaaK@STRIPCAPStelus.net                    The Rhythm has my soul.