Re: Are throwing default constructors bad style, and if so, why?
On 2008-09-20 08:23, Andrei Alexandrescu wrote:
I asked the question because I'm coming from a slightly unusual angle.
I've been using a garbage-collected environment for a while. When a GC
is in the equation *together* with deterministic destruction,
Interesting Things(TM) happen. One of them is that it's a great idea to
separate teardown (destruction) from deallocation. For example:
Foo * p = new Foo(...);
...
delete p;
In a GC environment, it makes a lot of sense if the delete statement
only invokes Foo's destructor against *p and then... leaves the object
alone in a "valid but non-resource-consuming" state.
Such an approach is not foreign to many on this group; there's been many
a discussion about what was descriptively called "zombie" object states.
The advantage of separating teardown from deallocation is that there are
no more dangling pointers, and consequently even incorrect programs
could achieve predictable, reproducible, and consequently debuggable
behavior. That way you get to use GC for what it's best at (recycling
memory) and deterministic teardown for what it's best at (timely release
of resources). In fact I'd go as far as saying that I think separating
teardown from deallocation an essential ingredient in successfully
marrying deterministic teardown with garbage collection. Other schemes
could be conceived, and I've thought of a few, but at least those I
could imagine lead to a m??salliance at most.
This really is not my area of expertise, but I fail to see how leaving
the object in an zombie-state will improve when it comes to dangling
pointers. After all, sooner or later the GC will come and collect the
used memory and it might be used by some other object. So at most you
manage to hide the fact that you are using an invalid pointer a bit longer.
I do agree that separating teardown from deallocation makes sense,
perhaps not only in a GC (I can imagine batching deallocations with
without a GC) but I still think that after the destructor has run the
object should be considered no longer existent.
--
Erik Wikstr??m
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