destructors

[Daniel Keep]

Daniel Giddings wrote:
> Bill Baxter wrote:
>> Is this ok in D?
>>
>>
>> class MyClass
>> {
>>    OtherClass a;
>>    this() {
>>       a = new OtherClass;
>>    }
>>    ~this() {
>>       delete a;
>>    }
>> }
>>
>> I was just trying to fix the crash on exit in the tinyXPath code on
>> dsource, and it does the above a lot.
>>
>> --bb
> 
> I would expect that to work (it is something I would want to be able to
> do - otherwise you would need a finalise method - yuck), I can't recall
> if I have done it though?
> 
> a should still be detected as having objects referencing it until the
> MyClass destructor is finished I would think (if the delete a wasn't
> there). Otherwise you would not be able to rely on any class properties
> as still existing in its destructor (ie a.method() could cause an
> exception as well).

Exactly.  You can't.

Long story short is thus: the *only* thing you can say for sure still
exists when your destructor is called, is your own object's memory.
Look at it like this:

> class ObjPtr
> {
>     ObjPtr obj;
>     this(ObjPtr obj = null) { this.obj = obj; }
>     ~this() { ... }
> }
>
> auto a = new ObjPtr;
> auto b = new ObjPtr(a);
> auto c = new ObjPtr(b);
>
> c = null;
> gc.fullCollect();

Question: what order do the objects get destroyed in?
Answer: no one knows.

The problem is that in order for the GC to destroy objects in "the right
order", it would have to build a complete dependency graph between the
various objects, then destroy them in the right order.

But what happens if you've got a *lot* of objects, or those objects
don't care what order they're destroyed in, or worse: cycles.  If you've
got cycles, the GC is stuffed, and there *is* no correct order to
destroy them in.

There was a thread quite some time ago that demonstrated a hacked
version of phobos that allowed destructors to take a bool argument that
told the destructor if the object was being destroyed deterministically
(ie: via the delete statement, or because the object was scoped) or not.
 Sadly, it never caught on :(

	-- Daniel

-- 
int getRandomNumber()
{
    return 4; // chosen by fair dice roll.
              // guaranteed to be random.
}

http://xkcd.com/

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