Please help with GC exception!

[Steven Schveighoffer]

On Wed, 09 May 2012 12:12:55 -0400, Gor Gyolchanyan  
<gor.f.gyolchanyan at gmail.com> wrote:

> I'm not deleting the member. I'm deleting memory, allocated by the  
> member.

The member is a struct, so it's fully contained within the class's block.   
Therefore, the memory block is a member, even if indirectly so.

> If GC deleted it before the object, the same error would appear when I
> forced a GC collection cycle.
> Also, docs clearly say, that I'm free to delete the memory myself.
> This means, that I shouldn't care if a collection cycle went before my
> deletion!

Here is how it works.  This is not your example, it's an even simpler one.

Memory block A has a pointer to memory block B.  Both are GC allocated.  A  
is an object with a dtor that calls GC.delete(B).  Since A has the only  
pointer to B, B is not removed by the GC as long as A is pointed at.

Now, A is no longer pointed at, and a GC collection runs.

The GC marks all memory, does not mark A, and therefore, does not mark B,  
so now A and B are scheduled for deletion.

After the mark period, the GC cycles through all deletable (unmarked)  
memory blocks, and finds *B first*.  Since B has no dtor, it's simply  
deallocated.

However, A has a destructor.  So first, the runtime runs A.__dtor(), which  
*again* deletes B.  This is where the failure occurs.

For this reason, you cannot have a destructor which deletes GC-allocated  
memory.  This is even in the spec.

-Steve