(Semi) precise GC [was: Re: Std Phobos 2 and logging library?]

[Frits van Bommel]

Robert Jacques wrote:
> On Tue, 14 Apr 2009 09:27:09 -0400, Frits van Bommel 
> <fvbommel at remwovexcapss.nl> wrote:
>> Robert Jacques wrote:
>>> On Tue, 14 Apr 2009 06:04:01 -0400, Frits van Bommel 
>>> <fvbommel at remwovexcapss.nl> wrote:
>>>> Using D2 structs with a moving GC would need some extra bookkeeping 
>>>> data anyway, to work out things like their postblit call.
>>>  Postblit is only called when generating an actual copy. For example 
>>> it is not called on assignment is the source is no longer used. So I 
>>> don't see any reason why it should, or it would be expected that 
>>> postblit would run when a struct was moved using the GC.
>>
>> Oh, I didn't know that. (I haven't done much of anything with D2, I 
>> mostly stick to D1)
>> I just presumed they were like C++ copy constructors.
>>
>> As an aside: I can certainly think of some places where it would be 
>> useful to have them get called whenever the address changes...
>> (Though "move constructors" would be even better for most of those cases)
> 
> Could you document this use case? (i.e. give some examples as I can't 
> think of any)

Any situation in which structs register themselves somewhere for one reason or 
another.

For example, I read that C++'s shared_ptr<> could be implemented by having the 
instances keep a doubly-linked list of themselves instead of using an extra heap 
allocation for the reference count. Such an implementation would need to update 
the pointers in neighboring nodes when moved, or insert itself before or after 
the original when copied.
Note that shared_ptr<> is not only useful for memory resources, it could also be 
used to e.g. keep a file handle or socket open until all users are done with it 
(and not longer, as you might get with a GC'ed file class).
Of course, in this case the more traditional approach with a heap-allocated 
reference (or even storing it in the Monitor structure each object has a pointer 
to) would be just as viable.

But you could also implement weak references in a similar way, to let the GC 
find them in a linked list and allow them to be nulled when referred-to objects 
get collected.

There are probably other use cases...