www.digitalmars.com         C & C++   DMDScript  

digitalmars.D.learn - Soft/weak references?

reply Robert Fraser <fraserofthenight gmail.com> writes:
Hi all,

In Java, there's the concept of a "soft" reference to garbage-collectable data.
Basically, a soft reference is a refrence to data that can be collected once
there are no more strong references to it. It's useful for implementing, say,
caches bounded by actual usage and memory availability, rather than some
arbitrary limit.

Out of curiosity, is there any way to emulate this with the D GC?

Thanks,
Fraser
Jun 21 2007
parent reply BCS <ao pathlink.com> writes:
Reply to Robert,


 Hi all,
 
 In Java, there's the concept of a "soft" reference to
 garbage-collectable data. Basically, a soft reference is a refrence to
 data that can be collected once there are no more strong references to
 it. It's useful for implementing, say, caches bounded by actual usage
 and memory availability, rather than some arbitrary limit.
 
 Out of curiosity, is there any way to emulate this with the D GC?
 
 Thanks,
 Fraser


fill in the dummy function and this might work

struct soft(T)
{
  uint hashOfValue; // used to test if memeory was GCed and realloced
  T** ptr;
   
  set(T* t)
  {
    ptr = gcAllocaNonPtrMemory(void*.sizeof);
    *ptr = t;
     hash = t.hashOf;
  }
  get()
  {
    if(wontAV(ptr) && **ptr.hashOf == has)
      return *ptr;
    else
      retrun null;
  }
}
Jun 21 2007
next sibling parent reply Robert Fraser <fraserofthenight gmail.com> writes:
BCS Wrote:


   T** ptr;


But this means there's still a reference to it.
Jun 21 2007
parent reply BCS <ao pathlink.com> writes:
Reply to Robert,


 BCS Wrote:
 

 T** ptr;
 


 But this means there's still a reference to it.
 


It's a total hack. You get a reference to a chunk of memory that contains 
a reference to the data, but you lie to the GC and tell it that the chunk 
of memory doesn't contain any references so it won't notice the refernce.
Jun 21 2007
next sibling parent reply Sean Kelly <sean f4.ca> writes:
BCS wrote:

 Reply to Robert,
 

 BCS Wrote:


 T** ptr;


 But this means there's still a reference to it.


 
 It's a total hack. You get a reference to a chunk of memory that 
 contains a reference to the data, but you lie to the GC and tell it that 
 the chunk of memory doesn't contain any references so it won't notice 
 the refernce.


The missing piece is that this reference won't be invalidated if the 
object is collected.  Phobos has Object.notifyRegister() for this 
purpose, but it currently does not exist in Tango because such a 
callback could easily deadlock the app if it enters a "synchronized" 
block as a part of its processing.  In short, I don't know of any 
entirely safe means of implementing a weak pointer in D.  If someone can 
suggest one, I'd be glad to hear it.


Sean
Jun 21 2007
parent reply Myron Alexander <someone somewhere.com> writes:
Sean Kelly wrote:

 BCS wrote:

 Reply to Robert,


 BCS Wrote:


 T** ptr;


 But this means there's still a reference to it.


 It's a total hack. You get a reference to a chunk of memory that 
 contains a reference to the data, but you lie to the GC and tell it 
 that the chunk of memory doesn't contain any references so it won't 
 notice the refernce.


 
 The missing piece is that this reference won't be invalidated if the 
 object is collected.  Phobos has Object.notifyRegister() for this 
 purpose, but it currently does not exist in Tango because such a 
 callback could easily deadlock the app if it enters a "synchronized" 
 block as a part of its processing.  In short, I don't know of any 
 entirely safe means of implementing a weak pointer in D.  If someone can 
 suggest one, I'd be glad to hear it.
 
 
 Sean


By pure coincidence, I have just discovered a need for weak references.

I am implementing chained exceptions that use property bags for storing 
additional information about the exception. Each exception class in the 
chain has it's own property bag. When I get a property, I want to search 
all the bags starting at the first exception in the chain, iterating 
through the chain until the first property with the given name is found.

At the moment, the chain is a single linked list without predecessor 
reference so a get on middle node reference will not be able to search 
the preceding node bags.

To fix this, I want to create a double linked list but for the sake of 
the GC, I want strong references in the forward direction, and weak 
references in the reverse direction.

I am using Phobos and have no intention of moving the code to Tango 
(nothing against Tango, my project is still in the baby steps phase).

If anyone could point me to a functional weak reference implementation 
for Phobos, I would be most thankful.

Thanks ahead and best regards,

Myron.
Jun 21 2007
parent reply Bill Baxter <dnewsgroup billbaxter.com> writes:
Myron Alexander wrote:

 Sean Kelly wrote:

 BCS wrote:

 Reply to Robert,


 BCS Wrote:


 T** ptr;


 But this means there's still a reference to it.


 It's a total hack. You get a reference to a chunk of memory that 
 contains a reference to the data, but you lie to the GC and tell it 
 that the chunk of memory doesn't contain any references so it won't 
 notice the refernce.


 The missing piece is that this reference won't be invalidated if the 
 object is collected.  Phobos has Object.notifyRegister() for this 
 purpose, but it currently does not exist in Tango because such a 
 callback could easily deadlock the app if it enters a "synchronized" 
 block as a part of its processing.  In short, I don't know of any 
 entirely safe means of implementing a weak pointer in D.  If someone 
 can suggest one, I'd be glad to hear it.


 Sean


 
 By pure coincidence, I have just discovered a need for weak references.
 
 I am implementing chained exceptions that use property bags for storing 
 additional information about the exception. Each exception class in the 
 chain has it's own property bag. When I get a property, I want to search 
 all the bags starting at the first exception in the chain, iterating 
 through the chain until the first property with the given name is found.
 
 At the moment, the chain is a single linked list without predecessor 
 reference so a get on middle node reference will not be able to search 
 the preceding node bags.
 
 To fix this, I want to create a double linked list but for the sake of 
 the GC, I want strong references in the forward direction, and weak 
 references in the reverse direction.
 
 I am using Phobos and have no intention of moving the code to Tango 
 (nothing against Tango, my project is still in the baby steps phase).
 
 If anyone could point me to a functional weak reference implementation 
 for Phobos, I would be most thankful.
 
 Thanks ahead and best regards,
 
 Myron.



Look at the code in Phobos's std.signals for an example of how to use 
the notifyRegister thing to implement weak references.

Note that this does not give you general purpose weak references.  It 
will only work for class instances.  It will not give you weak 
references to structs or arrays.

--bb
Jun 22 2007
next sibling parent Myron Alexander <someone somewhere.com> writes:
Bill Baxter wrote:

 Look at the code in Phobos's std.signals for an example of how to use 
 the notifyRegister thing to implement weak references.
 
 Note that this does not give you general purpose weak references.  It 
 will only work for class instances.  It will not give you weak 
 references to structs or arrays.
 
 --bb


Thanks.

Myron.
Jun 22 2007
prev sibling parent Myron Alexander <someone somewhere.com> writes:
Bill Baxter wrote:

 Look at the code in Phobos's std.signals for an example of how to use 
 the notifyRegister thing to implement weak references.
 
 Note that this does not give you general purpose weak references.  It 
 will only work for class instances.  It will not give you weak 
 references to structs or arrays.
 
 --bb


I have attached my implementation of weak references. It has been a long 
time since I used pointers and I'm not quite sure of my understanding of 
D's handling of pointers so if you see something wrong, or if you know 
of a better way to do this, please let me know.

Best regards,

Myron.
dprogramming...myron...alexander...com
replace the first ... with  , remove the second, and replace the third 
with ".".

P.S. I canceled the original message as a comment in the source was 
inaccurate.
Jun 22 2007
prev sibling parent reply Robert Fraser <fraserofthenight gmail.com> writes:
BCS Wrote:


 Reply to Robert,
 

 BCS Wrote:
 

 T** ptr;
 


 But this means there's still a reference to it.
 


 
 It's a total hack. You get a reference to a chunk of memory that contains 
 a reference to the data, but you lie to the GC and tell it that the chunk 
 of memory doesn't contain any references so it won't notice the refernce.
 
 


All right, I'll give it a shot. Thanks!
Jun 21 2007
parent BCS <ao pathlink.com> writes:
Reply to Robert,


 BCS Wrote:
 

 Reply to Robert,
 

 BCS Wrote:
 

 T** ptr;
 


 But this means there's still a reference to it.
 


 It's a total hack. You get a reference to a chunk of memory that
 contains a reference to the data, but you lie to the GC and tell it
 that the chunk of memory doesn't contain any references so it won't
 notice the refernce.
 


 All right, I'll give it a shot. Thanks!
 


BTW I haven't even tried to complile that code so...
Jun 21 2007
prev sibling parent Myron Alexander <someone somewhere.com> writes:
Hello.

Has anyone implemented soft/weak/phantom references for D?

Is it possible to implement these without changing the current Phobos GC?

Regards,

Myron.
Jun 21 2007