Re: [boost] Re: Cyclic smart pointers (holy grail: the uber-pointer)

From: Gregory Colvin (gregory.colvin_at_[hidden])
Date: 2003-05-28 14:33:11

On Wednesday, May 28, 2003, at 13:04 America/Denver, Chuck Messenger 
wrote:

> Larry Evans wrote:
>> Chuck Messenger wrote:
>> [snip]
>>> collections, put them in object heirarchies, etc).  This freedom 
>>> should ideally apply both internally (within library L code) and 
>>> most importantly, externally (in the code of users of library L).  
>>> Crucially,
>> Would you require the users to use a smart pointer instead of a raw?
>> If not, then you're only option, that I can tell, is use a 
>> conservative
>> collector like Boehm's.
>
> No -- users don't use pointers -- only objects.  The objects each have 
> one pimpl_ (which will be whatever "smart" type "we" - the 
> uber-pointer framework designers - want), pointing to the underlying 
> object.
>
> Note:  This is essentially a plain-vanilla Java-style object system 
> which I am describing.
>
>>>  The consequence of mis-identification is
>>> that you may fail to destroy some unused objects.  If this leads to 
>>> nothing worse than some leaked memory, then it's not a real problem 
>>> -- it will be a vanishingly small amount of memory.
>> This is the justification for Boehm's conservative collector.
>>  http://www.hpl.hp.com/personal/Hans_Boehm/gc/
>
> Does the Boehm collector likewise do a full scan of the heap?  I 
> assume so...

It marks all objects on the heap pointed to by the machine registers, 
and
all objects pointed to by those objects, and so on.  It is conservative 
in
the sense that it cannot generally tell whether a register or memory 
cell
contains a pointer or an integer.

>>> One big problem with this approach is that you end up having to scan 
>>> all of your memory.  This could (and for me, would) be an outrageous 
>>> proposition, as only a tiny portion of memory relates to my object 
>>> set.  Most of it will be raw data (e.g. images, etc).

Boehm's collector includes functions for allocating pointer-free memory.

>> This is what Christopher's (alias= global rc mark-scan) algorithm 
>> does.
>> It's also what mark-sweep algorithms do, i.e. they have to mark all 
>> reachable
>> objects, and then scan thru the whole heap sweeping the unmarked into 
>> the
>> garbage collector. However, since this global scan is done, usually,
>> infrequently, the time isn't that big a factor, at least compared 
>> with the
>> updating of reference counts during each pointer copy.
>> That's why Boehm's gc should work faster than refcounting.

It does.