Re: [boost] request for interest in a garbage collection library

Subject: Re: [boost] request for interest in a garbage collection library
From: Achilleas Margaritis (axilmar_at_[hidden])
Date: 2009-04-16 16:19:45

David Abrahams wrote:
> on Mon Apr 13 2009, Achilleas Margaritis <axilmar-AT-gmail.com> wrote:
> 
>> I would appreciate it if people could take a look and share their
>> opinions on it.
> 
> I haven't looked at it, but IMO the biggest obstacle to effective GC for
> C++ isn't the lack of a library (Boehm's collector is pretty good
> AFAIK). 

I have to disagree with that. According to Boehm himself 
(http://www.hpl.hp.com/personal/Hans_Boehm/gc/gcinterface.html):

"The C++ interface is implemented as a thin layer on the C interface. 
Unfortunately, this thin layer appears to be very sensitive to 
variations in C++ implementations, particularly since it tries to 
replace the global ::new operator, something that appears to not be 
well-standardized. Your platform may need minor adjustments in this 
layer (gc_cpp.cc, gc_cpp.h, and possibly gc_allocator.h). Such changes 
do not require understanding of collector internals, though they may 
require a good understanding of your platform. (Patches enhancing 
portability are welcome. But it's easy to break one platform by fixing 
another.) "

I have tried to use the Boehm GC in C++ and there were many problems:

1) can not be integrated with threading libraries such as Qt threads or 
boost threads. For example, under Windows, Boehm GC supposes you use 
Win32 threads, and it provides functions that replace Win32 calls.

2) the replacement of the global operator new creates many problems with 
  3rd party libraries which provide their own operator global new (MFC, 
for example).

3) there are few details on what is supposed to work with dlls and 
globals initialized before main(). I tried asking on the newsgroup, they 
did not know the answer. They said that it's platform specific, and 
sometimes allocating objects before main is ok, sometimes it is not, 
sometimes you have to call gc_init(), sometimes you have not to etc.

I can point you to the relevant discussions on the boehm gc newsgroup, 
if you wish.

> It's the lack of a programming model that integrates
> destructors and peoples' legitimate expectations that they will be
> called to release non-memory resxources.  What can you say about the
> non-memory resources owned by a GC'd object that contains, say, a mutex?
> 

Personally, I don't see why resources like mutexes or files should be 
managed like memory. Resources like that are "binary" resources: they 
have two states (locked/unlocked, opened/closed), which is perfectly 
suited for RAII.

Can you please elaborate on the problem of destructors? I have read some 
papers that say that gc and destructors don't play well together, but I 
did not really agree with that. An example would be appreciated.