Re: [boost] Shared_ptr "mini garbage collector"

From: Larry Evans (jcampbell3_at_[hidden])
Date: 2003-01-07 20:11:00

Peter Dimov wrote:
> From: "Larry Evans" <jcampbell3_at_[hidden]>
> 
>>>1. Find the two X objects (let's call them x1 and x2) on the heap, and
>>
> scan
> 
> 
>>Wouldn't this scan have to be either conservative, like BW, or use some
> 
> way
> 
>>to determine the precise location of the shared_ptr's within, .e.g.
> 
> p1->get()?
> 
> Currently I use a conservative scan that assumes that shared_ptr instances
> have a layout of
> 
> T * px;
> counted_base * pi;
> int id;
> 
> with pointers being aligned on a DWORD boundary. 'pi' must be in the count
> map, and 'id' must be detail::shared_count::id. (It's possible to test 'px'
> for validity too.) False positives should be extremely rare, but could not
> be ruled out entirely, and hence, breaking cycles may potentially corrupt
> the object.
Would false positives be any less frequent than in the BW conservative collector?
Why not use the Delef approach as demonstrated in shared_cyclic_ptr to avoid
false positives altogether?
> 
> 
>>This scan will also have to follow plain pointers.
> 
> 
> Plain pointers would have to be followed by a real collector, but why should
> a "simple cycle-breaker" bother?
> 
Because the following would require it in order to find the cycle:

struc Y;
struct X
{
   Y* y; //cyclic involves this arc.
};
struct Y
{
   boost::shared_ptr<X> x;
};

int main()
{
     boost::shared_ptr<X> p1(new X);
     boost::shared_ptr<X> p2(new X);

     p1->y.x = p2;
     p2->y.x = p1;

}