Re: [boost] Shared_ptr "mini garbage collector"

From: Larry Evans (jcampbell3_at_[hidden])
Date: 2003-01-07 20:55:58

Peter Dimov wrote:
> From: "Larry Evans" <jcampbell3_at_[hidden]>

[snip]
> 
> This doesn't look correct to me... did you mean something like
> 
> struct X
> {
>     Y * p;
> 
>     explicit X(Y * p): p(p) {}
>     ~X() { delete p; }
> };
> 
> struct Y
> {
>     shared_ptr<X> p;
> };
> 
> int main()
> {
>     Y * py = new Y;
>     shared_ptr<X> px(new X(py));
>     py->p = px;
>     px.reset();
> }
> 
> ?
Yes.  I had thought about the need to delete the Y* in the
X::DTOR, but that was days ago and I forgot to include it.

I also thought a little more about it and what I was suggesting
was more like:

struc Y;
struct X
{
    boost::unshared_cyclic_ptr<Y> y; //cyclic involves this arc.
    explicit X(void): y(new Y) {}
};
struct Y
{
    boost::shared_cyclic_ptr<X> x;
};

Where {unshared|shared}_cyclic_ptr use the detlef method to
record their offsets and hence enable tracing the pointer graph.

Then I'd imagine you'd ask, why not just use a shared_ptr<Y> instead
of bothering with unshared<Y>.  The only answer I can think of is
that maybe that reflects better the meaning of the variable y.
The programmer knows it's never shared; so, he decides to
be explicit about it with unshared_ptr<Y>.  If he had used
shared_ptr<Y>, then someone maintaining his code would have to wonder
who else is sharing this Y?  Also, the overhead for unshared_ptr
would be at least a little less than shared_ptr (no reference count
or mark bit and no updating the reference count).