[pdal] Shared ptr madness

[Howard Butler]

I can't discuss this too much as I am on the road, but see libLAS' use of boost::optional for managing the point's liblas::Header.  It would maybe keep the semantics cleaner than the shared_ptr.  

On Jul 13, 2011, at 10:12 PM, "Michael P. Gerlek" <mpg at flaxen.com> wrote:

> We now have a factory that creates stages for us, e.g.
> 
>    boost::shared_ptr<Reader> ptr1 =
> factory.createReader("drivers.las.reader", Options::none());
> 
> Observe that this returns a shared_ptr, which is probably the Right Thing To
> Do.
> 
> Now consider the next line of code, where we create a filter using that
> pointer to a reader we just got:
> 
>    boost::shared_ptr<Filter> ptr2 = factory.createFilter("filters.crop",
> *ptr1, Options::none());
> 
> Observe that we are passing in to the filter "*ptr1" as the "previous stage"
> parameter, because the ctor for a Filter takes a "const Stage&".  We need to
> deref the shared_ptr to get the real ptr.
> 
> 
> Question: should the filter stages take shared_ptr<Stage> parameters instead
> of Stage& parameters?  It seems like if we don't pass in a shared ptr, we're
> sort of mixing models -- if you are going to have a shared_ptr to something,
> you should use that shared_ptr semantics everywhere.
> 
> 
> Possible opposing view: actually the Filters take a const reference (const
> Stage&), not a true pointer, so we need not use or worry about the
> shared_ptr semantics.
> 
> 
> Possible opposing opposing view: the MultiFilter class takes a N previous
> stages as a parameter, typed as "const std::vector<const Stage*>".  Note
> this is an actual pointer, not a reference, because you can't have a
> std::vector of reference objects.  So now we have three different kinds of
> pointers to a Stage (shared, real * ptr, and &-ref ptr) all being used,
> unless we switch to "std::vector<shared_ptr<Stage>>".
> 
> 
> -mpg
> 
> 
> _______________________________________________
> pdal mailing list
> pdal at lists.osgeo.org
> http://lists.osgeo.org/mailman/listinfo/pdal