Re: %factory and templates

From: kris.f.thielemans 
Sent: 10 October 2020 00:23

 

From: William S Fulton <[email protected] <mailto:[email protected]> > 
Sent: 09 October 2020 19:03

 

On Fri, 9 Oct 2020 at 16:42, <kris.f.thielemans > wrote:

Hi

 

We are trying to use %factory (see https://github.com/swig/swig/blob/master/Lib/typemaps/factory.swg) to let SWIG do automatic downcasting to the actual types, as opposed to just returning the base class. However, at present it isn’t working for us. Our hunch is that this is due to templates and typemaps (which I don’t understand well enough). We haven’t boiled it down to a simple example yet, but I thought maybe would know the answer straight-away 😊

 

We have a hierarchy like this (apologies for long names, but I kept the ones in our actual code)

 

stir::GeneralisedPrior<TargetT >

stir::QuadraticPrior<elemT> : public stir::GeneralisedPrior<DiscretisedDensity<3,elemT> >

stir::RelativeDifferencePrior<elemT> : public stir::GeneralisedPrior<DiscretisedDensity<3,elemT> >

 

(cutting out some intermediate classes)

 

We then have some other templated class that has a member

 

stir::GeneralisedPrior<TargetT > *GeneralisedObjectiveFunction<TargetT>::get_prior_ptr()

 

So, in our SWIG interface file we say

 

%newobject *::get_prior_ptr;

%factory(stir::GeneralisedPrior *stir::GeneralisedObjectiveFunction::get_prior_ptr, stir::QuadraticPrior, stir::PLSPrior, stir::RelativeDifferencePrior);

 

i.e. without any template stuff at all, but it seems that the typemap generated by %factory isn’t picked up.

 

Anyone any suggestions?

 

 

 

You need to instantiate your templates with %template if you aren't already.

 

William

 

Thanks William,

 

Sorry to have posted incomplete code, but it is rather long due to the somewhat complicated hierarchy. We can try to make a reduced example. The %templates are there. Indeed, all the classes are callable from Python. It is just that the “factory” (get_prior_ptr) still returns a base object (GeneralisedPrior), as opposed to one of the leaf classes (QuadraticPrior etc).

 

Relevant bits of our interface file are:

-        %newobject, %factor, %include at https://github.com/UCL/STIR/blob/9c72b9ee8d584f4fd5729f496b7927fec8f55e4e/src/swig/stir.i#L1582-L1595

-        %template stuff at https://github.com/UCL/STIR/blob/9c72b9ee8d584f4fd5729f496b7927fec8f55e4e/src/swig/stir.i#L1634-L1650

 

I’m not sure how to debug the typemap question but will have a look at the debug switches of SWIG, unless there are any easier suggestions.

 

 

After including factory.i (sorry!), it does work if I add complete template specifications, as in

 

%factory(stir::GeneralisedPrior<TargetT> *stir::GeneralisedObjectiveFunction< TargetT >::get_prior_ptr,

         stir::QuadraticPrior<elemT>,

         stir::PLSPrior<elemT>,

         stir::RelativeDifferencePrior<elemT>);

 

where I have a %define for TargetT and elemT. I suppose I would have to repeat this for every instantiation that I want. (Indeed, it seems unlikely that SWIG can figure this out if I don’t specify the template arguments, as it wouldn’t know what to insert in the wrapper code).

 

Great!

 

However, now I stumble on another problem. In our actual code, the derived class can itself be a template with 2 or more arguments, e.g. in a simple example

 

%factory(Base* Base::create, mytemplate<int,int>);

 

When running SWIG, I get an error 

 

Error: Bad $descriptor() macro.

 

I presume this is because of the comma. So I tried William’s trick he told me once about: use %arg(). As in

 

%factory(Base* Base::create, %arg(mytemplate<int,int>));

 

But that didn’t help either. Conceivably this could be because the factory macro would itself have to use %arg somehow, e.g. here

https://github.com/swig/swig/blob/b6c2438d7d7aac5711376a106a156200b7ff1056/Lib/typemaps/factory.swg#L84

but I have no idea really.

 

Can anyone help?

 

Kris

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