Hi, That’s not really possible :) In your case, Method is already a pointer (with arity 1). Which means Method * Is in fact a pointer to a pointer (arity 2), something like "void**" This is a typical case, but you are not “having the two cases”, you still have one :) UFFI will try to box/unbox that for you, btw… but sometimes this is not what you want (because pointers to pointers is a typical way to answer lists, for example). And UFFI does not know what you want there… in this cases (when you want the list), the recommended way is to get the pointer and create the list yourself. Instead: Method *methodList() You call void *methodList() To avoid the unbox attempt, and then you process your answer “in raw” (for example using the #readArrayOf:until: method). If it happens that you use both SomeStructure, and SomeStructure*, then is not an opaque type, and you need to map it by subclasses FFIExternalStructure and implementing fieldsDesc, etc. Esteban

On 23 May 2020, at 20:00, Sean P. DeNigris <sean@clipperadams.com> wrote:

Esteban Lorenzano wrote

...

The easiest way I have to explain is:

You use an OpaqueObject when you will refer to the opaque type in calls as: someFunction(opaqueobject *someArg). You use an ExternalObject when you will refer to the opaque type in calls as: someFunction(externalobject someArg).

Very clear! We're getting somewhere :) But...

What if a type is used both ways?

For example: ^ self ffiCall: #(Method * class_copyMethodList(Class self, uint *outCount)) and ^ self ffiCall: #(struct objc_method_description * method_getDescription(Method self))

----- Cheers, Sean -- Sent from: http://forum.world.st/Pharo-Smalltalk-Developers-f1294837.html