On Fri, 22 May 2020 at 15:25, Esteban Lorenzano <esteba...@netc.eu> wrote:
> On 22 May 2020, at 04:24, Sean P. DeNigris <s...@clipperadams.com> wrote: > > Esteban Lorenzano wrote > > The class comments are not clear? > > > Maybe it's me ;-) > > No. I struggled with this also, and still don't have it fully grasped. I could follow Eteban's advice in a particular case I asked about, but still feel hard to reason about next time. http://forum.world.st/typedef-pointerArity-for-FFIOpaqueObjects-td4914975.html http://forum.world.st/UFFI-isPointer-absolute-or-versus-naturalPointerArity-td4917236.html > > > Esteban Lorenzano wrote > > Opaque object = https://en.wikipedia.org/wiki/Opaque_data_type > <https://en.wikipedia.org/wiki/Opaque_data_type> > External object = a pointer to something. > > > For example, why is ObjCClass a subclass of FFIExternalObject instead of > FFIOpaqueObject when the docs say that a "Class [is] An opaque type that > represents an Objective-C class"? Also, is the distinction just conceptual? > I don't see what either class "buys" you in behavior since they are > siblings > that both pretty much have no methods. > > > This is because ObjCClass = SomeOpaqueStructurePointer (is already a > pointer), same as ObjCSelector and ObjCMethod. > Difference is subtle, because is just a different way of representing the > same. > > The easiest way I have to explain is: > As a prelude to my next comment, how does "External" distinguish between the two case? Aren't both cases dealing with external C data? 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). > That makes the distinction really clear. So if now IIUC, both cases deal with "opaque types" with the *only* difference between the two being the arity? Then I propose the following naming would be more intention revealing... OpaqueObject when you will refer to "the opaque type" in calls as: someFunction(opaqueobject *someArg). OpaquePointer when you will refer to "the opaque type" in calls as: someFunction(opaquepointer someArg). And to test my new understanding, the matching typdefs and FFI calls would be... OPAQUE OBJECT - typedef MyOpaqueType opaqueobject. - int someFunction(opaqueobject *someArg) - FFIOpaqueObject subclass: #MyOpaqueType. - self ffiCall: #(int someFunction( MyOpaqueType *ot ) ). - opaque type has arity 0, so FFI needs to take a pointer to it OPAQUE POINTER - typedef MyOpaqueObject *opaquepointer. - int someFunction(opaquepointer someArg) - FFIOpaquePointer subclass: # MyOpaqueType. - self ffiCall: #(int someFunction( MyOpaqueType ot ) ). - opaque type has arity of 1, so FFI uses it directly The latter FFI call is without the *… because FFIOpaquePointer is already a pointer. In practice, both FFI calls are identical, and you use the one that matches the C header definitions. ...or maybe I still don't get it. cheers -ben
