On 22 May 2020, at 04:24, Sean P. DeNigris <sean@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.��
��
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.