Hi, The class comments are not clear? Opaque object = https://en.wikipedia.org/wiki/Opaque_data_type <https://en.wikipedia.org/wiki/Opaque_data_type> External object = a pointer to something.

On 21 May 2020, at 21:53, Sean P. DeNigris <sean@clipperadams.com <mailto:sean@clipperadams.com>> wrote:

I'm working on improving the documentation. I'd like examples of situations when you'd use one or the other and why...

Also, the FFIOpaqueObject class comment could be fleshed out a bit. Check out this excerpt: "this means we always access them through a reference. Now, we could declare the pointer to the structures, but then our declarations wouldn't be as close to C as we want."

What would be an example of "declar[ing] the pointer to the structures" and the resulting declarations that "wouldn't be as close to C as we want"?

There are plenty of cases in C where you do: struct MyStruct *s; Buy tou cannot do (or you cannot use): struct MyStruct s; This is like that because most modern frameworks use the opaque type as a place to keep state, but to interact with them you use functions. e.g: MyOpaqueType *ot = newOpaqueType(); opaque_type_someFunction(ot, “a parameter”); Then an opaque object would be: FFIOpaqueObject subclass: #MyOpaqueType. And you can do an ffi call like this: self ffiCall: #(void opaque_type_someFunction(MyOpaqueType *ot, char *aString)). Note the declaration of "MyOpaqueType *ot” This is how C declaration looks. We can say that an opaque type is a pointer type with arity 0. Now, an FFIExternalObject (which should be called FFIExternalPointer), has already an arity of 1. Which means, if you declare: FFIExternalObject subclass: #MyOpaqueTypePtr. The same ffiCall you did before would be: self ffiCall: #(void opaque_type_someFunction(MyOpaqueTypePtr ot, char *aString)). Note the declaration without the *… this is because FFIExternalObject is already a pointer. Of course, you can write the function call both ways and it works identically… but the first one is closer to what you do in C regularly. Esteban

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