[Openmcl-devel] How to access Cocoa objects returned via indirect references (i.e. NSError)
pbpublist at comcast.net
Wed Jul 26 05:41:25 EDT 2006
On Jul 25, 2006, at 2:14 AM, Gary Byers wrote:
> ;; First, we (stack) allocate a pointer to a pointer to an
> ;; NSObject (something of foreign type :id).
> (rlet ((error-ptr :id))
> ;; At this point, the value of ERRROR-PTR is well-defined (it's
> ;; an address on a stack, and if we run this in the same thread
> ;; and in the same execution context, it'll always be pointing
> ;; to the same address.)
> ;; Since we haven't done anything to initialize the word of
> ;; memory that ERROR-PTR points to, the result of doing
> ;; (pref error-ptr :id) at this point is a little less
> ;; the value is "whatever (garbage) was in that stack location".
> ;; We don't care; all that we've done is to reserve a word of
> ;; memory. We'll pass the address of that word to an ObjC method,
> ;; and it may store the address of some (constant or other)
> ;; object there.
> ;; On second thought, let's be defensive and store a NULL pointer
> ;; there.
> (setf (pref error-ptr :id) (%null-ptr))
> (let* ((the-url-object
> (send (@class "NSURL")
> :url-with-string #@"http:/blah"))
> (make-objc-instance 'ns:ns-xml-document
> :with-contents-of-url the-url-object
> :options (logior #$NSXMLNodePreserveWhitespace
> :error error-ptr)))
> ;; Were we able to create an NSXMLDocument ?
> (if (%null-ptr-p the-xml-document)
> (error "Error creating NSXMLDocument, NSError = ~s"
> (pref error-ptr :id))
> The above is untested code; I'm working on something else at the
> and can't easily test it (the only machine that I have free is running
> 10.3.9); I think that that's the general idea, however, and barring
> typos or other brain damage it should work fine.
> It's hard to know just by looking at a C/ObjC function prototype
> whether something passed by reference is an input, output, or both.
> In the case of this message, the parameter associated with the
> :error component of the messagee is something returned by the message,
> so the caller needs to reserve space into which the callee can store
> that extra return value.
> Actually, I was able to get that to work (so just beware of the fact
> that the code above neglects to use ccl:: prefixes where it
> should ...),
> and running it produced:
Thanks very much, that filled in the blanks for me.
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