On 08/09/2009, at 1:31 AM, Paulo F. Andrade wrote:
I don't get this. Won't the call
[sender value]
be dealt by objc_msgSend? Will it not traverse the isa pointer to
get the method table and find the method named "value" and call
that? At least from the UISlider class hierarchy there seems to be
only one method named "value", no ambiguity there.
Yes, but the return type will cause different compilation of
objc_msgSend. There are several variants of that (obc_msgSend_fpret,
objc_msgSend_stret, etc) depending on which registers are expected to
return what sorts of values - structs vs. floats vs. integers/objects
for example. So the right method will get called but within the wrong
context of what will be returned. You can verify this yourself by
disassembling some variants of calling a test method - if only the
return type varies and the compiler can't disambiguate based on class
of the receiver, it will compile the first version it finds.
What's probably occurring in your case is that the compiler has used a
version (such as -(id) value) that returns its value in a certain
register, whereas the -(float) value method returns it in a different
register (an fp register, generally). It just so happens that the
register that the caller pulls the result from holds the slider object.
I know this. I just want to fully understand why this is happening
for this particular name "value" and not for "maximumValue".
Probably because -value is ambiguous and -maximumValue is not.
Trust me - this happens. It's bitten me once or twice and I won't let
that happen again, as it's so hard to understand what's going on and
debug it.
--Graham
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