So I’m reviewing a book, and it happens to cover the way method lookups are performed in a particular language’s object model. I’m not going to say what the book is because it’s not relevant, and nothing discussed here has anything to do with that publication. But reading it made me realise just why even when the language allows it, my internal pain aversion system will always skip multiple inheritance as an option. Consider this Python code (with built-in python shell promptness):
>>> class One(object):
... def doStuff(self):
... print "One.doStuff()"
...
>>> class Two(One):
... pass
...
>>> class Three(One):
... def doStuff(self,x):
... print "Three.doStuff(%d)" % x
...
>>> class Four(Two,Three):
... pass
...
>>> class Five(Three,Two):
... pass
...
>>> a=Four()
>>> b=Five()
so, what is the method signature of a.doStuff() and b.doStuff()? Well, it depends entirely on the order in which the class hierarchy gets inspected, Python does things by leftright-bottomtop so both a and b inherit Three.doStuff(self,x). In the case of a, Four doesn’t have doStuff() so it looks at Two, which doesn’t so it looks at Three and finds it. However, Python also has another lookup mechanism (which was the only one available pre-2.3 or possibly 2.4, and can still be triggered by omitting the “object” superclass from One) which is left-bottomtop-right, so a inherits One‘s doStuff() and b inherits Three‘s. In the words of Peter Cook, that could confuse a stupid person…
So luckily Objective-C avoids this problem by only having an inheritance tree, and we can’t add method implementations via protocols either. And my specific example is moot, because -doStuff and -doStuff: are different selectors.
