Re: Function calling
Re: Function calling
- Subject: Re: Function calling
- From: Ondra Cada <email@hidden>
- Date: Thu, 26 May 2005 03:23:18 +0200
Thomas,
On 26.5.2005, at 2:56, Thomas Davie wrote:
Quite agree - the part I was disputing was ``The object knows.
That's called [...] "polymorphism".''
The part was completely right. What is polymorphism?
Polymorphism in fact means that you can send the same message to
different objects, and they would behave accordingly. With strong
typing, you just limit "different objects" to subclasses of some
class, or to implementers of some protocol. (Or, in other langauges,
to other similar primitives, but I thought we are discussing ObjC
here, are we not?)
For example, in a plain function
int sum(id o) {
int sum=0;
for (NSEnumerator *en=[a objectEnumerator];o=[en nextObject];) sum
+=[o intValue];
return sum;
}
could we perhaps benefit from strong typing, having the compiler to
check that the argument is an NSArray and its items are NSNumnbers?
Nope, we could not -- actually, the very opposite: it would *cripple*
the very idea of polymorphism. The way it is written now it would
work all right with a set containing NSStrings, or, with a few simple
categories, with, say, an NSImage and its NSImageReps or with an
NSDocument and its NSWindowControllers -- or, truly, with *whatever*.
That is polymorphism at its best: the fact we can send *any* object
messages "objectEnumerator" or "intValue" and presume it would behave
accordingly (which may also mean "reports and error": that's
definitely an appropriate reaction in case the object cannot support
the appropriate service). The strong typing just *limits*
polymorphism, never ever helps it.
---
Ondra Čada
OCSoftware: email@hidden http://www.ocs.cz
private email@hidden http://www.ocs.cz/oc
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