Hi Greg,
hope the runtime is being docile. :-)
On May 4, 2009, at 23:24 , Greg Parker wrote:
That's right. In some discussions of object-oriented programming, a
distinction is made between "value objects" and "reference objects".
Two value objects can be "equal" if they share the same "value",
even if they are separately allocated objects with different pointer
addresses in memory. For reference objects, "equal" means they have
the same pointer address.
Minor nit: the distinction between identity and equality isn't
really tied to having value objects, though there is some overlap, and
value objects in Cocoa are somewhat ad-hoc. Smalltalk is a little
more consistent here, in that it has there is a simple distinction
between identity and equality, with = message for equality and == for
identity.
NSNumber acts as a value object by this terminology, because
NSNumber's -isEqual: and -hash methods operate on the represented
number value, ignoring the actual allocation addresses. On the other
hand, NSWindow is a reference object; nobody would consider two
different NSWindow objects "equal" even if they shared the same name
and contents and position on screen.
Actually, the distinction tends to be somewhat different:
- for value objects, when they are equal they are also considered
identical, so two instances of the number 2 are considered
indistinguishable. In Cocoa-practice, that they are in fact
indistinguishable doesn't necessarily hold, though the frameworks
pretty much assume that and so they should be treated as such.
- for other objects, equality does not imply identity, so two windows
could be considered equal if they have the same content, but no-one
would consider them to be the same object ( not that -isEqual: is
necessarily implemented that way).
NSDictionary uses -isEqual: and -hash everywhere, so the contained
objects get to decide whether they act as value objects or reference
objects.
Well, this also depends on how they react to copy.
NSWindow's -isEqual: implementation simply compares the pointer
addresses and returns that result, while NSNumber's -isEqual:
implementation also looks at the represented number values stored
the objects.
If it's a true value object, it needs to *only* look at the value, not
"also".
A CFDictionary can be configured to treat all of its objects as
reference objects, ignoring any -isEqual: implementations. (Such a
dictionary would no longer be compatible with NSDictionary, though.)
It would be an IdentityDictionary.
Cheers,
Marcel
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