On 23 Feb 2014, at 10:05, Graham Cox <graham....@bigpond.com> wrote:
>
> You might find it happens to work today but it is not contracted to do so
>
I agree that there is some uncertainty going forward.
Part of the issue I think is in the nature of class clusters.
We rely on the behaviour of an unspecified collection of private subclasses of
a public abstract super class.
The public interface supplies the contract yet changes to the exact
implementation of the private subclasses can affect how the contract is
honoured (as the subclass is free to fulfil the public contract any way it
likes.
This leads to the sort of concerns that have been put forward.
The NSNumbers docs say:
Note that number objects do not necessarily preserve the type they are created
with.
and:
As with any class cluster, subclasses of NSNumber must override the primitive
methods of its superclass, NSValue. In addition, there are two requirements
around the data type your subclass represents:
• Your implementation of objCType must return one of “c”, “C”, “s”,
“S”, “i”, “I”, “l”, “L”, “q”, “Q”, “f”, and “d”. This is required for the other
methods of NSNumber to behave correctly.
• Your subclass must override the accessor method that corresponds to
the declared type—for example, if your implementation of objCType returns “i”,
you must override intValue.
The first is a logical cause for concern, but perhaps(!) it only becomes
relevant in non primitive representations like NSDecimalNumber.
The second provides some support for the idea that the NSNumber private
subclasses do indeed return an invariant -objCType.
Ultimately clarity on the matter still eludes me.
If I end up writing an NSNumber clone class cluster then at least I will now
have a decent idea of what it may entail.
At present I am still wrangling with precisely what behaviour such a cluster
would have.
> 1(int) and 1(float) can be represented by the same NSNumber object perfectly
> legally.
Is that true?
Tagged pointers slipperiness. These are very private indeed.
(lldb) p na
(NSNumber *) $0 = 0x0000000000000127 (int)1
(lldb) p nb
(NSNumber *) $1 = 0x0000000100400b40 (float)1.000000
Jonathan
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