On 17.12.2012, at 13:14, Roland King wrote:
>
> On 17 Dec, 2012, at 7:20 PM, Andreas Grosam <[email protected]> wrote:
>
>>
>> On 17.12.2012, at 11:25, Andreas Grosam wrote:
>>
>>> I would like to use the property access syntax for a (readonly) property
>>> which is defined in a category and sent to an object whose compile type
>>> info is just "id". Use case:
>>>
>>> id obj = …;
>>> if (obj.isFoo) { // <== error: property 'isFoo' not found for object of
>>> type '__strong id'.
>>> ...
>>> }
>>>
>>>
>>> So, why does message send syntax compiles fine - but property syntax not??
>>
>> Well, after thinking a while about the issue, I can answer this myself:
>>
>> The compiler requires the type information in order to figure the actual
>> getter signature. When declaring the a property, the getter and setter may
>> be explicitly specified, for example:
>>
>> @property (readonly, getter=getFooStateOrWhatever) isFoo;
>>
>> Thus, the compiler cannot deduce the setter and getter methods from the
>> property name unless it knows the type. So, in the case above where the type
>> is not known at compile time, I cannot use dot syntax but needs to use
>> method send syntax with the actual getter signature.
>>
>> Andreas
>>
>
> Don't see that at all, sorry. The dot syntax a.foo is just another way of
> saying [ a foo ], they are equivalent.
No, this is not the case. The compiler requires the declaration of the property
when it encounters a dot syntax, in order to figure the *actual* method
signature of the getter or setter. Most often, the method names for the setter
and getter are the "canonical names" (`foo`, `setFoo:`), since per default the
setter and getter are not specified.
For example, the property could have been declared as
@property (readonly, getter=foo) BOOL isFoo;
So, in this case, in order to compile properly, the compiler will issue code
for
if (obj.isFoo) {
}
that is equivalent to
if ([obj foo]) {
}
and NOT
if ([obj isFoo]) {
}
When the compiler does not, or cannot find the declaration of the property, it
cannot just guess the name of the setter/getter. So, it signals an error. It
cannot find the declaration of the property if the type is just `id`, for
example. So, this will issue an error:
id obj = …
if (obj.isFoo) {
}
Whether it should signal an error or just issue a warning is debatable, IMO.
But, we are allowed to send any message to any object - using method send
syntax:
id obj = …
if ([obj isFoo]) { <== ohps!
}
id obj = …
if ([obj foo]) { <== fine!
}
Andreas
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