Objective-C: id, accessing instance properties, synthesize?

Question

I'm very new to Objective-C, I'm wondering if there is a simple way to set an id to be an object instance (which has synthesized properties), and directly get/set those properties like:

id myID = myInstance;

myID.myProperty = something;

Where myInstance is an object with a synthesized property called myProperty. When I just do:

myInstance.myProperty = something;

It works, but when I've switched it for an id I get the error

Property 'myProperty' not found on object of type '_strong id'

Do I have to manually make getter/setter methods instead of using synthesize when using an id? Because I do seem to be able to make the id perform the instances methods.

Dismay

Is it an error, or a compiler warning? It will work at runtime but the reason for the compiler warning is simply that you are using the generic id type to refer to the object; it doesn't know until runtime if the message will be handled by the object or not. If you want to avoid the warning then use the correct class pointer. – Madame

Thanks, it labelled it as a semantic issue, so not sure which that is, but it didn't run in the iphone simulator when I tried it. – Dismay

Answer 1

If the object must be of type id, you can use messages (rather than dot notation) to access getters/setters:

id myID = ...;
NSString *prop = [myID property];
[myID setProperty:@"new value"];

But you have better alternatives:

Declaring a new variable

If you know the object's class, just make a variable with that type.

id myID; // defined elsewhere
MyClass *obj = (MyClass *)myID; // if you know the class, make a variable with that type
obj.property = @"new value";

Casting

Use an inline cast to tell the compiler what the type is without making a new variable.

id myID; // defined elsewhere
((MyClass *)myID).property = @"new value";

Protocols

If you don't know the exact class of the object but you know that it must implement certain methods, you can create a protocol:

id<MyProtocol> myID; // the compiler knows the object implements MyProtocol
myID.property = @"new value";

Banquette

Ah thanks, not sure I understand those yet, but I'll have a play with it and see how I get on. – Dismay

More information on protocols: developer.apple.com/documentation/Cocoa/Conceptual/ObjectiveC/… – Banquette

Answer 2

Properties need more information respect to simple messages. So the answer is.. you can't call a property on an id object. But you can use messages, casting (if you are not sure, use reflection to find out the object type), protocols...

Lordosis