How do I dynamically define an instance variable?

Question

Hi everyone,

I have two classes (class1 and class2) that just store data, no methods. I have a third class that has an instance variable that, depending on some user input, will be set to one of the two classes. So, in the third class I declare the variable generically as

NSObject *aClass;

and during runtime set it to whatever it should be.

aClass = [[Class1 alloc] init]; // or 
aClass = [[Class2 alloc] init];

However, when I try to access fields from aClass

NSString *str = aClass.field1;

It gives me the error: request for member 'field1' in something not a structure or a union. Field1 is declared in both class1 and class2. When I try to cast aClass

aClass = (Class1 *) aClass;

it gives the same error. What am I doing wrong, is there a better way to do this?

Answer 1

It's because properties only work when the compiler knows the class it has to deal with. NSObject has no property called field1, so it doesn't work.

You can, however, use the field1 method the property implicitly created:

NSString* str = [aClass field1];

But if you're doing it on a NSObject, it's going to emit a warning.

If you don't know which of the two classes you will have and they don't have any class relationship, I suggest you use the id type instead of NSObject. Using id, the compiler won't emit warnings about method calls that could potentially fail (but still won't resolve your properties).

Answer 2

All the compiler knows about aClass is that its an NSObject, so you can only treat it as such. Otherwise, what should happen if you try to access field1 when aClass points to an NSString? Make a superclass with the instance variables you want and declare aClass to be of that type. That ensures that aClass will contain a field1 variable so you can carry on without the poor compiler getting confused.

The reason you can't cast down from NSObject to Class1 follows the same principle. aClass could be anything, and the compiler doesn't know how to anything to a Class1. You can cast the other way because all aClass are NSObjects, but not all NSObjects are aClass. :D

Answer 3

Why not have both of the possible classes inherit from one base class that implements the property you are trying to define?

Or, create a category with that property that both classes implement, and instead of defining an NSObject variable say:

id <MyClassCategory> aClass;