objective c - When to use instance variables and when to use properties

Question

When using Objective-C properties can you stop creating instance variables altogether or do explicit instance variables (not the ones synthesized by the properties) still serve a purpose where properties would be inappropriate?

Answer 1

can you stop creating instance variables altogether

No, you can't (in a sense). What you can do is stop declaring them if you have properties. If you synthesize a property and you haven't declared the instvar, it will get declared for you, so you are creating an instance variable, just not explicitly.

do they still serve a purpose where properties would be inappropriate?

It used to be the advice to create properties for everything because having synthesized properties does almost all of the retains and releases for you. However, with ARC that reason for using properties to wrap the memory management has gone away. The advice now (for ARC) is, I believe, use properties to declare your external interface, but use direct instance variables where the variable is part of the object's internal state.

That's a good reason to adopt ARC: properties revert to their true purpose only of being part of the class's API and it's no longer necessary to use them as a hacky way to hide memory management work.

Edit

One more thing: you can now declare instance variables in the @implementation so there is now no need to leak any implementation details in the @interface. i.e.

@implementation MyClass
{
    NSString* myString;
}
// method definitions
@end

And I'm pretty sure it works in categories too. - see comment below