Question

I’m using AppCode and it flagged an interesting situation in the code of a very large project. Pre-ARC.

A subclass defines and synthesizes a property called delegate. Actually the property declaration had been commented out! But the @synthesize delegate = delegate_; statement was left behind.

The code compiles, presumably because the base class defines and synthesizes a property also called delegate and synthesizes it with a backing variable with the same name: @synthesize delegate = delegate_;

My question is: What happens with a message is sent to the delegate in

a) the base class methods and

b) in the sub-class methods.

AppCode flags the synthesize statement in the sub-class as an error:

Accessors of property ‘delegate’ were already synthesized with
instance variable ‘delegate_’

Answer 1

The @synthesize directive is shorthand for creating accessor methods and an ivar according to the specifications (atomicity, memory management) of the property of the same name. Given that, re-synthesizing a property in a subclass (without re-declaring it) works exactly like overriding the accessor methods — the subclass’s implementations are used instead of the superclass’s. Since the implementations are created by the compiler in both cases, there’s no noticeable difference in behavior.

The one difference is that a synthesized ivar has the same visibility as a @private ivar, so subclasses can’t access it, including to use it as the backing variable for a property. This means that the re-synthesis in the subclass must use a different ivar name. If the superclass has @synthesize wildHorses = wildHorses_;, then the compiler requires the subclass to do something like @synthesize wildHorses = equusFerus;.*

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*If the superclass uses the default name for the created ivar, @synthesize wildHorses; then the subclass must still synthesize a new variable: @synthesize wildHorses = wildHorses_;