Why does a declared property use both retain and readonly?

Question

I've noticed that some of Apple's examples include both a retain and readonly modifier on properties. What's the point of including retain if no setter gets generated when we're using the readonly modifier?

Example: @property (retain, readonly) NSString *title; from the AnimatedTableView sample.

Chuck · Accepted Answer · 2009-12-19 17:10:21Z

up vote 17 down vote accepted

You can include a second, private readwrite declaration in a class extension. The memory management scheme for all references needs to match IIRC, so you get silliness like "readonly, retain".

answered Dec 19 '09 at 17:10

Chuck
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3

It isn't entirely silly; the matrix of modifiers vs. @synthesize is such that the getter's code can change. Trying to suss out exactly which modifiers should be able to be added vs. which should #warn would rife with minutia and hard to ultimately understand. Better to simply go with "must be same except readonly->readwrite". – bbum Dec 19 '09 at 19:28

Figured i should expand on my comment.... – bbum Feb 23 '10 at 22:20

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score 36 · Answer 2

Or, more specifically, (readonly, retain) enables a pattern like this:

Foo.h:

@interface StuffHolder:NSObject
@property(readonly, retain) MyStuff *stuff;
@end

Foo.m:

@interface StuffHolder()
@property(readwrite, retain) MyStuff *stuff;
@end

@implementation StuffHolder
@synthesize stuff;
@end

The end result is a property that is publicly readonly while being readwrite within the implementation and for whom both setter and getter are synthesized automatically by the compiler.

A warning could be generated in the case of no (readwrite, retain) override in the class extension -- something akin to statement without an effect -- but it would be more confusing than beneficial. There are also a whole slew of different edge cases across the combinations that would equally warrant a warning, but don't really indicate an actual problem. The decision was made to largely accept the various patterns without complaint for simplicity's sake (since they aren't correctness issues).

Why does a declared property use both retain and readonly?

Why does a declared property use both retain and readonly?

2 Answers