Error accessing generated ivars when I override setters and getters in Modern Objective-C

Question

I know now the new Objective-C compiler lets you not need to synthesize your properties anymore. I have one file that has two classes in it. My .h for a simple helper class looks like this:

@interface ViewFrameModel : NSObject

@property (nonatomic, strong) UIView *view;
@property (nonatomic, assign) CGRect frame;

- (id)initWithView:(UIView *)view frame:(CGRect)frame;

@end

In the same .h file, for my other class (class 2), I have:

@property (nonatomic, strong) ViewFrameModel *viewFrameModel;

In class 2.m, I can do this:

- (void)setViewFrameModel:(ViewFrameModel *)viewFrameModel {
    _viewFrameModel = viewFrameModel;        
    [self pushViewFrameModel:viewFrameModel];
}

This works fine with no complaints from the compiler, however, when I add this:

- (ViewFrameModel *)viewFrameModel {
    return _viewFrameModel;
}

I get two complaints, one on the first method setViewFrameModel:

"Use of undeclared identifier _viewFrameModel, did you mean viewFrameModel"

And the other on return _viewFrameModel:

"Use of undeclared identifier _viewFrameModel, did you mean viewFrameModel" "Reference to local variable viewFrameModel' declared in enclosing context"

Why do I get these errors when I add in the

- (ViewFrameModel *)viewFrameModel {
    return _viewFrameModel;
}

method? I want to override this method with some custom info, but it's complaining at me :-. Thoughts? TIA.

"I know now the new Objective-C compiler lets you not need to synthesize your properties anymore" - **Thank you** for saying "compiler" and not "Xcode". Finally, somebody! — , Jan 08 '13 at 20:08

score 35 · Accepted Answer · answered Jan 08 '13 at 20:09

35

If you override both the setter and the getter, the compiler will not automatically create the instance variable for you anymore. You can add it to your class implementation like so:

@implementation ClassName {
    ViewFrameModel *_viewFrameModel;
}
...
@end

answered Jan 08 '13 at 20:09

Carl Veazey

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Do you have any references for this? Where did you read this? Thanks. – sole007 Dec 07 '15 at 16:42
@sole007 I think I saw it in the modern objective C WWDC video from some years back. – Carl Veazey Dec 08 '15 at 17:44

score 5 · Answer 2 · edited May 23 '17 at 11:47

5

Here is the results of some testing I did last year: iOS automatic @synthesize without creating an ivar.

In short, you need to use @synthesize or declare an iVar explicitly.

edited May 23 '17 at 11:47

Community

1
1

answered Jan 08 '13 at 20:15

Jeffery Thomas

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1

You do not *need* to use @synthesize. You can declare your own ivar instead. – rmaddy Jan 08 '13 at 20:18
1

Huh, I would say you don't need to declare iVars, you can use @ synthesize. I guess it's what you prefer. – Jeffery Thomas Jan 08 '13 at 20:40
Before the latest compiler, `@synthesize` only synthesized the setter and getter methods, not the ivar. Since the OP is providing the methods, there is no need to use `@synthesize`. It is clearer, in this case, to add your own ivar. Why use `@synthesize` when the bulk of what it does isn't being used? But, in the end, with the latest compiler, `@synthesize` will work. My main objection was to your use of the word "need" since that is in fact not true. It's an option, not a need. – rmaddy Jan 08 '13 at 20:53
@rmaddy No, `@synthesize` has always generated an iVar if it was not present. I've written lots of `@synthesize prop = _prop;` lines without ever needing to explicitly declare an iVar. I understand your nit about the word 'need', I'll change the wording. – Jeffery Thomas Jan 08 '13 at 21:56

score 3 · Answer 3 · edited Apr 14 '21 at 16:23

To summarize the answers:

If you override both the setter and the getter, the compiler will not create the instance variable for you.

Why? In that case, the compiler assumes that the property is dynamic: that it might be a property that relies on other properties for storage / computation, or that it will be created in other ways, for example, at runtime using Objective-C runtime functions.

To help the compiler understand the situation better there are two potential solutions:

@implementation Class
@synthesize property = _property;
...
@end

or

@implementation Class {
    PropertyClass *_property;
}
...
@end

Error accessing generated ivars when I override setters and getters in Modern Objective-C

3 Answers3

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