Under what conditions is @synthesize automatic in Objective-c?

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As of clang 3.2 (circa February 2012), “default synthesis” (or “auto property synthesis”) of Objective-C properties is provided by default. It’s essentially as described in the blog post you originally read: http://www.mcubedsw.com/blog/index.php/site/comments/new_objective-c_features/ (except that that post describes the feature as “enabled, then disabled”; I don’t know if that’s an issue with Xcode or if the clang developers themselves have gone back and forth on the question).

As far as I know, the only case in which properties will not be default-synthesized in clang 3.2 is when those properties have been inherited from a protocol. Here’s an example:

#import <Foundation/Foundation.h>

@protocol P
@property int finicky;
@end

@interface A : NSObject <P>
@property int easygoing;
@end

@implementation A
@end

int main() { A *a = [A new]; a.easygoing = 0; a.finicky = 1; }

If you compile this example, you’ll get a warning:

test.m:11:17: warning: auto property synthesis will not synthesize property
      declared in a protocol [-Wobjc-protocol-property-synthesis]
@implementation A
                ^
test.m:4:15: note: property declared here
@property int finicky;
              ^
1 warning generated.

and if you run it, you’ll get an error from the runtime:

objc[45820]: A: Does not recognize selector forward:: (while forwarding setFinicky:)
Illegal instruction: 4

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