Why don't we use strong for an enum in Objective-C?

Junior PRO

Essence

strong, weak, copy are ownership attributes: they describe how a property retains an object (a reference-counted heap pointer). An enum in Objective-C is a plain C integer (NSInteger/NSUInteger via NS_ENUM/NS_OPTIONS): it is stored directly in the ivar, has no retain count and there is nothing to retain. The same applies to BOOL, CGFloat, NSTimeInterval, structs (CGRect), SEL, C pointers.

For such properties the ownership attribute is assign (also the default for non-objects; written explicitly for readability):

#import <Foundation/Foundation.h>

typedef NS_ENUM(NSInteger, LoadState) {
    LoadStateIdle,
    LoadStateLoading,
    LoadStateFailed,
};

@interface Loader : NSObject
@property (nonatomic, assign) LoadState state;      // scalar
@property (nonatomic, assign) CGFloat progress;
@property (nonatomic, strong) NSError *lastError;   // object
@end

If you write strong for an enum, the compiler reports "property with 'retain (or strong)' attribute must be of object type".