Objective-c readonly copy properties and ivars -

i'm try grok properties declared both copy , readonly in objective-c, , specifically, whether have copy myself. in init methods. evidence suggests do:

@interface : nsobject      @property(nonatomic, copy, readonly) nsdata *test;     - (instancetype)initwithdata:(nsdata *)data; @end  @implementation  - (instancetype)initwithdata:(nsdata *)data {     if ((self = [super init]) != nil) {         _test = data;     }     return self; }  @end   int main (void) {     nsdata *d1 = [nsmutabledata datawithbytes:"1234" length:5];     *a = [[a alloc] initwithdata:d1];      nslog(@"%lx", (unsigned long)d1);     nslog(@"%lx", (unsigned long)a.test);     return 0; } 

i had thought self.test = data in init method, not permitted because it's readonly (not unexpectedly). of course, self.test = [data copy] ensures 2 different objects.

so: there way create readonly property in objective-c copies incoming value, or sufficiently edge case combination pointless , have copying myself manually anyway?

a @property declaration merely shorthand accessor/mutator method declarations, , (in cases) synthesized implementations said accessor/mutator methods.

in case, @property(nonatomic, copy, readonly) nsdata *test declaration expands equivalent code:

@interface : nsobject {     nsdata* _test; } - (nsdata*)test; @end  @implementation - (nsdata*)test {     return _test; } @end 

there no settest: mutator method because property declared readonly, copy attribute has no effect.

you can implement own mutator method:

- (void)settest:(nsdata*)newvalue {     _test = [newvalue copy]; } 

or, can have compiler synthesize mutator method declaring read/write property in private class extension in implementation file:

// a.m:  @interface a()  @property (nonatomic, copy) nsdata* test; @end 

both cases allow use test mutator method copy value _test instance variable:

- (instancetype)initwithdata:(nsdata *)data {     if ((self = [super init]) != nil) {         self.test = data;     }     return self; } 

the end result is:

@interface : nsobject @property(nonatomic, copy, readonly) nsdata* test; - (instancetype)initwithdata:(nsdata*)data; @end  @interface a() @property (nonatomic, copy) nsdata* test; @end  @implementation - (instancetype)initwithdata:(nsdata*)data {     if ((self = [super init]) != nil) {         self.test = data;     }     return self; } @end