thread safety - How to implement lazy evaluation of objective-C property which may be nil? -

in objective-c, how can implement lazy evaluation of property may nil?

the usual practice demonstrated below, not work properties may nil - initialization won't performed once.

@interface lzclass: nsobject @property (strong, readonly) nsstring * lazyprop; @end  @implementation lzclass  @synthesize lazyprop = _lazyprop; - (nsstring *)lazyprop {     if (_lazyprop == nil) {         _lazyprop = <some calculation>;     }     return _lazyprop }  @end 

update: answer isn't safe. see yan's comment below. leaving here people have same idea can beware.

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use dispatch_once instead of checking nil.

@interface lzclass: nsobject {     dispatch_once_t _oncetoken; } @property (strong, readonly) nsstring * lazyprop; @end  @implementation lzclass  @synthesize lazyprop = _lazyprop; - (nsstring *)lazyprop {     dispatch_once(&_oncetoken, ^{         _lazyprop = <some calculation>;     }     return _lazyprop }  @end 

this has advantage of being thread-safe. threads trying read property while initialization block being executed block, , continue after initialization block returns. therefore correct result guaranteed returned in threads.

also beware: standard code snippet dispatch_once defines static oncetoken, won't work correctly here; initialization block executed first object of class.