cocoa - What is difference between retain and strong in objective-c? -


this question has answer here:

looking forward response other : - arc , non-arc environment

strong , weak retain-release cycles (rrc), form of memory leak. ios uses called automatic reference countin (arc) know when object in use , should kept in memory, or no longer in use , should deleted gain resources. arc works because runtime knows each object, how many objects referencing it. when found reaches 0, object deleted.

issues arise when have 2 objects hold references each other. because object holds reference object b, , b a, reference count both , b never 0, , b in memory. it's possible there no other objects holding references or b, we've created memory leak.

getting strong , weak, these keywords used "denote ownership", if will. eliminate retain-release cycles limiting objects increment reference count object. strong property 1 increment reference count of object. if object has strong reference b, , no other object referencing b, b has count 1 (a owns, or needs exist b). now, if b wants have reference a, want use weak reference. weak references don't increment reference count of object. in particular case, if has no other objects referencing b, a's count 0 given b's weak reference.

can see how eliminating rrc? assuming no external references , not using strong/weak references, , b perpetually reside in memory. using strong , weak references outlined above, have count 0, removed memory. in turn deincrement b's reference count 1 0, causing b removed memory.

nonatomic used denote object being referenced in not thread safe. means object not able deal multiple requests @ same time. atomicity idea once make request, either happens or doesn't. when operation atomic, you're guaranteeing entity you're applying operation never in intermediate state. regardless of how @ entity, either looks way did before requested operation, or looks way once operation done. (when thinking atomicity, think of atoms. word means indivisible. atomic operations can't divided smaller operations.)


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