# iOS ## ARC Retain Cycles Source: * Strong reference - a reference that increments the reference count by 1 and thereby prevents the referring object to be deallocated by ARC. Strong references are safe to use when the object hierarchy is linear, i.e parent -> child only * Weak reference - reference that does not increment the ref count and thus does NOT protect the object from being deallocated by ARC. Good for child to refer to parent object - so when parent object goes out of scope, the child will also go out of scope automatically. In Swift, weak references have to be Mutable and Optional (because it should be possible to be set to nil after initial value). Use this for object-> delegate relationship and for objects that use NSNotificationCenter, NSTimer etc. * Retain cycle - Two objects have strong reference to each other. Both objects won't be deallocated. * Unowned reference - a reference that does not increment ref count (like weak reference), but is not an Optional (unlike weak reference). Use this only when you are sure that the object that defines the reference (for eg: closure) and the object that it refers to (instance that it captures) will be deallocated at the same time. For example, lazily defined closure properties. --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://kb.annjose.com/mobile-dev/ios-home.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.