When does automatic memory release occur for an object pointed to by a strong reference counter, and it becomes inaccessible?

Automatic memory release for an object with strong references occurs when the strong reference count to that object becomes zero — that is, no strong references point to the object anymore.

At this point, the object becomes inaccessible and can be removed by the memory management system (ARC in iOS). If at least one strong reference exists, the object remains in memory.

Example:

class MyClass {}

var a: MyClass? = MyClass() // strong reference count = 1
var b = a                   // strong reference count = 2

a = nil                    // strong reference count = 1
b = nil                    // strong reference count = 0, object is released

Thus, memory is released automatically when the object is no longer needed and there are no strong references to it.