What keeps a reference to an object in memory?

In iOS (and generally in Swift and Objective-C), a reference to an object in memory is stored in a variable or constant that contains a pointer to the memory area where the object itself is located.

For example, in Swift:

class MyClass {}

let obj = MyClass() // obj is a reference to an object in memory

The value of the variable obj is the address in memory where the instance of MyClass is located. The object itself is stored on the heap, and the variable is a pointer (reference) to that object.