What is a memory leak and how does it occur?

Memory leak is a situation where a program allocates memory for objects, but after they are no longer needed, references to them are not removed, and the garbage collector cannot free that memory. In iOS, where ARC (Automatic Reference Counting) is used, memory leaks often occur due to cyclic references, for example, when two objects reference each other through strong references, and neither is released.

Example of a cyclic reference in Swift:

class A {
    var b: B?
}

class B {
    var a: A?
}

let a = A()
let b = B()

a.b = b
b.a = a
// Both objects reference each other, ARC cannot automatically free them

To avoid leaks, weak (weak) or unowned (unowned) references are used, which do not increase the reference count and allow objects to be released correctly.