What is a retain cycle and how do closures cause memory leaks in Swift?

A retain cycle occurs when two or more objects hold strong references to each other, preventing deallocation.

Closures capture variables from their surrounding context. If a class instance captures self strongly inside a closure, and that closure is stored on the same instance, ARC cannot deallocate it. This creates a memory leak. Swift solves this using capture lists like [weak self] or [unowned self].