What are strong references necessary for in the Swift programming language?

Strong references in Swift are necessary to keep an object in memory as long as there is at least one strong reference to it. They are the default behavior for reference types (classes).

When a variable or constant references an instance of a class, this reference is strong by default. This increases the reference count for that instance. As soon as all strong references to the instance are gone (for example, variables go out of scope or are assigned nil), the reference count reaches zero, and the memory management system (ARC - Automatic Reference Counting) frees the memory occupied by that instance.

The main purpose of strong references is to ensure that an object is not deallocated prematurely while it is still in use. Without strong references, objects could be deleted at any time, leading to dangling pointer errors.

However, strong references can lead to retain cycles, where two or more objects hold strong references to each other, preventing them from being deallocated even if there are no other external references. To prevent such cycles, weak and unowned references are used.