Unowned references for guaranteed lifetime in Swift - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an unowned reference in Swift?

An unowned reference is a non-owning pointer to an instance that is expected to always have a valid value during its lifetime. It does not increase the reference count and assumes the referenced object will not be deallocated while the unowned reference exists.

intermediate

How does an unowned reference differ from a weak reference?

An unowned reference assumes the referenced object will never be nil during its lifetime, so it is non-optional and does not become nil automatically. A weak reference is optional and automatically becomes nil when the referenced object is deallocated.

beginner

When should you use unowned references?

Use unowned references when you know the referenced object will always outlive the reference, such as in parent-child relationships where the child references the parent but the parent owns the child.

intermediate

What happens if you access an unowned reference after the referenced object is deallocated?

Accessing an unowned reference after the referenced object is deallocated causes a runtime crash because the reference points to invalid memory.

beginner

Example: How do you declare an unowned reference in Swift?

You declare an unowned reference using the keyword <code>unowned</code> before the variable, for example: <br><pre>class Child {
  unowned let parent: Parent
  init(parent: Parent) {
    self.parent = parent
  }
}</pre>

What keyword declares an unowned reference in Swift?

A unowned

B weak

C strong

D optional

The keyword unowned declares a reference that does not increase the reference count and assumes the referenced object will always exist.

Which statement about unowned references is true?

A They do not increase the reference count and are non-optional.

B They are always optional types.

C They become nil automatically when the referenced object is deallocated.

D They increase the reference count to keep the object alive.

Unowned references do not increase the reference count and are non-optional, assuming the object will always be valid.

When is it safe to use an unowned reference?

A When the referenced object might be deallocated anytime.

B When the referenced object is guaranteed to outlive the reference.

C When you want the reference to become nil automatically.

D When you want to create a strong ownership cycle.

Unowned references are safe only if the referenced object will outlive the reference, preventing dangling pointers.

What happens if you access an unowned reference after its object is deallocated?

A Nothing happens; it continues normally.

B The reference becomes nil safely.

C Swift automatically recreates the object.

D The program crashes at runtime.

Accessing an unowned reference after deallocation causes a runtime crash due to invalid memory access.

Which of these is a typical use case for unowned references?

A A parent object referencing its child.

B Two unrelated objects referencing each other strongly.

C A child object referencing its parent.

D A global variable referencing a local variable.

A child referencing its parent with an unowned reference avoids strong reference cycles when the parent owns the child.

Explain what an unowned reference is and when you should use it in Swift.

Think about parent-child relationships and memory safety.

Describe the difference between unowned and weak references and why that difference matters.

Consider what happens if the referenced object is deallocated.