Rethrowing functions in Swift - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does it mean when a function is marked as rethrows in Swift?

A rethrows function can only throw an error if one of the functions it calls (passed as parameters) throws an error. It does not throw errors on its own.

beginner

How do you declare a function that can rethrow errors from a closure parameter?

You add the rethrows keyword after the parameter list. For example: <br>func perform(action: () throws -> Void) rethrows { try action() }

beginner

Can a rethrows function throw an error by itself without calling a throwing function?

No. A rethrows function can only throw if the closure or function it calls throws. It cannot throw errors independently.

intermediate

Why use rethrows instead of throws in Swift?

Using rethrows makes your function safer and clearer. It only throws errors if the passed-in function throws, so callers don’t have to handle errors if the passed function doesn’t throw.

intermediate

Example: What is the output of this Swift code?<br>

func callTwice(action: () throws -> Void) rethrows { try action(); try action() }<br>try? callTwice { print("Hello") }

Output:<br>Hello<br>Hello<br><br>Explanation: The closure does not throw, so callTwice runs both times and prints "Hello" twice without error.

What keyword do you use to declare a function that only throws if a passed-in function throws?

A catch

B rethrows

C try

D throws

The rethrows keyword means the function only throws if the function it calls throws.

Can a rethrows function throw an error on its own without calling a throwing function?

A Yes, always

B Only if marked with <code>throws</code>

C No, never

D Only if the closure is non-throwing

A rethrows function cannot throw errors by itself; it only throws if the passed-in function throws.

Which of these is a valid declaration of a rethrowing function?

A func example(action: () -> Void) throws { action() }

B func example() rethrows { throw Error() }

C func example() throws { }

D func example(action: () throws -> Void) rethrows { try action() }

Only option A correctly declares a rethrowing function that calls a throwing closure.

What happens if you call a rethrows function with a non-throwing closure?

A It runs without throwing

B It throws an error

C You must use try

D It crashes

If the closure does not throw, the rethrows function runs without throwing.

Why might you prefer rethrows over throws?

A To allow throwing only when needed

B To avoid using try

C To force error handling always

D To catch errors automatically

rethrows lets the function throw only if the passed-in function throws, making error handling conditional.

Explain in your own words what a rethrowing function is and why it is useful.

Think about when the function throws errors and when it doesn't.

Write a simple Swift function that uses rethrows to call a throwing closure twice.

Use the example: func callTwice(action: () throws -> Void) rethrows { try action(); try action() }