27.2 · defer: what runs, when, and what it can change

defer looks simple until it meets a return value, a loop or a throw. Most puzzles hide in when the returned value is computed and whether the defer statement was reached.

Mental model. Three facts: the return value is computed first, a defer runs every time its scope exits, and a defer only exists once execution has passed the line.

Return values and scopes

What a defer can and cannot change, and how it behaves in loops, throws and early exits.

1. A function returns x and a defer changes x afterwards. What does the caller receive, for a value, an inout, a class instance and a string being built? 27.2.1 · L2
   The return expression is evaluated first, the result is held, then the deferred blocks run, then the function actually returns. So for plain() the defer does…
2. When does a defer inside a loop, a throwing function or an early-exit branch run, and when is it never registered? 27.2.2 · L2
   A defer belongs to the scope it is written in and runs whenever that scope is left: normal end, return, break, continue or a thrown error. In a loop body the…

More in Puzzles

• 27.1 · Objects, closures and weak references
• 27.3 · Queue ordering, deadlocks and work items
• 27.4 · Actor reentrancy, task ordering, SwiftUI identity, and how to answer
• 27.5 · Values, optionals, patterns, order and isolation: ten more checked puzzles