1.11 · Static vs dynamic dispatch

Every call has to be resolved. The question is when — compile time or runtime — and that single answer explains performance, overriding, and the “my method never ran” bug.

Mental model. Static = the compiler already knows. Dynamic = something is only known at runtime: a subclass, an existential, or an Objective-C receiver.

Static vs dynamic

How a call finds its implementation.

1. Static vs dynamic dispatch in Swift? 1.11.1 · L2
   Static — the compiler knows the exact implementation: structs, enums, generics after specialization, final classes, and protocol extension methods that are not…
2. Why does final (and whole-module optimization) help performance? 1.11.2 · L3
   final tells the compiler no subclass can override the method, so it can devirtualize the call into a direct one — and then inline it. Whole-module optimization…

The protocol extension trap

Same call, different method, depending on the static type.

1. You added a method in a protocol extension and overrode it on a concrete type, but calling through any P does not hit your override. Why? 1.11.3 · L5
   Because the method is not a protocol requirement. Only requirements get a slot in the witness table. A method that exists only in an extension is a plain…

More in Swift language

• 1.1 · Value vs reference types
• 1.2 · Optionals, pattern matching, control flow
• 1.3 · Protocols, extensions, associated types
• 1.4 · Generics, <code>some</code>, <code>any</code>
• 1.5 · Closures, escaping, capture lists
• 1.6 · Properties, observers, wrappers
• 1.7 · Access control
• 1.8 · Error handling, <code>defer</code>, ownership
• 1.9 · Strings and Unicode
• 1.10 · Codable, Sendable, macros, ABI