Protocol Extensions: Why Static Dispatch?Jason Sadler
I wrote last year about Swift protocol extensions. If you define a method in a protocol extension that isn’t defined as a protocol requirement, it is dispatched statically, whereas methods defined as protocol requirements are dispatched dynamically. (See the original article for a detailed explanation.)
In March, there was an update – an acknowledgement by the Swift team of this shortcoming and a possibility for a change in behaviour.
So essentially, while protocols have a virtual function table, protocol extensions do not, and cannot easily have one because a type adopting the protocol won’t necessarily know about all extensions at compile time and therefore cannot add the extension methods to its own vtable. Opinions may vary whether dispatching protocols dynamically all the time would be a viable alternative, but it’s clearly not what the Swift team has in mind for the language.
Kevin’s post is very informative and worth reading in its entirety.