Comment by NobodyNada
6 years ago
FYI, that’s a pretty expensive line of code. The compiler has to search myObject’s type metadata and protocol conformance records to find the conformance, then it has to create a new SomeProtocol existential container, copy myObject to the new container (potentially incurring retain/release traffic), use a witness table to dynamically call the method, and finally destroy the existential container. Dynamic casts are slow; if you can restructure your code to avoid the cast then it won’t have to do a bunch of that extra work.
Yes, with all that in mind the complexity I see in the generated code still far exceeds my intuitive expectations. Of course I'll end up removing protocols from critical parts of my code, but like I said in the other comments, then what's the point of writing those parts in Swift? Protocols are the core part of the language, they are offered as a substitute for multiple inheritance and even for e.g. enforcing abstract methods (no other idiomatic way in Swift), they are elegant and look cheap except they are not!
The really expensive part here is not the use of a protocol, it’s the downcast (which isn’t really idiomatic Swift). Static dispatch is always faster than dynamic dispatch/polymorphism, but protocols are usually reasonably efficient (even more so if you can use generics instead of existentials).