How does iOS Swift Runtime work

Question

In ObjC, it is using Messaging, static binding, dynamic typing, dynamic binding, dynamic method resolution, dynamic loading, introspector and so on.

Importantly, the core method objc_msgSend is responsible for taking the selector you're sending and the object you're sending it to, and looking that up in the class method tables to figure out exactly which piece of code is supposed to handle it.

My concerns here are:

1. Is Swift doing something similar like ObjC on runtime?

2. How does Swift runtime find the implementation code for some object/class method?

3. In ObjC, classes and objects are compiled to some runtime types such as C struct on runtime. Then what are such classes and objects compiled to on runtime in Swift?

4. Does Swift runtime have something like class / meta class / isa pointer / super pointer?

Answer 1

In short there are Dynamic and Static types of method call dispatching.

1. Static - the function address to be called is determined in the compilation time, so that expense of such call is similar to C-function calling. This mechanism is used for private methods or final classes methods call dispatching.

2. Dynamic dispatching is mechinism which allows to implement polymorphism concept of OOP - the function address to be called is determined in running time. Swift has two subtypes of it:

   2.1. Obj-C - you already described in the question. This mechanism is used when object inherits from NSObject or calling method has @objc prefix.

   2.2. Virtual table based (like in C++) - there is similar witness tables. What it does during method call dispatching is just single arithmetic operation - calculation of actual function address based on function offset in the base class witness table and the object class witness table location. So that's a relatively cheap operation comparing to Obj-C. It explains why "pure" Swift approximates to C++ performance.

If you don't mark you method with private keyword or your class is not final and same time you class is "pure" Swift (it does not inherit NSObject) then this virtual table based mechanism is used. It means that all the methods by default are virtual.

P.S. Helpful link for proving my vision regarding "Types":

https://developer.apple.com/swift/blog/?id=27

"Subtypes" explanation is based on my understanding.