运行时派发机制

• Unlike C++, it is not necessary to designate that a method is virtual in Swift. The compiler will work out which of the following to use: 

  (the performance metrics of course depend on hardware)

  • Inline the method : 0 ns
  • Static dispatch: < 1.1ns
  • Virtual dispatch 1.1ns (like Java, C# or C++ when designated). 
  • Dynamic Dispatch 4.9ns (like Objective-C). 

  Objective-C of course always uses the latter.

https://stackoverflow.com/questions/24022172/does-swift-use-message-dispatch-for-methods/25438321