What’s the difference between a protocol and a class?

Swift version: 5.4

Paul Hudson @twostraws May 28th 2019

When writing protocol-oriented Swift, protocols and classes become fairly similar, but they are never the same.

In their basic form, a protocol describes what an unknown type of object can do. You might say it has two or three properties of various types, plus methods. But that protocol never includes anything inside the methods, or provides actual storage for the properties.

In a more advanced form, you can write extensions on your protocols that provide default implementations of the methods. You still can’t provide storage for properties, however.

In comparison, classes are concrete things. While they might adopt protocols – i.e., say they implement the required properties and methods – they aren’t required to do that. You can create objects from classes, whereas protocols are just type definitions. Try to think of protocols as being abstract definitions, whereas classes and structs are real things you can create.