choosing-between-structs-and-classes.md

Choosing Between Structures and Classes

In addition to the Apple guidelines, we generally prefer to use struct for:

• Value types like the Networking Models
• Stateless helpers

But consider using class instead if:

• You need to manage mutable states. Especially if there are more than a few mutating functions, the struct becomes harder to reason about.
• You have to set a struct property declaration as var because it has a mutating function. In this case, a constant (let) class property may be easier to reason about.

Classes should be marked as Final by default

When using classes, these should be marked as final by default so their behavior cannot be altered by subclassing and/or overriding. There's a few reasons for that:

• It favors composition over inheritance
• The class could be missused or abused when subclassed or overriden, which could be problematic if the class is doing anything important. By making it final we disallow this at compiler time, and enforce that the class has to be used as it was written.
• Marking a class as final tells the Swift compiler that the class methods should be called directly rather than looking them up in a method table (static vs dynamic dispatch), which reduces function call overhead and increases performance
• If inheritance is necessary later on, it can always be allowed, but the final modifier will create a reminder of its necessity and refactoring