Use structures by default.
Use classes when you need Objective-C interoperability.
Use classes when you need to control the identity of the data you’re modeling.
Use structures along with protocols to adopt behavior by sharing implementations.
Choose Structures by Default
Using structures makes it easier to reason about a portion of your code without needing to consider the whole state of your app. Because structures are value types—unlike classes—local changes to a structure aren’t visible to the rest of your app unless you intentionally communicate those changes as part of the flow of your app. As a result, you can look at a section of code and be more confident that changes to instances in that section will be made explicitly, rather than being made invisibly from a tangentially related function call.
Use Classes When You Need Objective-C Interoperability
If you use an Objective-C API that needs to process your data, or you need to fit your data model into an existing class hierarchy defined in an Objective-C framework, you might need to use classes and class inheritance to model your data. For example, many Objective-C frameworks expose classes that you are expected to subclass.
Use Classes When You Need to Control Identity
Classes in Swift come with a built-in notion of identity because they’re reference types. This means that when two different class instances have the same value for each of their stored properties, they’re still considered to be different by the identity operator (===). It also means that when you share a class instance across your app, changes you make to that instance are visible to every part of your code that holds a reference to that instance.
Important
Treat identity with care. Sharing class instances pervasively throughout an app makes logic errors more likely. You might not anticipate the consequences of changing a heavily shared instance, so it’s more work to write such code correctly.
Use Structures and Protocols to Model Inheritance and Share Behavior
Structures and classes both support a form of inheritance. Structures and protocols can only adopt protocols; they can’t inherit from classes. However, the kinds of inheritance hierarchies you can build with class inheritance can be also modeled using protocol inheritance and structures.