According to the very popular WWDC 2015 talk Protocol Oriented Programming in Swift (video, transcript), Swift provides a number of features that make structs better than classes in many circumstances.
Structs are preferable if they are relatively small and copiable because copying is way safer than having multiple references to the same instance as happens with classes. This is especially important when passing around a variable to many classes and/or in a multithreaded environment. If you can always send a copy of your variable to other places, you never have to worry about that other place changing the value of your variable underneath you.
With Structs, there is much less need to worry about memory leaks or multiple threads racing to access/modify a single instance of a variable. (For the more technically minded, the exception to that is when capturing a struct inside a closure because then it is actually capturing a reference to the instance unless you explicitly mark it to be copied).
Classes can also become bloated because a class can only inherit from a single superclass. That encourages us to create huge superclasses that encompass many different abilities that are only loosely related. Using protocols, especially with protocol extensions where you can provide implementations to protocols, allows you to eliminate the need for classes to achieve this sort of behavior.
The talk lays out these scenarios where classes are preferred:
- Copying or comparing instances doesn't make sense (e.g., Window)
- Instance lifetime is tied to external effects (e.g., TemporaryFile)
- Instances are just "sinks"--write-only conduits to external state (e.g.CGContext)
It implies that structs should be the default and classes should be a fallback.
On the other hand, The Swift Programming Language documentation is somewhat contradictory:
Structure instances are always passed by value, and class
instances are always passed by reference. This means that they are
suited to different kinds of tasks. As you consider the data
constructs and functionality that you need for a project, decide
whether each data construct should be defined as a class or as a
structure.
As a general guideline, consider creating a structure when one or more
of these conditions apply:
- The structure’s primary purpose is to encapsulate a few relatively simple data values.
- It is reasonable to expect that the encapsulated values will be copied rather than referenced when you assign or pass around an
instance of that structure.
- Any properties stored by the structure are themselves value types, which would also be expected to be copied rather than referenced.
- The structure does not need to inherit properties or behavior from another existing type.
Examples of good candidates for structures include:
- The size of a geometric shape, perhaps encapsulating a width property and a height property, both of type Double.
- A way to refer to ranges within a series, perhaps encapsulating a start property and a length property, both of type Int.
- A point in a 3D coordinate system, perhaps encapsulating x, y and z properties, each of type Double.
In all other cases, define a class, and create instances of that class
to be managed and passed by reference. In practice, this means that
most custom data constructs should be classes, not structures.
Here it is claiming that we should default to using classes and use structures only in specific circumstances. Ultimately, you need to understand the real world implication of value types vs. reference types and then you can make an informed decision about when to use structs or classes. Also, keep in mind that these concepts are always evolving and The Swift Programming Language documentation was written before the Protocol Oriented Programming talk was given.
