In this post I’ll take a look at three of the program design constructs that Swift provides: Classes, Structures, and Protocols. I’ll discuss how they can contribute to well-designed Object-Oriented (OO) programs in a way that might be interesting to dynamic language programmers, and show how aspects of their design reflect concerns of the platform for which they were developed.
Classes and Structures
The best source of information about Swift so far is The Swift Programming Language, a free e-book provided by Apple. It contains a first pass introduction to the language, a language reference, and a good deal of example code. After introducing the basic facilities of the language - its philosophy, the basic available types and data structures, control flow, closures, and so on - we get to the section on Classes and Structures.
“Classes and structures are general-purpose, flexible constructs that become the building blocks of your program’s code.” 
This sounds similar to the functionality of a Class or Module in Ruby. From this list of the features of Classes and Structures, we see even more similarities. Classes and Structures can both:
- Define properties to store values
- Define methods to provide functionality
- Define initializers to set up their initial state
- Be extended to expand their functionality beyond a default implementation
This means that Swift provides similar functionality to familiar OO Programming Languages. We can model problems using objects which can encapsulate data and functionality, and build complex relationships between these objects in a modular fashion. Curious readers might have the same reaction to this as I did - if it’s the case that both classes and structures can do the above, which is a reasonable set of functionality to expect from classes, why are there two constructs? What are the differences between them, and why does Swift as a programming language need both?
First, let’s take a look at what Classes can do that Structures cannot. Classes provide functionality for two crucial pieces of OO functionality that Ruby developers, for instance, tend to rely on somewhat heavily:
- Inheritance, where one class can inherit the characteristics of another
- Type casting, where you can check and interpret the type of a class instance at runtime
Additionally, classes provide the following facilities which are stated in terms familiar to those with experience managing resources and memory manually:
- Deinitializers, to enable an instance of a class to free up any resources it has assigned
- Reference counting, to allow for more than one reference to a class instance
Stated in other terms, inheritance, type casting, deinitializers, and reference counting make it possible to expressively create designs which employ the full compliment of OO techniques.
From my perspective, Classes and Structures have just the right amount of overlap in functionality, leaving enough room for developers to make reasoned decisions about which construct to employ according to the purpose it may be used for. The differences hinge on how instances of Classes and Structures are represented in memory after they are initialized – structure instances are always passed by value and class instances are always passed by reference. This distinction is something that dynamic language developers typically do not have to spend much time thinking about - on a mobile platform, however, this kind of thinking becomes very important.
Different memory semantics is not the only advantage of having these distinct types, however. Because structures are simpler than classes, and cannot be as heavily modified after declaration, they provide an opportunity to create value objects which represent pieces of data independent from their behavior. This is very useful.
While Classes and Structures cover much of the familiar ground with respect to OO functionality in Ruby or Python, there is one more construct that might not be so familiar that I’d like to point out before we draw any conclusions on Swift’s OO design capabilities.
Protocols are an interesting addition to the already rich world of classes and structures. They provide a way to define behavior separately from the classes which implement them:
“A protocol defines a blueprint of methods, properties, and other requirements that suit a particular task or piece of functionality”
Similar constructs exist in Java, Go, and other languages, but many dynamic languages do not embrace this design style. Protocols embrace flexible design by encapsulating the necessary data and behavior for a domain idea outside the scope of a Class or Structure definition. This means that a concept can be represented separately from its implementation, allowing for more creative reuse, composition, and more.
To illustrate the usefulness of Protocols, consider the development of a small networked process to record some information from a system over time. You might want to have the system write to a Key-Value store, or to disk, or to STDOUT, depending on a variety of circumstances. In this case, you would simply define a Protocol:
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This protocol requires that anything which implements them must contain at least two instance methods -
get, which accepts an Integer and returns an Integer, and
set, which accepts an Integer and returns a Boolean value. A class that implements this Protocol might look something like the following:
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You could imagine similar classes for the other backends I mentioned above. This is a very convenient solution for swapping out backends when you need to run your process in different locations and under different circumstances - simply implement new ones as needed, and pass them in wherever a Backend is needed.
Defining a Protocol and implementing it is really the tip of the iceberg, however - things can get very subtle and complex. Protocols can be treated as types, and intricate hierarchies can be created with them. While it is interesting to look through the Swift book to see these examples, I don’t believe they are the real selling points of protocols. Used simply and in combination with classes and structures, protocols provide a missing piece of the design puzzle that often vexes programmers in dynamic languages.
Swift is a very interesting programming language that at first blush appears to have all of the necessary components to build expressive OO programs. In addition to classes, structures, and protocols, Ruby developers might also want to look into
Extensions which offer mixin-like capabilities.
 Apple Inc. “The Swift Programming Language.” iBooks.