What are generics?

Swift version: 5.6

Paul Hudson @twostraws May 28th 2019

Generics are a way of making one data type act in a variety of ways depending on how it is created. You’ve already used them whether you realized or not: Swift has an Array type, but it is generic – it doesn’t contain any sort of specific data. Instead, you ask for arrays that hold specific kinds of data by using things like [String] to get a string array.

It’s not hard to create generics of your own, and to demonstrate that we’re going to create a simple Queue type. These are first-in, first-out data structures (FIFO), which means you add things to the back and remove them from the front – much like a real-life queue.

We want this queue to be generic, and in Swift you do that by writing the name of a generic placeholder inside angle brackets, like this: struct Queue<T> {. That T doesn’t mean anything special – it could have been R or Element – but T is commonly used.

Inside the queue we’re going to have an internal array tracking the items we’re storing, and we’ll write methods to add and remove items.

Here’s the complete Queue struct:

struct Queue<T> {
    private var internalArray = [T]()

    var count: Int {
        return internalArray.count
    }

    mutating func add(_ item: T) {
        internalArray.append(item)
    }

    mutating func remove() -> T? {
        if internalArray.count > 0 {
            return internalArray.removeFirst()
        } else {
            return nil
        }
    }
}

You can now create a queue to store any object you want. For example, this create a queue of integers: