Swift Generics: Writing Flexible and Reusable Code

What are Generics?

Generics are a means to write code in Swift so that it is not type-specific and may be used with any type. This enables you to create reusable, adaptable code that may be applied in a variety of situations.

Writing generic functions

A simple generic function that switches the values of two variables is shown here as an example:

func swapTwoValues<T>(_ a: inout T, _ b: inout T) {
    let temporaryA = a
    a = b
    b = temporaryA

The generic type T, which can be used to represent any type inside the function, is defined using the syntax. If both arguments are of the same type, the function can then be called with any kind of arguments:

var x = 5
var y = 10
swapTwoValues(&x, &y)
// x is now 10, and y is now 5

var a = "hello"
var b = "world"
swapTwoValues(&a, &b)
// a is now "world", and b is now "hello"

Writing generic types

Generic types, like structs and classes, can be made in addition to generic functions. Here is an illustration of a generic Stack type that stores its values in an array:

struct Stack<Element> {
    var items = [Element]()
    mutating func push(_ item: Element) {
    mutating func pop() -> Element {
        return items.removeLast()

When you create a new instance of this Stack type, you must define the type of the elements it will store in order to use it:

var stackOfInts = Stack<Int>()
let poppedInt = stackOfInts.pop()
// poppedInt is 2

var stackOfStrings = Stack<String>()
let poppedString = stackOfStrings.pop()
// poppedString is "world"

How to refactor legacy code with generics

If we want to apply the capabilities of the generics in legacy code, we will have to take into account some points. For example, you can start by identifying parts of legacy code that would profit from being made more flexible or reusable before applying generics to them. The code in those areas can then be refactored to use generic types and functions.

Consider a function that takes an array of integers as input and outputs the sum of all the array’s elements:

func sumArray(_ array: [Int]) -> Int {
    var result = 0
    for number in array {
        result += number
    return result

To use generics, you can restructure this method as follows:

func sumArray<T: Numeric>(_ array: [T]) -> T {
    var result: T = 0
    for number in array {
        result += number
    return result

Now that the function has been updated, it can be used with any type of numeric data, including doubles, floats, and integers.

Some more generics complex examples

Surely you know about the Result type, which represents the result of an operation, which may succeed or fail. It has two generic types: SuccessType and FailureType, which indicate the type of result that will be returned if the operation is successful and unsuccessful, respectively.

enum Result<SuccessType, FailureType> {
    case success(SuccessType)
    case failure(FailureType)

Here is an illustration of how to retrieve a list of people from a database using the Result type:

func getUsers() -> Result<[User], Error> {
    do {
        let users = try database.fetchUsers()
        return .success(users)
    } catch {
        return .failure(error)

We can set another case for generics. For example, we can set a Tree type that describes a tree structure in which each node can have any number of child nodes and has a value of generic type T:

class Tree<T> {
    var value: T
    var children: [Tree<T>] = []
    init(value: T) {
        self.value = value
    func addChild(_ child: Tree<T>) {

Here is an example of how to make a simple family tree using the Tree type:

let grandfather = Tree(value: "Mark")
let grandmother = Tree(value: "Pam")
let father = Tree(value: "Robert")
let mother = Tree(value: "Sue")
let son = Tree(value: "Charles")
let daughter = Tree(value: "Anne")


Using typealias with generics

The typealias keyword in Swift allows you to designate an alias for a type, which comes in handy when working with generics among other things. For example:

struct Stack<Element> {
    typealias ItemType = Element
    var items = [ItemType]()
    mutating func push(_ item: ItemType) {
    mutating func pop() -> ItemType {
        return items.removeLast()

In this example, the generic type Element is represented by the typealias ItemType, which is a property of the Stack struct. This enables you to refer to the type of the items in the stack using the more evocative word ItemType rather than Element.

Set type constaints for generics

In Swift, constraints can be used to provide the specifications for the generic types that are utilized in a function or type. These restrictions make that the generic type complies with certain specifications, such as adhering to a specific protocol or being a subclass of a specific class. For example:

protocol Comparable {
    func isGreaterThan(_ other: Self) -> Bool

func findLargest<T: Comparable>(in array: [T]) -> T {
    var largest = array.first!
    for element in array {
        if element.isGreaterThan(largest) {
            largest = element
    return largest

The findLargest(in:) function in this illustration has a generic type T that is restricted to the Comparable protocol. As a result, the function can only be applied to types that follow the Comparable standard, and it can also invoke the isGreaterThan(_:) method on those types.

Type constraints for generic types can also be specified using typealiases. For instance:

struct Queue<Element: Equatable> {
    typealias ItemType = Element
    var items = [ItemType]()
    mutating func enqueue(_ item: ItemType) {
    mutating func dequeue() -> ItemType? {
        guard !items.isEmpty else { return nil }
        return items.removeFirst()

The ItemType typealias in this iteration of the Queue struct is restricted to the Equatable protocol, therefore the Queue can only be used with types that comply with Equatable. This enables you to compare items in the queue using the == operator.

Multiple constaints

Additionally, more than one constraint may be used in a single generic type declaration. For instance:

func display<T: CustomStringConvertible & Comparable>(item: T) {

In this example, the generic type T of the display(item:) function is restricted to the CustomStringConvertible and Comparable protocols. This indicates that only types that abide by both of these protocols can be utilized with the function.


In conclusion, Swift’s use of generics enables the development of reusable and adaptable code by enabling programmers to create functions and types that operate on any sort of data rather than just a few predefined types.

The developer can have better control over the data utilized and catch mistakes at build time rather than run time thanks to the ability to impose a type constraint on generics. This enhances the readability, maintainability, and safety of the code. Swift developers may write more reliable and effective code by utilizing generics, making it a crucial skill to acquire when using the language.

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