Queue Example on iOS, written in Swift..

Queue.swift

import UIKit

public protocol Queue {
    associatedtype Element
    mutating func enqueue(_ element: Element)
    mutating func dequeue() -> Element?
    var isEmpty: Bool { get }
    var peek: Element? { get }
}

public struct QueueArray<T>: Queue {
    
    private var array: [T] = []
    
    public init() {}
    
    public var isEmpty: Bool {
        array.isEmpty
    }
    
    public var peek: T? {
        array.first
    }
    
    public mutating func enqueue(_ element: T) {
        array.append(element)
    }
    
    public mutating func dequeue() -> T? {
        isEmpty ? nil : array.removeFirst()
    }

}

extension QueueArray: CustomStringConvertible {

    public var description: String {
        String(describing: array)
    }

}

var queue = QueueArray<String>()
queue.enqueue("A")
queue.enqueue("B")
queue.enqueue("C")
print(queue)
queue.dequeue()
print(queue)
print(queue.peek)