generics.swift

struct IntStack {
    var items = [Int]()
    mutating func push(item: Int) {
        items.append(item)
    }
    mutating func pop() -> Int {
        return items.removeLast()
    }
}

//
// using generics instead of Int:
//

struct Stack<T> {

    var items = [T]()
    mutating func push(item: T)  {
        items.append(item)
    }
    
    mutating func pop() -> T {
        return items.removeLast()
    }
}

// using it:

var stackOfStrings = Stack<String>()
stackOfStrings.push("uno")
stackOfStrings.push("dos")
stackOfStrings.push("tres")
stackOfStrings.push("cuatro")

//
// extending a generic type
//

extension Stack {
    var topItem: T? {
        return items.isEmpty ? nil : items[items.count - 1]
    }
}

if let topItem = stackOfStrings.topItem {
    print(topItem)
}