generic_type.swift

struct Buffer<Element> {
    
    enum BufferError: Error {
        case outOfBounds
    }
    
    private var items: [Element] = []
    
    mutating func add(_ item: Element) {
        items.append(item)
    }
    
    func get(index: Int) throws -> Element {
        guard index < items.count else {
            throw BufferError.outOfBounds
        }
        return items[index]
    }
    
}

var stringBuffer = Buffer<String>()
stringBuffer.add("New String")
stringBuffer.add("ViVa")

(0...3).forEach {
    do {
        let itemFromBuffer = try stringBuffer.get(index: $0)
        print("itemFromBuffer: '\(itemFromBuffer)' at index: \($0)")
    } catch {
        print("\(error) at index: \($0)")
    }
}
/*
 itemFromBuffer: 'New String' at index: 0
 itemFromBuffer: 'ViVa' at index: 1
 outOfBounds at index: 2
 outOfBounds at index: 3
 */

var intBuffer = Buffer<Int>()
intBuffer.add(1)
intBuffer.add(100)
intBuffer.add(3000)

(0...3).forEach {
    let intItem = try? intBuffer.get(index: $0)
    print("intItem: '\(String(describing: intItem))' at index: \($0)")
}
/*
 intItem: 'Optional(1)' at index: 0
 intItem: 'Optional(100)' at index: 1
 intItem: 'Optional(3000)' at index: 2
 intItem: 'nil' at index: 3
 */