associatedType

associatedType.swift

protocol ItemStoring {
    associatedtype DataType

    var items: [DataType] { get set }
    mutating func add(item: DataType)
}

extension ItemStoring {
    mutating func add(item: DataType) {
        items.append(item)
    }
}

struct NameDataBase: ItemStoring {
    var items = [String]()
}

var names = NameDataBase()

names.add(item: "Test 1")
names.add(item: "Item 2")

print(names)