Constraints
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Constraints
When you define a generic function or type, you can set constraints on the types that may be used. Constraints can be protocols or classes, and the syntax is <T: Protocol> or <T: Class>.
<T: Protocol>
The generic function swapValues<T> works with all types, but consider this generic function:
func max<T>(lhs: T, rhs: T) -> T {
return lhs > rhs ? lhs : rhs
}
func max<T: Comparable>(lhs: T, rhs: T) -> T {
return lhs > rhs ? lhs : rhs
}
class FamilyCar: Vehicle { ... }
let myCar = FamilyCar()
max(lhs: familyCar, rhs: myCar)
max(lhs: familyCar.maxSpeed, rhs: myCar.maxSpeed)
<T: Class>
Another class in the same module is VehicleType, which has a move method:
class VehicleType {
...
func move(direction: Direction, duration: TimeInterval, speed: Double) { ... }
}
func moveAtoB<T: VehicleType>(vehicle: T, locationA: CLLocation, locationB: CLLocation) { ... }
Where Clause
Use where when you need to set more than one constraint, for example, if T must inherit from the VehicleType class and also conform to the Comparable protocol:
<T where T: VehicleType, T: Comparable> // illustration only
init<S>(_ s: S) where Element == S.Element, S : Sequence
extension Setting {
mutating func save(from untypedValue: Any) {
if let value = untypedValue as? T {
self.value = value
}
}
}
extension Setting where T: Decodable {
mutating func save(from untypedValue: Any) {
if let value = untypedValue as? T {
self.value = value
}
}
mutating func save(from json: Data) throws {
let decoder = JSONDecoder()
self.value = try decoder.decode(T.self, from: json)
}
}
extension Setting {
mutating func save(from untypedValue: Any) {
if let value = untypedValue as? T {
self.value = value
}
}
mutating func save(from json: Data) throws where T: Decodable {
let decoder = JSONDecoder()
self.value = try decoder.decode(T.self, from: json)
}
}