# Type Casting
# Downcasting
A variable can be downcasted to a subtype using the type cast operators as?, and as!.
The as? operator attempts to cast to a subtype.
It can fail, therefore it returns an optional.
let value: Any = "John"
let name = value as? String
print(name) // prints Optional("John")
let age = value as? Double
print(age) // prints nil
The as! operator forces a cast.
It does not return an optional, but crashes if the cast fails.
let value: Any = "Paul"
let name = value as! String
print(name) // prints "Paul"
let age = value as! Double // crash: "Could not cast value…"
It is common to use type cast operators with conditional unwrapping:
let value: Any = "George"
if let name = value as? String {
print(name) // prints "George"
}
if let age = value as? Double {
print(age) // Not executed
}
# Type casting in Swift Language
# Type Casting
Type casting is a way to check the type of an instance, or to treat that instance as a different superclass or subclass from somewhere else in its own class hierarchy.
Type casting in Swift is implemented with the is and as operators. These two operators provide a simple and expressive way to check the type of a value or cast a value to a different type.
# Downcasting
A constant or variable of a certain class type may actually refer to an instance of a subclass behind the scenes. Where you believe this is the case, you can try to downcast to the subclass type with a type cast operator (as? or as!).
Because downcasting can fail, the type cast operator comes in two different forms. The conditional form, as?, returns an optional value of the type you are trying to downcast to. The forced form, as!, attempts the downcast and force-unwraps the result as a single compound action.
Use the conditional form of the type cast operator (as?) when you are not sure if the downcast will succeed. This form of the operator will always return an optional value, and the value will be nil if the downcast was not possible. This enables you to check for a successful downcast.
Use the forced form of the type cast operator (as!) only when you are sure that the downcast will always succeed. This form of the operator will trigger a runtime error if you try to downcast to an incorrect class type. Know more. (opens new window)
# String to Int & Float conversion : -
let numbers = "888.00"
let intValue = NSString(string: numbers).integerValue
print(intValue) // Output - 888
let numbers = "888.00"
let floatValue = NSString(string: numbers).floatValue
print(floatValue) // Output : 888.0
# Float to String Conversion
let numbers = 888.00
let floatValue = String(numbers)
print(floatValue) // Output : 888.0
// Get Float value at particular decimal point
let numbers = 888.00
let floatValue = String(format: "%.2f", numbers) // Here %.2f will give 2 numbers after decimal points we can use as per our need
print(floatValue) // Output : "888.00"
# Integer to String value
let numbers = 888
let intValue = String(numbers)
print(intValue) // Output : "888"
# Float to String value
let numbers = 888.00
let floatValue = String(numbers)
print(floatValue)
# Optional Float value to String
let numbers: Any = 888.00
let floatValue = String(describing: numbers)
print(floatValue) // Output : 888.0
# Optional String to Int value
let hitCount = "100"
let data :AnyObject = hitCount
let score = Int(data as? String ?? "") ?? 0
print(score)
# Downcasting values from JSON
let json = ["name" : "john", "subjects": ["Maths", "Science", "English", "C Language"]] as [String : Any]
let name = json["name"] as? String ?? ""
print(name) // Output : john
let subjects = json["subjects"] as? [String] ?? []
print(subjects) // Output : ["Maths", "Science", "English", "C Language"]
# Downcasting values from Optional JSON
let response: Any = ["name" : "john", "subjects": ["Maths", "Science", "English", "C Language"]]
let json = response as? [String: Any] ?? [:]
let name = json["name"] as? String ?? ""
print(name) // Output : john
let subjects = json["subjects"] as? [String] ?? []
print(subjects) // Output : ["Maths", "Science", "English", "C Language"]
# Manage JSON Response with conditions
let response: Any = ["name" : "john", "subjects": ["Maths", "Science", "English", "C Language"]] //Optional Response
guard let json = response as? [String: Any] else {
// Handle here nil value
print("Empty Dictionary")
// Do something here
return
}
let name = json["name"] as? String ?? ""
print(name) // Output : john
let subjects = json["subjects"] as? [String] ?? []
print(subjects) // Output : ["Maths", "Science", "English", "C Language"]
# Manage Nil Response with condition
let response: Any? = nil
guard let json = response as? [String: Any] else {
// Handle here nil value
print("Empty Dictionary")
// Do something here
return
}
let name = json["name"] as? String ?? ""
print(name)
let subjects = json["subjects"] as? [String] ?? []
print(subjects)
# Output : Empty Dictionary
# Casting with switch
The switch statement can also be used to attempt casting into different types:
func checkType(_ value: Any) -> String {
switch value {
// The `is` operator can be used to check a type
case is Double:
return "value is a Double"
// The `as` operator will cast. You do not need to use `as?` in a `switch`.
case let string as String:
return "value is the string: \(string)"
default:
return "value is something else"
}
}
checkType("Cadena") // "value is the string: Cadena"
checkType(6.28) // "value is a Double"
checkType(UILabel()) // "value is something else"
# Upcasting
The as operator will cast to a supertype. As it cannot fail, it does not return an optional.
let name = "Ringo"
let value = string as Any // `value` is of type `Any` now
# Example of using a downcast on a function parameter involving subclassing
A downcast can be used to make use of a subclass's code and data inside of a function taking a parameter of its superclass.
class Rat {
var color = "white"
}
class PetRat: Rat {
var name = "Spot"
}
func nameOfRat(🐭: Rat) -> String {
guard let petRat = (🐭 as? PetRat) else {
return "No name"
}
return petRat.name
}
let noName = Rat()
let spot = PetRat()
print(nameOfRat(noName))
print(nameOfRat(spot))