Swift Language Type Casting Downcasting
Example
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
}
