Type Casting in Swift

Kanika Dhiman

1 October 2020

What is 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.

class Subjects {
   var physics: String
   init(physics: String) {
      self.physics = physics
   }
}

class Chemistry: Subjects {
   var equations: String
   init(physics: String, equations: String) {
      self.equations = equations
      super.init(physics: physics)
   }
}

class Maths: Subjects {
   var formulae: String
   init(physics: String, formulae: String) {
      self.formulae = formulae
      super.init(physics: physics)
   }
}

let sa = [ Chemistry(physics: "solid physics", equations: "Hertz"),
   Maths(physics: "Fluid Dynamics", formulae: "Giga Hertz")]

let samplechem = Chemistry(physics: "solid physics", equations: "Hertz")
print("Instance physics is: \(samplechem.physics)")
print("Instance equation is: \(samplechem.equations)")

let samplemaths = Maths(physics: "Fluid Dynamics", formulae: "Giga Hertz")
print("Instance physics is: \(samplemaths.physics)")
print("Instance formulae is: \(samplemaths.formulae)")

class Subjects {

var physics: String

init(physics: String) {

self.physics = physics

}

class Chemistry: Subjects {

var equations: String

init(physics: String, equations: String) {

self.equations = equations

super.init(physics: physics)

}

class Maths: Subjects {

var formulae: String

init(physics: String, formulae: String) {

self.formulae = formulae

super.init(physics: physics)

}

let sa = [ Chemistry(physics: "solid physics", equations: "Hertz"),

Maths(physics: "Fluid Dynamics", formulae: "Giga Hertz")]

let samplechem = Chemistry(physics: "solid physics", equations: "Hertz")

print("Instance physics is: \(samplechem.physics)")

print("Instance equation is: \(samplechem.equations)")

let samplemaths = Maths(physics: "Fluid Dynamics", formulae: "Giga Hertz")

print("Instance physics is: \(samplemaths.physics)")

print("Instance formulae is: \(samplemaths.formulae)")

Checking Type

Use the type check operator (is) to check whether an instance is of a certain subclass type. The type check operator returns true if the instance is of that subclass type and false if it is not.

var chemCount = 0
var mathsCount = 0
for item in sa {
   if item is Chemistry {
      ++chemCount
   } else if item is Maths {
      ++mathsCount
   }
}

print("Subjects in chemistry contains \(chemCount) topics and maths contains \(mathsCount) topics")

var chemCount = 0

var mathsCount = 0

for item in sa {

if item is Chemistry {

++chemCount

} else if item is Maths {

++mathsCount

}

print("Subjects in chemistry contains \(chemCount) topics and maths contains \(mathsCount) topics")

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!).’

Conditional down casting (as?): as?’ returns an optional value of the type you are trying to downcast to.
Forced down casting (as!): ‘as!’ attempts the downcast and force-unwraps the result as a single compound action.

var chemCount = 0
var mathsCount = 0

for item in sa {
   if let print = item as? Chemistry {
      print("Chemistry topics are: '\(print.physics)', \(print.equations)")
   } else if let example = item as? Maths {
      print("Maths topics are: '\(example.physics)', \(example.formulae)")
   }
}

var chemCount = 0

var mathsCount = 0

for item in sa {

if let print = item as? Chemistry {

print("Chemistry topics are: '\(print.physics)', \(print.equations)")

} else if let example = item as? Maths {

print("Maths topics are: '\(example.physics)', \(example.formulae)")

}

Type Casting: Any and Any Object

Swift provides two special types for working with nonspecific types:

Any can represent an instance of any type at all, including function types.

var chemCount = 0
var mathsCount = 0

for item in sa {
   if let print = item as? Chemistry {
      print("Chemistry topics are: '\(print.physics)', \(print.equations)")
   } else if let example = item as? Maths {
      print("Maths topics are: '\(example.physics)', \(example.formulae)")
   }
}

var exampleany = [Any]()

exampleany.append(12)
exampleany.append(3.14159)
exampleany.append("Example for Any")
exampleany.append(Chemistry(physics: "solid physics", equations: "Hertz"))

for print in exampleany {
   switch print {
      case let someInt as Int:
         print("Integer value is \(someInt)")
      case let someDouble as Double where someDouble > 0:
         print("Pi value is \(someDouble)")
      case let someString as String:
         print("\(someString)")
      case let phy as Chemistry:   
         print("Topics '\(phy.physics)', \(phy.equations)")
      default:
         print("None")
   }
}

var chemCount = 0

var mathsCount = 0

for item in sa {

if let print = item as? Chemistry {

print("Chemistry topics are: '\(print.physics)', \(print.equations)")

} else if let example = item as? Maths {

print("Maths topics are: '\(example.physics)', \(example.formulae)")

}

var exampleany = [Any]()

exampleany.append(12)

exampleany.append(3.14159)

exampleany.append("Example for Any")

exampleany.append(Chemistry(physics: "solid physics", equations: "Hertz"))

for print in exampleany {

switch print {

case let someInt as Int:

print("Integer value is \(someInt)")

case let someDouble as Double where someDouble > 0:

print("Pi value is \(someDouble)")

case let someString as String:

print("\(someString)")

case let phy as Chemistry:

print("Topics '\(phy.physics)', \(phy.equations)")

default:

print("None")

}

AnyObject can represent an instance of any class type.

class Subjects {
   var physics: String
   init(physics: String) {
      self.physics = physics
   }
}

class Chemistry: Subjects {
   var equations: String
   init(physics: String, equations: String) {
      self.equations = equations
      super.init(physics: physics)
   }
}

class Maths: Subjects {
   var formulae: String
   init(physics: String, formulae: String) {
      self.formulae = formulae
      super.init(physics: physics)
   }
}

let saprint: [AnyObject] = [Chemistry(physics: "solid physics", equations: "Hertz"),
   Maths(physics: "Fluid Dynamics", formulae: "Giga Hertz"),
   Chemistry(physics: "Thermo physics", equations: "Decibels"),
   Maths(physics: "Astro Physics", formulae: "MegaHertz"),
   Maths(physics: "Differential Equations", formulae: "Cosine Series")]

let samplechem = Chemistry(physics: "solid physics", equations: "Hertz")
print("Instance physics is: \(samplechem.physics)")
print("Instance equation is: \(samplechem.equations)")

let samplemaths = Maths(physics: "Fluid Dynamics", formulae: "Giga Hertz")
print("Instance physics is: \(samplemaths.physics)")
print("Instance formulae is: \(samplemaths.formulae)")

var chemCount = 0
var mathsCount = 0

for item in saprint {
   if let print = item as? Chemistry {
      print("Chemistry topics are: '\(print.physics)', \(print.equations)")
   } else if let example = item as? Maths {
      print("Maths topics are: '\(example.physics)', \(example.formulae)")
   }
}

var exampleany = [Any]()
exampleany.append(12)
exampleany.append(3.14159)
exampleany.append("Example for AnyObject")
exampleany.append(Chemistry(physics: "solid physics", equations: "Hertz"))

for print in exampleany {
   switch print {
      case let someInt as Int:
         print("Integer value is \(someInt)")
      case let someDouble as Double where someDouble > 0:
         print("Pi value is \(someDouble)")
      case let someString as String:
         print("\(someString)")
      case let phy as Chemistry:
         print("Topics '\(phy.physics)', \(phy.equations)")
      default:
         print("None")
   }
}

class Subjects {

var physics: String

init(physics: String) {

self.physics = physics

}

class Chemistry: Subjects {

var equations: String

init(physics: String, equations: String) {

self.equations = equations

super.init(physics: physics)

}

class Maths: Subjects {

var formulae: String

init(physics: String, formulae: String) {

self.formulae = formulae

super.init(physics: physics)

}

let saprint: [AnyObject] = [Chemistry(physics: "solid physics", equations: "Hertz"),

Maths(physics: "Fluid Dynamics", formulae: "Giga Hertz"),

Chemistry(physics: "Thermo physics", equations: "Decibels"),

Maths(physics: "Astro Physics", formulae: "MegaHertz"),

Maths(physics: "Differential Equations", formulae: "Cosine Series")]

let samplechem = Chemistry(physics: "solid physics", equations: "Hertz")

print("Instance physics is: \(samplechem.physics)")

print("Instance equation is: \(samplechem.equations)")

let samplemaths = Maths(physics: "Fluid Dynamics", formulae: "Giga Hertz")

print("Instance physics is: \(samplemaths.physics)")

print("Instance formulae is: \(samplemaths.formulae)")

var chemCount = 0

var mathsCount = 0

for item in saprint {

if let print = item as? Chemistry {

print("Chemistry topics are: '\(print.physics)', \(print.equations)")

} else if let example = item as? Maths {

print("Maths topics are: '\(example.physics)', \(example.formulae)")

}

var exampleany = [Any]()

exampleany.append(12)

exampleany.append(3.14159)

exampleany.append("Example for AnyObject")

exampleany.append(Chemistry(physics: "solid physics", equations: "Hertz"))

for print in exampleany {

switch print {

case let someInt as Int:

print("Integer value is \(someInt)")

case let someDouble as Double where someDouble > 0:

print("Pi value is \(someDouble)")

case let someString as String:

print("\(someString)")

case let phy as Chemistry:

print("Topics '\(phy.physics)', \(phy.equations)")

default:

print("None")

}