Swift Programming - Type Casting

To validate the type of an instance 'Type Casting' comes into play in Swift language. It is used to check whether the instance type belongs to a particular super class or subclass or it is defined in its own hierarchy.

Swift type casting provides two operators 'is' to check the type of a value and 'as' and to cast the type value to a different type. Type casting also checks whether the instance type follows particular protocol conformance standard.

Defining a Class Hierarchy

Type casting is used to check the type of instances to find out whether it belongs to particular class type. Also, it checks hierarchy of classes and its subclasses to check and cast those instances to make it as a same 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")

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

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

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

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

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

Result:

Instance physics is: solid physics

Instance equation is: Hertz

Instance physics is: Fluid Dynamics

Instance formulae is: Giga Hertz

Type Checking

Type checking is done with the 'is' operator. The 'is' type check operator checks whether the instance belongs to particular subclass type and returns 'true' if it belongs to that instance else it will return 'false'.

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"),

   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")

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

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

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

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

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

var chemCount = 0

var mathsCount = 0

for item in sa {

   if item is Chemistry {

      ++chemCount

   }else if item is Maths {

      ++mathsCount

   }

}

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

Result:

Instance physics is: solid physics

Instance equation is: Hertz

Instance physics is: Fluid Dynamics

Instance formulae is: Giga Hertz

Subjects in chemistry contains 2 topics and maths contains 3 topics

Downcasting

Downcasting the subclass type can be done with two operators (as? and as!).'as?' returns an optional value when the value returns nil. It is used to check successful downcast.

'as!' returns force unwrapping as discussed in the optional chaining when the downcasting returns nil value. It is used to trigger runtime error in case of downcast failure

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"),

   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")

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

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

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

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

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

var chemCount = 0

var mathsCount = 0

for item in sa {

   if let print = item as? Chemistry {

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

   }else if let example = item as? Maths {

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

   }

}

Result:

Instance physics is: solid physics

Instance equation is: Hertz

Instance physics is: Fluid Dynamics

Instance formulae is: Giga Hertz

Chemistry topics are: 'solid physics', Hertz

Maths topics are: 'Fluid Dynamics', Giga Hertz

Chemistry topics are: 'Thermo physics', Decibels

Maths topics are: 'Astro Physics', MegaHertz

Maths topics are: 'Differential Equations', Cosine Series

Typecasting: Any and Any Object

The keyword 'Any' is used to represent an instance which belongs to any type including function types.

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"),

   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")

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

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

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

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

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

var chemCount = 0

var mathsCount = 0

for item in sa {

   if let print = item as? Chemistry {

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

   }else if let example = item as? Maths {

      println("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:

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

   case let someDouble as Double where someDouble > 0:

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

   case let someString as String:

      println("\(someString)")

   case let phy as Chemistry:

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

   default:

      println("None")

   }

}

Result:

Instance physics is: solid physics

Instance equation is: Hertz

Instance physics is: Fluid Dynamics

Instance formulae is: Giga Hertz

Chemistry topics are: 'solid physics', Hertz

Maths topics are: 'Fluid Dynamics',  Giga Hertz

Chemistry topics are: 'Thermo physics', Decibels

Maths topics are: 'Astro Physics',  MegaHertz

Maths topics are: 'Differential Equations',  Cosine Series

Integer value is 12

Pi value is 3.14159

Example for Any

Topics 'solid physics', Hertz

AnyObject

To represent the instance of any class type, 'AnyObject' keyword is used.

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")

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

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

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

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

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

var chemCount = 0

var mathsCount = 0

for item in saprint {

   if let print = item as? Chemistry {

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

   }else if let example = item as? Maths {

      println("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:

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

   case let someDouble as Double where someDouble > 0:

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

   case let someString as String:

      println("\(someString)")

   case let phy as Chemistry:

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

   default:

      println("None")

   }

}

Result:

Instance physics is: solid physics

Instance equation is: Hertz

Instance physics is: Fluid Dynamics

Instance formulae is: Giga Hertz

Chemistry topics are: 'solid physics', Hertz

Maths topics are: 'Fluid Dynamics',  Giga Hertz

Chemistry topics are: 'Thermo physics', Decibels

Maths topics are: 'Astro Physics',  MegaHertz

Maths topics are: 'Differential Equations',  Cosine Series

Integer value is 12

Pi value is 3.14159

Example for Any

Topics 'solid physics', Hertz