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Swift - Type Casting


To validate the type of an instance 'Type Casting' comes into play in Swift 4 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 4 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") 
 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)")

When we run the above program using playground, we get the following 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")
   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 sa { 
     if item is Chemistry { 
        ++chemCount 
     } else if item is Maths {  
       ++mathsCount  
    } 
  } 
    print("Subjects in chemistry contains 
 \(chemCount) topics and maths contains
  \(mathsCount) topics")

When we run the above program using playground, we get the following 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") 
  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 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)") 
     } 
  }

When we run the above program using playground, we get the following 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")  
 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 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")  
   }  }

When we run the above program using playground, we get the following 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") 
 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 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")     }  }

When we run the above program using playground, we get the following 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


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Hi I am Pluto.