Swift - ARC Overview

Memory management functions and its usage are handled in Swift language through Automatic reference counting (ARC). ARC is used to initialize and deinitialize the system resources thereby releasing memory spaces used by the class instances when the instances are no longer needed. ARC keeps track of information about the relationships between our code instances to manage the memory resources effectively.

Functions of ARC

• ARC allocates a chunk of memory to store the information each and every time when a new class instance is created by init().
• Information about the instance type and its values are stored in memory.
• When the class instance is no longer needed it automatically frees the memory space by deinit() for further class instance storage and retrieval.
• ARC keeps in track of currently referring class instances properties, constants and variables so that deinit() is applied only to those unused instances.
• ARC maintains a 'strong reference' to those class instance property, constants and variables to restrict deallocation when the class instance is currently in use.

ARC Program

class StudDetails {
   var stname: String!
   var mark1: Int!
   init(stname: String, mark1: Int) {
      self.stname = stname
      self.mark1 = mark1
   }

   deinit {
      println("Deinitialized \(self.stname)")
      println("Deinitialized \(self.mark1)")
   }
}

let stname = "swift"
let mark1 = 98

println(stname)
println(mark1)

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

swift
98

ARC Strong Reference Cycles Class Instances

class studmarks {
   let name: String
   var stud1: student?

   init (name: String) {
      println("Initializing: \(name)")
      self.name = name
   }

   deinit {
      println("Deallocating: \(self.name)")
   }
}

class student {
   let name: String
   var strname: studmarks?

   init (name: String) {
      println("Initializing: \(name)")
      self.name = name
   }
    
   deinit {
      println("Deallocating: \(self.name)")
   }
}

var shiba: studmarks?
var mari: student?

shiba = studmarks(name: "Swift")
mari = student(name: "ARC")

shiba!.stud1 = mari
mari!.strname = shiba

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

Initializing: Swift
Initializing: ARC

ARC Weak and Unowned References

Class type properties has two ways to resolve strong reference cycles −

• Weak References
• Unowned References

These references are used to enable one instance to refer other instances in a reference cycle. Then the instances may refer to each and every instances instead of caring about strong reference cycle. When the user knows that some instance may return 'nil' values we may point that using weak reference. When the instance going to return something rather than nil then declare it with unowned reference.

Weak Reference Program

class module {
   let name: String
   init(name: String) { self.name = name }
   var sub: submodule?
   deinit { println("\(name) Is The Main Module") }
}

class submodule {
   let number: Int

   init(number: Int) { self.number = number }

   weak var topic: module?

   deinit { println("Sub Module with its topic number is \(number)") }
}

var toc: module?
var list: submodule?
toc = module(name: "ARC")
list = submodule(number: 4)
toc!.sub = list
list!.topic = toc

toc = nil
list = nil

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

ARC Is The Main Module
Sub Module with its topic number is 4

Unowned Reference Program

class student {
   let name: String
   var section: marks?

   init(name: String) {
      self.name = name
   }

   deinit { println("\(name)") }
}
class marks {
   let marks: Int
   unowned let stname: student
 
   init(marks: Int, stname: student) {
      self.marks = marks
      self.stname = stname
   }

   deinit { println("Marks Obtained by the student is \(marks)") }
}

var module: student?
module = student(name: "ARC")
module!.section = marks(marks: 98, stname: module!)
module = nil

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

ARC
Marks Obtained by the student is 98

Strong Reference Cycles for Closures

When we assign a closure to the class instance property and to the body of the closure to capture particular instance strong reference cycle can occur. Strong reference to the closure is defined by 'self.someProperty' or 'self.someMethod()'. Strong reference cycles are used as reference types for the closures.

class HTMLElement {
   let samplename: String
   let text: String?

   lazy var asHTML: () -> String = {
      if let text = self.text {
         return "<\(self.samplename)>\(text)</\(self.samplename)>"
      }else {
         return "<\(self.samplename) />"
      }
   }

   init(samplename: String, text: String? = nil) {
      self.samplename = samplename
      self.text = text
   }

   deinit {
      println("\(samplename) is being deinitialized")
   }
}

var paragraph: HTMLElement? = HTMLElement(samplename: "p", text: "Welcome to Closure SRC")
println(paragraph!.asHTML())

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

<p>Welcome to Closure SRC</p>

Weak and Unowned References

When the closure and the instance refer to each other the user may define the capture in a closure as an unowned reference. Then it would not allow the user to deallocate the instance at the same time. When the instance sometime return a 'nil' value define the closure with the weak instance.

class HTMLElement {
   let module: String
   let text: String?

   lazy var asHTML: () -> String = {
      [unowned self] in
      if let text = self.text {
         return "<\(self.module)>\(text)</\(self.module)>"
      }else {
         return "<\(self.module) />"
      }
   }

   init(module: String, text: String? = nil) {
      self.module = module
      self.text = text
   }

   deinit {
      println("\(module) the deinit()")
   }
}

var paragraph: HTMLElement? = HTMLElement(module: "Inside", text: "ARC Weak References")
println(paragraph!.asHTML())
paragraph = nil

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

<Inside>ARC Weak References</Inside>
Inside the deinit()