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README.md	README.md

Memory Management 1 ~ 14

1) What is the concept of the Memory Management?

Answer:

Memory Management is the process of controlling the lifetime of an object based on its necessity

var car: Car? = Car()
car.move()
car.blowHorn()
car.stop()
car = nil //release

2) What is Manual Memory Management?

Answer:

Prior to ARC, developer was responsible for retaining and releasing the objects’ memory using release, retain and autorelease.

3) What is ARC?

Answer:

Automatic Reference Counting is a concept of managing the memory automatically by the runtime system.
But still there are three different places where developer has to take of releasing the memory.
- Closures
- Delegates
- Outlets

4) What is Memory Leak?

Answer:

Memory Leak is a situation where the runtime system fails to release the memory of unused objects.

5) What is Strong?

Answer:

Strong shares the ownership of the object and increases the reference count by 1.

class Car {

  func move() {
		print("Car is moving")
  }

}

var car: Car? = Car()
car?.move()

// Strong Reference Occured
var anotherCar: Car? = car

car?.move() // Car is moving
anotherCar?.move() // Car is moving

car = nil
car?.move() // Not executing
anotherCar?.move() // Car is moving, not released
anotherCar!.move() // Car is moving, not released

6) What is Weak?

Answer:

weak shared the ownership of the object without increasing the reference count.

class Car {

  func move() {
		print("Car is moving")
  }

}

var car: Car? = Car(
// Strong Reference Occured
weak var anotherCar: Car? = car

car?.move() // Car is moving
anotherCar?.move() // Car is moving

car = nil
car?.move() // Not executing
anotherCar?.move() // Not executing. released
anotherCar!.move() // Crash happens

7) strong vs weak

Answer:

strong and weak are memory management related keywords.
When you assign a variable to another:
- strong shares the ownership of the object and increases the reference count by 1.
- weak shares the ownership and doesn’t increase the reference of that object.

8) What is Retain Cycle?

Answer:

When two objects are pointing each other strongly and ARC fails to release memory. It leads to memory leaks. This situation is called Retain / Reference Cycle.

9) What are the different places we use weak?

Answer:

Closures
Delegates
Outlets

class Human {

	var heart: Heart? = nil

	deinit {
			print("Human is released")
	}

}

class Heart {

	var owner: Human? = nil

	deinit {
			print("Heart is released")
	}

}

var person: Human? = Human()
var heart: Heart? = Heart()
person?.heart = heart
heart?.owner = person

person = nil // "Human is released" is not printed
heart = nil // "Heart is released" is not printed

class Human {

    weak var heart: Heart? = nil

    deinit {
        print("Human is released")
    }

}

class Heart {

    weak var owner: Human? = nil

    deinit {
        print("Heart is released")
    }

}

var person: Human? = Human()
var heart: Heart? = Heart()
person?.heart = heart
heart?.owner = person

person = nil // Human is released
heart = nil // Heart is released

10) weak vs unowned

Answer:

weak and unowned are keywords used to avoid retain cycles.
weak variable might become nil. So weak variables are declared as var and Optional.
When weak is used in closures, weak references act like optionals. So when you use them in closures, you have to use optional chain with them.
unowned variables never become nil. So unowned variables can be declared as let and stored.
You can use unowned when the other instance has the same life time or the longer lifetime.
Use unowned, when you are sure it is not nil.