ARC overview for memory management in Swift - Step-by-Step Execution

Concept Flow - ARC overview for memory management

Create Object

↓

Increase Reference Count

↓

Use Object

↓

Decrease Reference Count

↓

Reference Count == 0?

No→Keep Object Alive

Yes↓

Deallocate Object

This flow shows how ARC tracks how many references point to an object. When no references remain, ARC frees the object.

Execution Sample

Swift

class Person {
  var name: String
  init(name: String) { self.name = name }
  deinit { print("Person deallocated") }
}

var p1: Person? = Person(name: "Anna")
p1 = nil

This code creates a Person object, assigns it to p1, then sets p1 to nil, triggering ARC to deallocate the object.

Execution Table

Step	Action	Reference Count	Object State	Output
1	Create Person("Anna") and assign to p1	1	Object alive
2	Set p1 = nil (remove reference)	0	Object deallocated	Person deallocated
3	End	-	-	No references remain, object freed

💡 Reference count reached zero after p1 set to nil, so ARC deallocates the object.

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
p1	nil	Person instance	nil	nil
Reference Count	0	1	0	0

Key Moments - 3 Insights

Why does setting p1 to nil cause the object to be deallocated?

Because p1 was the only reference to the object, setting it to nil reduces the reference count to zero, triggering ARC to free the object (see execution_table step 2).

What happens if multiple variables reference the same object?

ARC increases the reference count for each variable. The object is only deallocated when all references are removed, meaning the count reaches zero.

Why do we see 'Person deallocated' printed?

The deinit method runs when ARC frees the object, showing that the object is cleaned up properly (see execution_table step 2 output).

Visual Quiz - 3 Questions

Look at the execution table, what is the reference count after creating the Person object?

A 0

B 1

C 2

D nil

💡 Hint

Check execution_table row 1 under 'Reference Count'

At which step does ARC deallocate the object?

A Step 1

B Step 3

C Step 2

D Never

💡 Hint

See execution_table row 2 where reference count becomes zero and object is deallocated

If another variable p2 also referenced the Person object, what would happen when p1 is set to nil?

A Reference count decreases but object stays alive

B Reference count increases

C Object is deallocated immediately

D Program crashes

💡 Hint

Recall that ARC deallocates only when reference count reaches zero; see key_moments about multiple references

Concept Snapshot

ARC (Automatic Reference Counting) tracks how many references point to an object.
When references increase, count goes up; when references are removed, count goes down.
If count reaches zero, ARC frees the object automatically.
Use optional variables to hold references; setting them to nil removes references.
Deinit runs when object is deallocated, useful for cleanup.

Full Transcript

This visual execution shows how ARC manages memory in Swift. First, creating a Person object sets its reference count to 1 because p1 points to it. When p1 is set to nil, the reference count drops to zero. ARC then deallocates the object, running the deinit method which prints 'Person deallocated'. Variables hold references; ARC counts these references to decide when to free memory. If multiple variables reference the same object, ARC waits until all are nil before deallocating. This automatic process helps prevent memory leaks and crashes by cleaning up unused objects safely.