Deinitializers for cleanup in Swift - Step-by-Step Execution

Concept Flow - Deinitializers for cleanup

Create instance

↓

Use instance

↓

Instance no longer needed

↓

Deinitializer called

↓

Cleanup done

↓

Memory freed

When an instance is no longer needed, Swift calls its deinitializer to clean up resources before freeing memory.

Execution Sample

Swift

class Person {
    var name: String
    init(name: String) {
        self.name = name
        print("\(name) is initialized")
    }
    deinit {
        print("\(name) is being deinitialized")
    }
}
var person: Person? = Person(name: "Anna")
person = nil

This code creates a Person instance, then sets it to nil, triggering the deinitializer to run.

Execution Table

Step	Action	Instance State	Output
1	Create Person instance with name 'Anna'	Person(name: "Anna") exists	"Anna is initialized"
2	person variable holds the instance	Person(name: "Anna") exists
3	Set person = nil (remove reference)	No instance referenced	"Anna is being deinitialized"
4	Instance deinitialized and memory freed	No instance

💡 person set to nil, no references remain, deinitializer runs and instance is cleaned up

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
person	nil	Person(name: "Anna")	Person(name: "Anna")	nil	nil

Key Moments - 2 Insights

Why does the deinitializer run only after setting person to nil?

Because the instance is kept alive as long as 'person' references it. When 'person' is set to nil (see execution_table step 3), no references remain, so deinit runs.

Can we call deinit manually?

No, Swift calls deinit automatically when the instance is about to be destroyed, as shown in step 3 and 4 of the execution_table.

Visual Quiz - 3 Questions

Look at the execution_table, what is the output at Step 1?

A "Anna is initialized"

B "Anna is being deinitialized"

C No output

D Error

💡 Hint

Check the Output column at Step 1 in the execution_table.

At which step does the instance get deinitialized?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the Action and Output columns in the execution_table for when deinit message appears.

If we never set person to nil, what happens to the deinitializer?

A It runs immediately after initialization

B It never runs because the instance is still referenced

C It runs twice

D It runs before initialization

💡 Hint

Refer to variable_tracker and execution_table steps showing when references are removed.

Concept Snapshot

Deinitializers run automatically when an instance is about to be destroyed.
Use deinit to clean up resources like files or network connections.
Deinit cannot be called manually.
Instance must have no references for deinit to run.
Setting variables to nil removes references and triggers deinit.

Full Transcript

This visual trace shows how Swift calls a deinitializer to clean up an instance. First, a Person instance named 'Anna' is created and initialized, printing a message. The variable 'person' holds this instance. When 'person' is set to nil, the instance has no references left. At this moment, Swift automatically calls the deinitializer, printing a cleanup message. Finally, the instance is removed from memory. This process ensures resources are freed properly without manual intervention.