Initializers and designated init in Swift - Step-by-Step Execution

Concept Flow - Initializers and designated init

Start: Create instance

↓

Call designated init

↓

Initialize all properties

↓

Call superclass init if subclass

↓

Instance ready to use

↓

End

The flow shows how a designated initializer sets up all properties and calls superclass initializers if needed, preparing the instance for use.

Execution Sample

Swift

class Person {
  var name: String
  init(name: String) {
    self.name = name
  }
}

let p = Person(name: "Anna")

This code creates a Person instance by calling its designated initializer to set the name property.

Execution Table

Step	Action	Property 'name' Value	Instance State
1	Call Person.init(name: "Anna")	uninitialized	Instance created, properties uninitialized
2	Assign self.name = "Anna"	Anna	Property 'name' initialized
3	Initializer completes	Anna	Instance ready to use
4	Return instance to caller	Anna	Instance assigned to variable p

💡 Initialization completes after all properties are set and initializer returns

Variable Tracker

Variable	Start	After Step 2	Final
self.name	uninitialized	Anna	Anna
p	nil	nil	Person instance with name 'Anna'

Key Moments - 2 Insights

Why must all properties be initialized before the initializer ends?

Because Swift requires all stored properties to have a value before the instance is used, as shown in step 3 of the execution_table where 'name' is set before completion.

What is a designated initializer?

It is the main initializer that fully initializes all properties and calls superclass initializers if needed, as shown in step 1 where Person.init is called.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'self.name' after step 1?

A empty string

B "Anna"

C uninitialized

D nil

💡 Hint

Check the 'Property 'name' Value' column at step 1 in execution_table

At which step does the instance become ready to use?

A Step 3

B Step 2

C Step 1

D Step 4

💡 Hint

Look at the 'Instance State' column in execution_table for when initialization completes

If the property 'name' was not assigned, what would happen?

A The initializer would complete successfully

B The compiler would give an error

C The instance would have a default name

D The instance would be nil

💡 Hint

Swift requires all properties to be initialized before initializer ends, see key_moments explanation

Concept Snapshot

class MyClass {
  var prop: Type
  init(prop: Type) {
    self.prop = prop  // Designated init sets all properties
  }
}
- Designated init fully initializes all stored properties
- Must call superclass init if subclass
- Instance ready after init completes

Full Transcript

This visual trace shows how Swift's designated initializer works. When creating an instance, the designated initializer is called first. It sets all stored properties, like 'name' in the example. Only after all properties are initialized does the initializer complete and the instance becomes ready to use. This ensures safe and predictable object creation. If any property is not set, Swift will give a compile-time error. The flow also includes calling superclass initializers if the class inherits from another. This step-by-step trace helps beginners see how initialization happens internally.