Convenience initializers in Swift - Step-by-Step Execution

Concept Flow - Convenience initializers

Call Convenience Init

↓

Perform Custom Setup

↓

Call Designated Init

↓

Complete Initialization

↓

Object Ready

A convenience initializer calls another initializer in the same class to reuse setup code, then completes object creation.

Execution Sample

Swift

class Car {
  var color: String
  var model: String

  init(model: String, color: String) {
    self.model = model
    self.color = color
  }

  convenience init(model: String) {
    self.init(model: model, color: "Black")
  }
}

let car = Car(model: "Sedan")

Creates a Car object using a convenience initializer that sets a default color.

Execution Table

Step	Action	Evaluation	Result
1	Call convenience init with model="Sedan"	convenience init(model: String)	Starts convenience initializer
2	Inside convenience init, call designated init with model="Sedan", color="Black"	self.init(model: model, color: "Black")	Calls designated initializer
3	Designated init sets self.model = "Sedan"	self.model = model	model property set to "Sedan"
4	Designated init sets self.color = "Black"	self.color = color	color property set to "Black"
5	Designated init completes	init complete	Object properties initialized
6	Convenience init completes	convenience init complete	Object ready for use

💡 Initialization ends after convenience initializer calls designated initializer and completes.

Variable Tracker

Variable	Start	After Step 3	After Step 4	Final
model	nil	"Sedan"	"Sedan"	"Sedan"
color	nil	nil	"Black"	"Black"

Key Moments - 2 Insights

Why does the convenience initializer call the designated initializer?

Because in Swift, convenience initializers must call a designated initializer to ensure all properties are properly initialized, as shown in step 2 of the execution_table.

Can a convenience initializer set properties directly without calling another initializer?

No, convenience initializers cannot set all properties directly; they must call a designated initializer first, which sets the properties, as seen in steps 3 and 4.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'color' after step 4?

A "Sedan"

B "Black"

C nil

D "Red"

💡 Hint

Check the variable_tracker row for 'color' after step 4.

At which step does the designated initializer complete its work?

A Step 2

B Step 4

C Step 5

D Step 6

💡 Hint

Look at the 'Action' column in the execution_table for when 'init complete' happens.

If the convenience initializer did not call the designated initializer, what would happen?

A The code would not compile because all properties must be initialized.

B The object would be fully initialized anyway.

C The convenience initializer would set default values automatically.

D The designated initializer would be called automatically.

💡 Hint

Recall Swift's rule that convenience initializers must call a designated initializer (see key_moments).

Concept Snapshot

Convenience initializers in Swift:
- Use 'convenience init' keyword
- Must call another initializer in the same class
- Help provide default or simpler ways to create objects
- Cannot fully initialize properties themselves
- Always call a designated initializer to complete setup

Full Transcript

This example shows how a convenience initializer in Swift works. When we create a Car object with only a model, the convenience initializer runs first. It calls the designated initializer with a default color. The designated initializer sets the model and color properties. Then initialization finishes, and the object is ready. Convenience initializers help avoid repeating code by reusing designated initializers. They must call a designated initializer to ensure all properties are set properly. This flow ensures safe and complete object creation.