Reference sharing and side effects in Swift - Step-by-Step Execution

Concept Flow - Reference sharing and side effects

Create Object A

↓

Assign Reference B = A

↓

Modify Object via B

↓

Check Object A's state

↓

Observe Side Effects

↓

End

This flow shows how two variables can point to the same object, so changing it through one affects the other.

Execution Sample

Swift

class Box {
    var value: Int
    init(value: Int) { self.value = value }
}

let boxA = Box(value: 10)
let boxB = boxA
boxB.value = 20
print(boxA.value)

This code creates a Box object, assigns it to two variables, changes the value via one, and prints the value via the other.

Execution Table

Step	Action	boxA.value	boxB.value	Explanation
1	Create boxA with value 10	10	-	boxA points to Box(value:10)
2	Assign boxB = boxA	10	10	boxB points to same Box as boxA
3	Change boxB.value = 20	20	20	Both see updated value because same object
4	Print boxA.value	20	20	Output is 20, showing side effect
5	End	20	20	Execution stops

💡 Execution stops after printing boxA.value which is 20 due to shared reference

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
boxA.value	-	10	10	20	20
boxB.value	-	-	10	20	20

Key Moments - 2 Insights

Why does changing boxB.value also change boxA.value?

Because boxB and boxA both point to the same Box object, changing the value through one reference affects the shared object, as shown in execution_table step 3.

Is boxB a copy of boxA or a reference to the same object?

boxB is a reference to the same object as boxA, not a copy. This is why changes via boxB affect boxA's value (see execution_table step 2).

Visual Quiz - 3 Questions

Look at the execution table, what is boxA.value after step 3?

A 20

B 10

C 0

D Undefined

💡 Hint

Check the 'boxA.value' column at step 3 in the execution_table.

At which step do boxA and boxB start pointing to the same object?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look at the 'Action' column in execution_table where boxB is assigned.

If we changed boxB.value to 30 instead of 20 at step 3, what would boxA.value be after step 3?

A 20

B 30

C 10

D Undefined

💡 Hint

Since boxA and boxB share the same object, changing boxB.value changes boxA.value too (see variable_tracker).

Concept Snapshot

Reference sharing means two variables point to the same object.
Changing the object via one variable affects the other.
In Swift, classes are reference types.
Use this to understand side effects.
Example: let b = a; b.value = x changes a.value too.

Full Transcript

This example shows how in Swift, when you assign one class instance to another variable, both variables refer to the same object in memory. Changing the object's property through one variable changes it for the other as well. The execution table traces creating a Box object with value 10, assigning it to boxB, changing boxB's value to 20, and printing boxA's value which is also 20. This demonstrates reference sharing and side effects clearly.