Capture lists in closures in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of closure capturing a variable

What is the output of this Swift code snippet?

Swift

var x = 10
let closure = { [x] in
    print(x)
}
x = 20
closure()

A 10

B 20

C Compilation error

D Runtime error

💡 Hint

Think about when the value of x is captured by the closure.

The closure captures the value of x at the time of its creation because of the capture list [x]. Later changes to x do not affect the captured value.

❓ Predict Output

intermediate

Effect of capture list on reference type

What will this Swift code print?

Swift

class Counter {
    var count = 0
}
let counter = Counter()
let closure = { [counter] in
    print(counter.count)
}
counter.count = 5
closure()

A 5

B Runtime error

C Compilation error

D 0

💡 Hint

Consider how reference types behave when captured in closures.

The capture list captures the reference to the Counter instance, not a copy. So changes to counter.count are visible inside the closure.

🔧 Debug

advanced

Why does this closure cause a retain cycle?

Examine the code below. Why does it cause a retain cycle?

Swift

class Person {
    var name: String
    lazy var greet: () -> Void = {
        print("Hello, \(self.name)!")
    }
    init(name: String) {
        self.name = name
    }
}
let p = Person(name: "Alice")
p.greet()

A The closure does not capture self, so no retain cycle occurs.

B The lazy property causes a syntax error.

C The closure captures name by value, causing a retain cycle.

D The closure captures self strongly, causing a retain cycle.

💡 Hint

Think about how closures capture self by default in classes.

The closure captures self strongly by default, and since self owns the closure, this creates a retain cycle preventing deallocation.

📝 Syntax

advanced

Correct capture list syntax

Which option shows the correct syntax for capturing a variable weakly in a closure?

Swift

var value = 10
let closure = { /* capture list here */ in
    print(value)
}

A { [weak var value] in print(value) }

B { [value weak] in print(value) }

C { [weak value] in print(value) }

D { [weak self] in print(value) }

💡 Hint

The capture list syntax uses square brackets with capture specifiers before the variable name.

The correct syntax is [weak value]. Option C is invalid syntax, C is invalid because 'var' is not used in capture lists, and D captures self, not value.

🚀 Application

expert

Predict the output with multiple captures and mutation

What is the output of this Swift code?

Swift

var a = 1
var b = 2
let closure = { [a, b] in
    print("a = \(a), b = \(b)")
}
a = 3
b = 4
closure()

A a = 3, b = 2

B a = 1, b = 2

C a = 1, b = 4

D a = 3, b = 4

💡 Hint

Captured values are copied at closure creation time.

The closure captures copies of a and b at the time of its creation, so later changes do not affect the captured values.