Sendable protocol for thread safety in Swift - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the Sendable protocol in Swift?

The Sendable protocol marks types as safe to be used across different threads without causing data races or undefined behavior.

intermediate

How does Swift check if a type conforms to Sendable?

Swift uses compile-time checks to ensure that all stored properties of a type are also Sendable, making the whole type safe for concurrent use.

intermediate

Can classes conform to Sendable automatically? Why or why not?

No, classes do not automatically conform because they have reference semantics and can cause data races. You must manually ensure thread safety and mark them as Sendable if safe.

beginner

What keyword do you use to declare a type as conforming to Sendable?

You use the Sendable protocol in the type declaration, for example: struct MyType: Sendable { ... }.

beginner

Why is Sendable important in Swift concurrency?

Sendable helps the compiler ensure that data passed between concurrent tasks is safe, preventing bugs caused by simultaneous access to mutable state.

What does the Sendable protocol guarantee in Swift?

A That a type is immutable

B That a type can be safely shared across threads

C That a type is a class

D That a type is only used on the main thread

Sendable ensures a type is safe to use across threads, not necessarily immutable or restricted to the main thread.

Which of these types automatically conforms to Sendable in Swift?

A Structs with only <code>Sendable</code> properties

B All classes

C Functions

D Enums with associated reference types

Structs with only Sendable properties automatically conform. Classes and enums with reference types do not.

If a class has mutable state, can it conform to Sendable safely without extra work?

A Only if it inherits from NSObject

B Yes, always

C No, you must ensure thread safety manually

D Only if it is final

Mutable state in classes requires manual thread safety measures before conforming to Sendable.

What happens if you try to pass a non-Sendable type between concurrent tasks?

A The program crashes immediately

B Nothing, it works fine

C The type is automatically copied

D The compiler will produce an error or warning

Swift's compiler checks and warns or errors if non-Sendable types are passed between concurrent contexts.

Which keyword is used to declare a type conforms to Sendable?

A Sendable

B threadsafe

C sendable

D concurrent

The protocol name Sendable is used exactly as is in the type declaration.

Explain what the Sendable protocol is and why it matters for thread safety in Swift.

Think about how Swift prevents data races when sharing data between threads.

Describe how you would make a custom struct conform to Sendable safely.

Consider what properties your struct has and their thread safety.

Practice

1. What does the Sendable protocol in Swift ensure?

easy

A. That a function runs asynchronously.

B. That a class can only be used on the main thread.

C. That a variable is immutable.

D. That data can be safely shared between threads without causing race conditions.

Solution

1. Step 1: Understand the purpose of Sendable

   The Sendable protocol marks types that can be safely passed between threads without causing data races.

2. Step 2: Compare options with the definition

   Only That data can be safely shared between threads without causing race conditions. correctly describes this safety guarantee. Other options describe unrelated concepts.

3. Final Answer:

   That data can be safely shared between threads without causing race conditions. -> Option D

4. Quick Check:

   Sendable = safe cross-thread data sharing [OK]

Hint: Sendable means safe to share data across threads [OK]

Common Mistakes:

• Confusing Sendable with immutability
• Thinking Sendable restricts to main thread only
• Assuming Sendable makes functions async

2. Which of the following is the correct way to declare a struct as Sendable in Swift?

easy

A. struct Data { var value: Int } implements Sendable

B. struct Data: Sendable { var value: Int }

C. struct Data Sendable { var value: Int }

D. struct Data { var value: Int } : Sendable

Solution

1. Step 1: Recall Swift protocol conformance syntax

   In Swift, a type conforms to a protocol by listing it after a colon following the type name.

2. Step 2: Match syntax to options

   struct Data: Sendable { var value: Int } uses correct syntax: struct Data: Sendable { ... }. Others are invalid Swift syntax.

3. Final Answer:

   struct Data: Sendable { var value: Int } -> Option B

4. Quick Check:

   Protocol conformance uses colon after type name [OK]

Hint: Use colon after struct name to conform to Sendable [OK]

Common Mistakes:

• Placing protocol after braces
• Using 'implements' keyword (not Swift)
• Omitting colon between type and protocol

3. What will happen when you try to compile this Swift code?

struct Counter: Sendable {
  var count: Int
}

let counter = Counter(count: 5)
Task {
  print(counter.count)
}

medium

A. It compiles and prints 5.

B. It causes a compile-time error because Counter is not Sendable.

C. It causes a runtime error due to thread safety violation.

D. It compiles but prints 0.

Solution

1. Step 1: Check if Counter conforms to Sendable

   The struct explicitly conforms to Sendable and contains only an Int, which is Sendable.

2. Step 2: Understand Task usage

   The Task runs asynchronously but can safely access counter because it is Sendable.

3. Final Answer:

   It compiles and prints 5. -> Option A

4. Quick Check:

   Sendable struct with Int prints value safely [OK]

Hint: Sendable structs with simple types compile and run safely [OK]

Common Mistakes:

• Assuming compile error without checking conformance
• Confusing runtime errors with compile errors
• Thinking default structs are not Sendable

4. Identify the error in this Swift code using Sendable:

class DataHolder: Sendable {
  var data: [Int]
  init(data: [Int]) {
    self.data = data
  }
}

medium

A. Missing explicit Sendable conformance on Array.

B. Class cannot conform to Sendable at all.

C. Array<Int> is not Sendable by default, causing a compile error.

D. No error; code compiles fine.

Solution

1. Step 1: Check Sendable conformance for class with mutable property

   Classes with mutable stored properties like arrays are not automatically Sendable.

2. Step 2: Understand why Array<Int> is not Sendable by default

   Array is a reference type and mutable, so compiler disallows automatic Sendable conformance without extra guarantees.

3. Final Answer:

   Array<Int> is not Sendable by default, causing a compile error. -> Option C

4. Quick Check:

   Mutable reference types block Sendable conformance [OK]

Hint: Mutable class properties like arrays block Sendable by default [OK]

Common Mistakes:

• Thinking classes cannot conform to Sendable at all
• Assuming arrays are always Sendable
• Ignoring mutability in Sendable checks

5. You want to make this struct safely shareable across threads. What is the best way to fix it?

struct UserData: Sendable {
  var name: String
  var preferences: [String: String]
}

hard

A. Make preferences an immutable dictionary and ensure keys and values are Sendable.

B. Remove Sendable conformance because dictionaries are never Sendable.

C. Change preferences to a class type and mark it as Sendable.

D. Add @Sendable attribute to the struct.

Solution

1. Step 1: Understand Sendable requirements for dictionary properties

   Dictionaries are Sendable only if their keys and values are Sendable and the dictionary is used immutably.

2. Step 2: Apply immutability and ensure key/value types conform

   Making preferences immutable (let) and using Sendable key/value types (String) satisfies Sendable.

3. Final Answer:

   Make preferences an immutable dictionary and ensure keys and values are Sendable. -> Option A

4. Quick Check:

   Immutable dictionary with Sendable types = Sendable struct [OK]

Hint: Use immutable dictionary with Sendable keys/values for Sendable struct [OK]

Common Mistakes:

• Thinking dictionaries are never Sendable
• Confusing @Sendable attribute with Sendable protocol
• Using mutable properties in Sendable structs