Mutating methods for value types in Swift - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the mutating keyword do in Swift?

It allows a method in a value type (like a struct or enum) to modify (change) the instance's properties.

beginner

Why do value types need mutating methods to change their properties?

Because value types are copied when assigned or passed, Swift requires explicit permission (mutating) to change their own data inside methods.

beginner

Can classes have mutating methods in Swift?

No. Classes are reference types and can always modify their properties inside methods without mutating.

beginner

Example: What will this code print?

struct Counter {
  var count = 0
  mutating func increment() {
    count += 1
  }
}
var c = Counter()
c.increment()
print(c.count)

It will print 1 because the increment() method changes the count property using mutating.

beginner

What happens if you remove mutating from a method that changes a property in a struct?

The Swift compiler will give an error because the method tries to change a property without permission to mutate the value type.

What keyword must you add to a method in a struct to modify its properties?

A override

B static

C final

D mutating

The mutating keyword allows methods in value types like structs to modify their own properties.

Which type requires mutating methods to change properties?

A Struct

B Protocol

C Class

D Enum without cases

Structs are value types and need mutating methods to change their properties.

What happens if you call a mutating method on a constant struct instance?

A The struct becomes mutable

B It works normally

C Compiler error: cannot use mutating method on let constant

D The method is ignored

You cannot call mutating methods on a struct instance declared with let because it is immutable.

Can enums have mutating methods in Swift?

A Only in classes

B Yes, to change their cases

C Only if they have raw values

D No, enums are immutable

Enums can have mutating methods to change their current case value.

Why don't classes need the mutating keyword?

A Because classes are reference types and can always modify their properties

B Because classes cannot change properties

C Because classes are immutable

D Because classes use <code>final</code> instead

Classes are reference types, so their methods can modify properties without mutating.

Explain why the mutating keyword is necessary for methods in Swift structs.

Think about how value types behave differently from classes.

Describe what happens if you try to modify a struct property inside a method without using mutating.

Consider Swift's safety rules for value types.