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A mutating method in Swift is an instance method defined within a value type (struct or enum) that explicitly permits the modification of its implicit self instance and its associated properties. By default, value types are immutable from within their own instance methods because Swift passes self as a constant (let). Applying the mutating keyword alters the method’s signature, instructing the compiler to treat self as an inout parameter.

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Underlying Mechanism

When a mutating method is invoked, Swift does not mutate the existing memory in place in the same way a reference type does. Instead, the following sequence occurs:
  1. The existing self is passed into the method as an inout variable.
  2. The method performs mutations on this local, mutable copy of self.
  3. Upon the method’s return, the modified copy completely overwrites the original instance in memory.

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Syntax and Implementation

struct StateContainer {
    var count: Int

    // Without 'mutating', the compiler throws: 
    // "Cannot assign to property: 'self' is immutable"
    mutating func increment() {
        self.count += 1
    }

    // Mutating methods can also reassign the entire 'self' instance
    mutating func reset() {
        self = StateContainer(count: 0)
    }
}

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Architectural Rules and Constraints

  • Value Types Exclusive: The mutating keyword is strictly reserved for value types (struct, enum, and protocols adopted by value types). Classes (class) are reference types; their properties can be modified without special keywords because self is a reference to a heap-allocated object, not a copied value.
  • Constant Instance Restriction: A mutating method cannot be invoked on an instance assigned to a constant (let). Because the method requires write access to overwrite the original instance, the compiler enforces mutability at the call site.
var mutableContainer = StateContainer(count: 0)
mutableContainer.increment() // Allowed

let immutableContainer = StateContainer(count: 0)
immutableContainer.increment() // Compiler Error: Cannot use mutating member on immutable value: 'immutableContainer' is a 'let' constant

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Protocol Conformance

When defining a protocol that requires a method to modify the conforming instance, the method must be declared as mutating in the protocol definition. 
protocol Resettable {
    mutating func reset()
}
If a class conforms to this protocol, the mutating keyword is omitted in the class’s implementation, as reference types do not require explicit mutation authorization. If a struct or enum conforms, the mutating keyword must be retained.
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