Hands-On Design Patterns with Swift

Mutation and value types

Some protocols may have methods that mutate the object members or properties. Consider the following protocol:

protocol Incrementing {    func increment() -> Int}

The Incrementing protocol requires the implementation of a single method, increment() -> Int, which returns the current value after the increment. This protocol may be used whenever we need to count linearly. It also suggests that it increments a local counter:

struct Counter: Incrementing {    private var value: Int = 0    func increment() -> Int {        value += 1        return value    }}

This implementation will not compile and will emit the following error:

Left side of mutating operator isn't mutable: 'self' is immutable

This error is emitted because the increment() function ...