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A key path is a typed reference to a property — like a pointer to a setter/getter, but compile-time-checked.

struct Person {
    var name: String
    var age: Int
}

let keyPath = \Person.name           // KeyPath<Person, String>

let ada = Person(name: "Ada", age: 36)
ada[keyPath: keyPath]                  // "Ada"

The \Type.property syntax creates a key path. Apply it to an instance with instance[keyPath: kp].

Higher-order use:

let people = [
    Person(name: "Ada", age: 36),
    Person(name: "Bob", age: 25),
]

// Sort by age:
let sorted = people.sorted { $0.age < $1.age }
// Or with key path:
let sortedByAge = people.sorted(using: KeyPathComparator(\.age))

// Map to a property:
let names = people.map(\.name)        // ["Ada", "Bob"]

map(\.name) is shorthand for map { $0.name }. Compile-time-checked, slightly more concise.

WritableKeyPath — for mutable properties:

let writeKp: WritableKeyPath<Person, String> = \Person.name
var p = Person(name: "Ada", age: 36)
p[keyPath: writeKp] = "Linus"

ReferenceWritableKeyPath — for class properties (since classes have reference semantics).

Use cases:

• Generic functions over arbitrary properties
• Reactive frameworks (Combine, SwiftUI) heavily use key paths to wire bindings
• Dependency injection containers
• Diff calculation in CoreData / SwiftData

Key paths are how \.property syntax works in @Binding(get:set:), sort/filter operations, and Combine's assign(to:on:).