What is the difference between a stored property and a computed property?

Answer: A stored property keeps a value in the instance's memory, while a computed property stores nothing and calculates its value every time it is read.

• Stored — let or var, part of the instance's layout. Available on structs and classes (not enums).
• Computed — always var, defined by a get (and optional set). Available on structs, classes, and enums, and can be added in extensions.

Code Example:

struct Rectangle {
    // Stored — takes up memory in every Rectangle
    var width: Double
    var height: Double

    // Computed — derived on every access, nothing stored
    var area: Double {
        width * height
    }
}

var rect = Rectangle(width: 4, height: 5)
print(rect.area) // 20.0
rect.width = 10
print(rect.area) // 50.0 — always in sync with its inputs

Key Points:

• Extensions can add computed properties but cannot add stored properties.
• A computed property can never fall out of sync with the values it derives from — that is the main reason to prefer it over caching a derived value in a stored property.
• Callers expect property access to be cheap. If the work is expensive, use a method (func calculateArea()) or cache the result in a lazy / stored property instead.