Swift Programming: The Big Nerd Ranch Guide, 3rd Edition

Type Properties

Up to now, you have been working with instance properties. When you create a new instance of a type, that instance gets its own properties that are distinct from other instances of that type. Instance properties are useful for storing and computing values on an instance of a type.

However, when you need to store a value that is common to all instances of a type, it is wasteful to store a copy of that value in memory for every instance. Instead, such properties can exist once, on the type itself, and are called type properties. For example, all instances of a Square type will have exactly four sides, so the number of sides for Square might be stored in a type property. If you create 1,000 instances of Square in your program, ...