iOS 10 Programming Fundamentals with Swift

Chapter 3. Variables and Simple Types

A variable is a named “shoebox” whose contained value must be of a single well-defined type. Every variable must be explicitly and formally declared. To put an object into the shoebox, thus causing the variable name to refer to that object, you assign the object to the variable. (As we know from Chapter 2, a function, too, has a type, and can be assigned to a variable.)

This chapter goes into detail about declaration and initialization of variables. It then discusses all the primary built-in Swift simple types. (I mean “simple” as opposed to collections; the primary built-in collection types are discussed at the end of Chapter 4.)

Variable Scope and Lifetime

Aside from the convenience of giving a reference a name, a variable, by virtue of where it is declared, endows its referent with a particular scope (visibility) and lifetime; assigning something to a variable is a way of ensuring that it can be seen by code that needs to see it and that it persists long enough to serve its purpose.

In the structure of a Swift file (see Example 1-1), a variable can be declared just about anywhere. It will be useful to distinguish several levels of variable scope and lifetime:

Global variables

A global variable, or simply a global, is a variable declared at the top level of a Swift file. A global variable lives as long as the file lives, which is as long as the program runs. A global variable is visible everywhere (that’s what “global” means). It is visible ...