iOS 13 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 a value into the shoebox, causing the variable name to refer to that value, you assign the value to the variable. The variable name becomes a reference to that value.

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

A variable not only gives its referent a name; it also, by virtue of where it is declared, endows its referent with a particular scope (visibility) and lifetime. (See “Scope and Lifetime”.) Assigning a value to a variable is a way of ensuring that this value can be seen by code that needs to see it and that it persists long enough to serve its purpose. There are three distinct 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 to all code within the same file, because it is at top level; any other code in the same file is therefore at the same level or at a lower ...