Chapter 5. Flow Control and More

This chapter is a miscellany. I’ll start by describing Swift’s flow control constructs for branching, looping, and jumping. Then I’ll summarize Swift’s privacy and introspection features, and talk about how to override operators and how to create your own operators. Next I’ll explain some specialized aspects of Swift memory management. Finally I’ll survey some recently added Swift language features: synthesized protocol implementations, key paths, dynamic members, property wrappers, custom string interpolation, reverse generics, function builders, and Result.

Flow Control

A computer program has a path of execution through its code statements. Normally, this path follows a simple rule: execute each statement in succession. But there is another possibility. Flow control can be used to make the path of execution skip some statements, or go back and repeat some statements.

Flow control is what makes a computer program “intelligent.” By testing in real time the truth value of a condition — an expression that evaluates to a Bool and is thus true or false — the program decides at that moment how to proceed. Flow control based on testing a condition may be divided into two general types:


The code is divided into alternative chunks, like roads that diverge in a wood, and the program is presented with a choice of possible ways to go; the truth of a condition is used to determine which chunk will actually be executed.


A chunk of code ...

Get iOS 13 Programming Fundamentals with Swift now with O’Reilly online learning.

O’Reilly members experience live online training, plus books, videos, and digital content from 200+ publishers.