Should Town be a Struct or a Class?

Swift Programming: The Big Nerd Ranch Guide (2nd E 15. Structs and Classes
1

I know that the book tells us to make Town a struct, but this appears to be causing problems. I use the code in the book to define Town, Monster, and Zombie (i.e., the code before the challenges). I then use the following in main.swift:

var myTown = Town()
myTown.changePopulation(by: 500)

let genericMonster = Monster()
genericMonster.town = myTown
genericMonster.terrorizeTown()
print("After generic terrorizes")
genericMonster.town?.printDescription() // population of generic monster's town
myTown.printDescription()               // population of myTown

let fredTheZombie = Zombie()
fredTheZombie.town = myTown
fredTheZombie.name = "Fred the Zombie"
fredTheZombie.terrorizeTown()
print("After Fred terrorizes")
fredTheZombie.town?.printDescription()  // population of Fred's town
myTown.printDescription()               // population of myTown

This gives the output:

population changing by 500
Monster is terrorizing a town!
After generic terrorizes
Population: 5922, number of stoplights: 4
Population: 5922, number of stoplights: 4
population changing by -10
Fred the Zombie is terrorizing a town!
After Fred terrorizes
Population: 5912, number of stoplights: 4
Population: 5922, number of stoplights: 4
Program ended with exit code: 0

Things are OK for the generic monster, but they are wrong after Fred the Zombie terrorize. Notice that Fred’s town has a population of 5912, whereas myTown has a population of 5922. From this, it appears that Fred has been given a copy of myTown, not (a pointer to) myTown.

I can fix this behavior by defining myTown to be a class, not a struct as suggested by someone in the Bronze & Silver Challenge. This issue seems to be affecting a number of people in the Bronze & Silver Challenge thread, but it appears to me to be a problem with the code in the actual chapter.

Is my interpretation correct or is something else going on?

2

Replying to my own thread to help others.
This question is answered in Chapter 18. Put simply, structs are passed by value, whereas classes are passed by reference. I don’t see how it makes any sense to have a Town be a struct, as multiple monsters could affect the population of the town. In the case described above, all of the monsters need to act on myTown, not copies of myTown.

3

Melissa, you raise some excellent points and questions here.

The short version is that you are 100% correct. While this was not meant to be any sort of “gotcha”, we make Town a struct specifically so that we can raise this issue in the challenges and chapters later on.

If you were to write this as an actual app today, you should absolutely make Town a class, so that multiple monsters can terrorize the same one.