Optional array vs. empty array in Swift

Question

I have a simple Person class in Swift that looks about like this:

class Person {
    var name = "John Doe"
    var age = 18
    var children = [Person]?

    \ init function goes here, but does not initialize children array
}

Instead of declaring children to be an optional array, I could simply declare it and initialize it as an empty array like this:

var children = [Person]()

I am trying to decide which approach is better. Declaring the array as an optional array means that it will not take up any memory at all, whereas an empty array has at least some memory allocated for it, correct? So using the optional array means that there will be at least some memory saving. I guess my first question is: Is there really any actual memory saving involved here, or are my assumptions about this incorrect?

On the other hand, if it is optional then each time I try to use it I will have to check to see if it is nil or not before adding or removing objects from it. So there will be be some loss of efficiency there (but not much, I imagine).

I kind of like the optional approach. Not every Person will have children, so why not let children be nil until the Person decides to settle down and raise a family?

At any rate, I would like to know if there are any other specific advantages or disadvantages to one approach or the other. It is a design question that will come up over and over again.

question from:https://stackoverflow.com/questions/26811787/optional-array-vs-empty-array-in-swift

Answer 1

The ability to choose between an empty array or an optional gives us the ability to apply the one that better describe the data from a semantic point of view.

I would choose:

• An empty array if the list can be empty, but it's a transient status and in the end it should have at least one element. Being non optional makes clear that the array should not be empty
• An optional if it's possible for the list to be empty for the entire life cycle of the container entity. Being an optional makes clear that the array can be empty

Let me make some examples:

• Purchase order with master and details (one detail per product): a purchase order can have 0 details, but that's a transient status, because it wouldn't make sense having a purchase order with 0 products
• Person with children: a person can have no children for his entire life. It is not a transient status (although not permanent as well), but using an optional it's clear that it's legit for a person to have no children.

Note that my opinion is only about making the code more clear and self-explainatory - I don't think there is any significant difference in terms of performance, memory usage, etc. for choosing one option or the other.