Swift Protocol get only settable?

Question

why can I do this without any error:

var testDto = ModelDto(modelId: 1)
testDto.objectId = 2

while I define this:

protocol          


        

Answer 1

_{I'm answering the question in it's generic sense.}

Before addressing the question you must know what does get & set mean.

(If you'r coming from an Objective-C world:) get means readOnly, that is I'm allowed to know the number of legs an animal has. I'm not allowed to set it. get & set together means readWrite ie I'm allowed to know the weight of an animal while I'm also able to set/change the weight of an animal

With the following example.

protocol Animal {
    var weight : Int { get set }
    var limbs : Int { get }
}

---

If you only have getter, and attempt to hide setter (by using private (set)... then you WON'T get an error ... it's likely what you wanted and how it must be done!

Likely what you intended:

class Cat : Animal {
    private (set) var limbs: Int = 4 // This is what you intended, because you only have get requirements...and don't want any conforming type to be able to set it ie don't want others do catInstance.limbs = 22
    var weight: Int = 15

}

var smallCat = Cat()
smallCat.weight = 20 // Good!

// attempting to set it will create an error!!!
smallCat.limbs = 5 // Error: Cannot assign to property: 'limbs' setter is inaccessible

Likely what you didn't intend:

class Panda : Animal {
    var limbs: Int = 4 // This is OK, but it kinda defeats the purpose of it being a get only
    var weight: Int = 200   
}

var littlPanda = Panda()

littlPanda.weight = 40 // Good
littlPanda.limbs = 30 // NO Error!!! Likely unintended

Basically with {get} there is still some extra work to be done which the compiler doesn't tell you ... YOU must add private (set) to achieve the intended behavior

---

If your property has setter and you attempt to hide setter then you will actually see an error.

class Dog : Animal {
    private (set) var limbs: Int = 4
    private (set) var weight: Int = 50  // Error: Setter for property 'weight' must be declared internal because it matches a requirement in internal protocol 'Animal'
}

You're not allowed to hide, because you promised to provide a setter...