Swift: Creating array of objects with generic type parameter

Question

I have a type called Setting that takes a generic type parameter as such:

Setting<T>

Every setting contains a value that can be an Int32, String, Bool, or a custom object type, etc. Here is some of the full implementation of Setting:

class Setting<T> {
    var key:String?
    var defaultValue:T?
    //...
}

This all works with various type params as expected, however, now there is a requirement for a collection that contains multiple Setting objects that could have various type parameters. When I declare an array variable of type [Setting], obviously the compiler expects a type which is unknown at runtime.

I've tried using a protocol and an extension on the types that could be used for the generic type parameter such as this:

protocol SettingProtocol {
    func getType() -> Self.Type
}

extension Int32:SettingProtocol {
    func getType() -> Int32.Type {
        return Int32.self
    }
}

extension String:SettingProtocol {
    func getType() -> String.Type {
        return String.self
    }
}
//...

and declaring my array as

var settings = [Setting<SettingProtocol>]()

but this does not work when I try to append a Setting instance to the array as follows:

var newSetting = Setting<String>()
newSetting.setDefaultValue(value: "SomeString")
settings?.append(newSetting) // compile error here

and results in the following compiler error:

Cannot convert value of type 'Setting<String>' to expected argument type 'Setting<SettingProtocol>'

Also, using the protocol/extension route might require an extension on every type that might be encountered when building these objects which seems really clunky.

I feel like there should be a way to accomplish this. Also hoping that when I pull these items out of the array that I can avoid a lot of type checking.

Can anyone offer any advice?

Anand · Accepted Answer · 2019-04-16 15:49:59Z

1

Change

class Setting<T>

to

class Setting<T:SettingProtocol>

and try compiling.

answered Apr 16, 2019 at 15:49

Anand

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3 Comments

Ahmad F Over a year ago

Won't resolve it. "Using 'SettingProtocol' as a concrete type conforming to protocol 'SettingProtocol' is not supported".

Pheepster Over a year ago

@AhmadF You're correct. I had to make my Setting class conform to a protocol, and now it works

Ahmad F Over a year ago

@Pheepster What?! How is a valid answer? Would you mention what have you done to make it work?!

Ahmad F · Accepted Answer · 2019-04-16 16:44:01Z

Actually, you can't define:

var settings = [Setting<SettingProtocol>]()

because the generic type of Setting must be one of the concrete types but not the protocol itself. For example, you could declare it as:

var settings = [Setting<String>]() // since you already implemented extension String:SettingProtocol { ...

Therefore you could append objects of type Setting<String>, however that's not what are you looking for, you need settings to be a heterogeneous container.

So what you could do is:

class Setting {
    var key:String?
    var defaultValue:SettingProtocol?
}

protocol SettingProtocol { }
extension Int32:SettingProtocol {}
extension String: SettingProtocol {}

At this point, you declared defaultValue to be of type SettingProtocol, without the need of dealing with a generic.

Therefore:

var newStringSetting = Setting()
newStringSetting.defaultValue = "My String"
settings.append(newStringSetting)

var newInt32Setting = Setting()
newInt32Setting.defaultValue = Int32(100)
settings.append(newInt32Setting)

for setting in settings {
    print(setting.defaultValue)
    // Optional("My String")
    // Optional(100)
}

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