Make instance of struct with a generic when the generic has a default value

问题

I wanted to store ListStyle in a property inside SupportOpts, so I added the <S> and where S: ListStyle to the struct declaration (from this answer).

struct SupportOpts<S> where S: ListStyle {
    var title: String = "Placeholder"
    var listStyle: S = InsetGroupedListStyle() as! S /// had to put an `as! S` or else it couldn't compile
}

I can make an instance of it like this:

let options: SupportOpts = SupportOpts(
    title: "Title",
    listStyle: InsetGroupedListStyle()
)

But, when I remove InsetGroupedListStyle() (I already set a default value inside SupportOpts):

let options: SupportOpts = SupportOpts(
    title: "Title"
)

I get:

Generic parameter 'S' could not be inferred, Explicitly specify the generic arguments to fix this issue [Fix]

So I pressed the Fix button, and it's now expecting a S: ListStyle:

What should I put here?

let options: SupportOpts = SupportOpts<ListStyle>(title: "Title")
/// Error: Value of protocol type 'ListStyle' cannot conform to 'ListStyle'; only struct/enum/class types can conform to protocols

let options: SupportOpts = SupportOpts<S>(title: "Title")
/// Error: Cannot find type 'S' in scope (well that's kind of obvious...)

let options: SupportOpts = SupportOpts<InsetGroupedListStyle>(title: "Title")
/// this works...

Do I have to specify SupportOpts<InsetGroupedListStyle>(title: "Title"), even if I already set a default inside SupportOpts?

回答1:

You can tell that something has gone wrong when you needed to add as!. That isn't the right approach. Consider the case if someone chose a different S:

let options = SupportOpts<InsetListStyle>(title: "Title")

This would have to crash.

Instead, you want an extension to make things more convenient.

struct SupportOpts<S> where S: ListStyle {
    var title: String = "Placeholder"
    var listStyle: S     // No default value here
}

Your goal is to have an init that just takes title, so you add that:

extension SupportOpts where S == InsetGroupedListStyle {
    init(title: String) {
        self.init(title: title, listStyle: InsetGroupedListStyle())
    }
}

And type inference will work out the rest:

let options = SupportOpts(title: "Title")

来源：https://stackoverflow.com/questions/64529932/make-instance-of-struct-with-a-generic-when-the-generic-has-a-default-value

标签

ios

swift

generics