如何指定组合类型 CustomStringConvertible 和 RawRepresentable 的函数参数的类型？

Question

I want a generic function that can instantiate object of a few different enum types I have by supplying the enum type and the Int raw-value. These enum s are also CustomStringConvertible .

I tried this:

func myFunc(type: CustomStringConvertible.Type & RawRepresentable.Type, rawValue: Int)

which results in 3 errors:

• Non-protocol, non-class type 'CustomStringConvertible.Type' cannot be used within a protocol-constrained type
• Non-protocol, non-class type 'RawRepresentable.Type' cannot be used within a protocol-constrained type
• Protocol 'RawRepresentable' can only be used as a generic constraint because it has Self or associated type requirements

Forgetting about 'CustomStringConvertible` for now, I also tried:

private func myFunc<T: RawRepresentable>(rawValue: Int, skipList: [T]) {
    let thing = T.init(rawValue: rawValue)
}

But, although code completion suggests it, leads to an error about the T.init(rawValue:) :

• Cannot invoke 'init' with an argument list of type '(rawValue: Int)'

How can I form a working generic function like this?

Answer 1

The issue is that T.RawValue can be something else than Int with your current type constraints. You need to specify that T.RawValue == Int in order to pass your rawValue: Int input parameter to init(rawValue:) .

func myFunc<T: RawRepresentable & CustomStringConvertible>(rawValue: Int, skipList: [T]) where T.RawValue == Int {
    let thing = T.init(rawValue: rawValue)
}