问题描述
我有一个泛型类型的类，泛型类型有一个子类型约束。然后我尝试将一个类的实例传递给一个方法，但最终出现以下编译时错误：
无法转换值类型'B< SomeType>'到期望的参数类型'B< NSObject>'
简单的例子：
class A：NSObject {
func testB（b：B< NSObject>） {
$ b}
}
class B< SomeType：NSObject> ;: NSObject {
func test（）{
让a = A（）
a.testB（b：self）//< -----上面提到的错误在这里显示
}
}
由于 SomeType 保证是 NSObject ，我预计编译器可以将 B > 转换为 B< NSObject> 并传递它。
类A只关心B的 SomeType 是 NSObject的而不是关于 SomeType 的实际类型。这就好像一个函数需要一个 UIView 参数，并且你传递了一个 UILabel 。
func testB（b： B< NSObject>）{
}
收件人：
func testB< SomeType：NSObject>（b：B< SomeType>）{
}
这将起作用，因为 SomeType 约束为 NSObject ，不等于 NSObject 。
所以你的函数的 SomeType 也应该约束为 NSObject ：
func testB< SomeType：NSObject>（b：B< SomeType>）{...}
I have a class with a generic type and the generic type has a subclass type constraint. I then try and pass an instance of the class to a method but end up with the following compile time error:
Cannot convert value of type 'B<SomeType>' to expected argument type 'B<NSObject>'
Below is a simple example:
class A: NSObject {
func testB(b: B<NSObject>) {
}
}
class B<SomeType: NSObject>: NSObject {
func test() {
let a = A()
a.testB(b: self) // <----- The error noted above is shown here
}
}
Because SomeType is guaranteed to be a subclass of NSObject, I would have expected the compiler would be ok with converting B<SomeType> to B<NSObject> and passing it along.
Class A only cares that B's SomeType is a subclass of NSObject and not about the actual type of SomeType. It's like if a function were to take a UIView parameter and you pass in a UILabel.
Change:
func testB(b: B<NSObject>) {
}
To:
func testB<SomeType: NSObject>(b: B<SomeType>) {
}
This will work, because SomeType is constrain to NSObject, not equal to NSObject.
So your function's SomeType should be constrain to NSObject too:
func testB<SomeType: NSObject>(b: B<SomeType>) { ... }
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