我可以指定泛型是一个值类型吗？ [英] Can I specify that a generic is a value type?

问题描述

我知道我们基本可以用 AnyObject 指定我们的泛型是任何引用类型：

I know that we can essentially specify that our generics be any reference type by using AnyObject:

class Foo<T: AnyObject> {
    // ...
}

但有没有办法指定我们的泛型只应该是 value 类型，并且不允许使用引用类型？

But is there a way to specify that our generics should only be value types, and not allow reference types?

推荐答案

// some code for testing    
class C { } // just a simple class as an example for a reference type
var c = C()
var d: Double = 0.9 // a value type

---

解决方案1通过扩展

---

Solution 1 via extension

protocol ValueType { }
extension Double : ValueType { }
extension Int : ValueType { }
// ... all value types to be added

func printT1 <T: ValueType> (input: T) {
    println("\(input) is value")
}
printT1(d) // Does work
//printT1(c) // Does not work

但正如评论中提到的那样，它正在工作但不可行，因为用户定义值类型必须实现此协议。

But as mentioned in the comments, it is working but not feasible, because user defined value types have to implement this protocol.

func printT <T: AnyObject> (input: T) {
    println("\(input) is reference")
}

func printT <T: Any> (input: T) {
    println("\(input) is value")
}

---

解决方案3通过 assert

另一个解决方案可以通过 assert

func printT <T: Any> (input: T) {
    print("\(input) is " + ((T.self is AnyObject) ? "reference" : "value"))
}

---

Solution4 via where 子句

我认为这将是最好的解决方案。不幸的是，不可能有

---

"Solution" 4 via where clauses

This would be the best solution, I think. Unfortunately, it is not possible to have

func printT <T: Any where T: ~AnyObject > (input: T) {
    println("\(input) is value")
}

或类似。也许在未来的Swift版本中可能会这样。

or similar. Maybe it will be possible in future releases of Swift.

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