[英]Generic Function without Input Parameter in Swift?
I have a generic Swift function like this: 我有一个通用的Swift函数,如下所示:
func toNSArray<T>() -> [T] {
...
}
The compiler gives no error but I do not know how to call this function. 编译器没有错误,但我不知道如何调用此函数。 I tried:
我试过了:
jList.toNSArray<String>()
jList.<String>toNSArray()
but it did not work. 但它不起作用。
How do I call a Generic function in Swift without input parameters? 如何在没有输入参数的情况下在Swift中调用Generic函数?
You need to tell Swift what the return type needs to be through some calling context: 你需要通过一些调用上下文告诉Swift返回类型是什么:
// either
let a: [Int] = jList.toNSArray()
// or, if you aren’t assigning to a variable
someCall( jList.toNSArray() as [Int] )
Note, in the latter case, this would only be necessary if someCall
took a vague type like Any
as its argument. 注意,在后一种情况下,只有当
someCall
采用类似于Any
的模糊类型作为其参数时,才需someCall
。 If instead, someCall
is specified to take an [Int]
as an argument, the function itself provides the context and you can just write someCall( jList.toNSArray() )
相反,
someCall
被指定为[Int]
作为参数,函数本身提供上下文,你可以只写someCall( jList.toNSArray() )
In fact sometimes the context can be very tenuously inferred! 事实上,有时可以非常推断出背景! This works, for example:
这有效,例如:
extension Array {
func asT<T>() -> [T] {
var results: [T] = []
for x in self {
if let y = x as? T {
results.append(y)
}
}
return results
}
}
let a: [Any] = [1,2,3, "heffalump"]
// here, it’s the 0, defaulting to Int, that tells asT what T is...
a.asT().reduce(0, combine: +)
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