swift - 使用或不使用泛型的 'where' 子句有什么区别？

Question

这两种使用或不使用“where”子句声明泛型父类(super class)的方法有什么区别？

func foo<T: SomeClass>(object: T) -> Array<T>

func foo<T>(object: T) -> Array<T> where T: SomeClass

Answer 1

这在 Swift guide 中有明确说明:

The requirements in a generic where clause specify that a type parameter inherits from a class or conforms to a protocol or protocol composition. Although the generic where clause provides syntactic sugar for expressing simple constraints on type parameters (for instance, <T: Comparable> is equivalent to <T> where T: Comparable and so on), you can use it to provide more complex constraints on type parameters and their associated types. For instance, you can constrain the associated types of type parameters to conform to protocols. For example, <S: Sequence> where S.Iterator.Element: Equatable specifies that S conforms to the Sequence protocol and that the associated type S.Iterator.Element conforms to the Equatable protocol. This constraint ensures that each element of the sequence is equatable.

简单地说，where允许您指定有关泛型参数关联类型的约束，而在 <> 中你不能这样做。