weak
时,有时会从Xcode收到以下错误消息:
或者'weak' may only be applied to class and class-bound protocol types
我想知道为什么关键字'weak' must not be applied to non-class-bound 'SomeProtocol'; consider adding a protocol conformance that has a class bound
weak
只能应用于类和类绑定协议类型?这个要求背后的原因是什么?