swift 4 : "cannot use mutating member on immutable value: ' self' is immutable"in mutating function

Question

这段代码

extension Collection {
  mutating func f() {
    removeFirst()
  }
}

处理错误

cannot use mutating member on immutable value: 'self' is immutable

我认为 mutating 关键字应该使“self”可变。为什么？

Answer 1

RangeReplaceableCollection是集合所遵循的协议(protocol)如果您可以添加或删除元素，则符合:

extension RangeReplaceableCollection {
    mutating func f() {
        self.removeFirst()
    }
}