在Swift中将常量字符串转换为变量字符串

考虑一下这个假设的Swift函数：

func putFirst(_ string: String) {
    var str = string
    let c = str.popFirst()
    print(c)
}

基于前面的问题，例如，

str

是一个变量，因此是可变的。但是，

str.popFirst（）

抛出编译错误

Cannot use mutating member on immutable value: 'str' is immutable

这是我不知道的微妙之处吗？这是Swift 4的新行为吗？我该如何处理它呢？

它是；问题是在

集合

上只定义了一个

popFirst（）

方法：

然后你会说：

func putFirst(_ string: String) {
  var str = string
  let c = str.attemptRemoveFirst()
  print(c)
}

---

先让c=str；str.dropFirst（）

extension RangeReplaceableCollection {

  /// Removes and returns the first element of the collection.
  ///
  /// Calling this method may invalidate all saved indices of this
  /// collection. Do not rely on a previously stored index value after
  /// altering a collection with any operation that can change its length.
  ///
  /// - Returns: The first element of the collection if the collection is
  ///   not empty; otherwise, `nil`.
  ///
  /// - Complexity: O(n)
  public mutating func attemptRemoveFirst() -> Element? {
    return isEmpty ? nil : removeFirst()
  }
}

func putFirst(_ string: String) {
  var str = string
  let c = str.attemptRemoveFirst()
  print(c)
}