Swift中Queue结构体popFirst报错但removeFirst正常，原因何在？

Great question! Let's break this down step by step.

First, let's clear up the core confusion: in modern Swift versions (4.2 and later), your dequeue() method using elements.popFirst() should not throw that error—both popFirst() and removeFirst() are mutating methods on Array, and since your dequeue() method is marked mutating, self (and its elements property) is mutable within the method.

That said, here are the most likely reasons you're seeing that error:

• You're using an outdated Swift version
  The popFirst() method for MutableCollection (which Array conforms to) was added in Swift 4.2. If you're working with Swift 4.1 or earlier, Array doesn't have this method at all. In this case, the compiler might be throwing a misleading error message (complaining about immutable self instead of telling you popFirst() doesn't exist). Updating your Swift version will fix this.

• Temporary compiler glitch
  Sometimes Swift's compiler can spit out incorrect error messages, especially if your project has cached build artifacts. Try cleaning your build folder (Cmd+Shift+K in Xcode) and recompiling, or restarting your compiler/Xcode—this often resolves false positive errors like this.

Also, a quick safety note about your dequeue2() method: it's risky! When elements is empty, removeFirst() will trigger a runtime crash (since it returns a non-optional T, not a T?). To fix this, add a check for emptiness first:

public mutating func dequeue2() -> T?{
    guard !elements.isEmpty else { return nil }
    return elements.removeFirst()
}

In contrast, popFirst() is inherently safe—it returns nil automatically when the array is empty, no extra checks needed.

内容的提问来源于stack exchange，提问作者mfaani