为什么Swift会在这里给我EXC_BAD_INSTRUCTION？

Question

When executed in the playground, the following piece of code causes a EXC_BAD_INSTRUCTION :

func greet(person: [String:String]?, age: Int?) {
    guard let name = person!["name"] where person != nil else {
        return
    }
}

greet(nil, age: nil)

Can anyone tell me what's going on?

Answer 1

The exclamation mark after person unwraps the optional which causes the crash because person is nil .

When you use optional binding you can omit the explicit checking for nil .

func greet(person: [String:String]?, age: Int?) {
  guard let name = person?["name"] else {
    return
  }
}