Why optional constant does not automatically have a default value of nil [duplicate]

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Not setting a read-only (constant) field with either an:

  • initialization expression
  • initializer

is almost certainly an indication of an error in your program.

Since you have no other opportunity to set the value of your let field, the value of the field is going to remain nil (or some other default). It is rather unlikely that a programmer would find such behavior desirable, and request it on purpose.

That is why Swift marks this situation as an error. On the other hand, if you actually wanted your String constant to remain nil, you could add an expression to set it to nil, and silence the error:

let owner: String? = nil // Pretty useless, but allowed

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