When should I use optionals and when should I use non-optionals with default values?

问题

I know the recommended way in Swift is to use:

class Address {
var firstLine : String?
var secondLine : String?
}

but sometimes I see other developers write their code this way:

class Address {
var firstLine : String = ""
var secondLine : String = ""
}

Is this the unrecommended way because whenever you have nil you will just crash and there's no outlet for your to recover. Is that right? Or there are some use cases where using non-optionals with default can be good. If so then where?

I saw this other question which is asking about efficiency rather than which better suits your needs. I'm looking for an answer where it says "This is a good place to use non-optionals and this is a good place to use optionals". Sometimes I see folks just dump optionals everywhere and it makes me think do we not ever need non-optionals? Sometimes I see people trying to avoid optionals as much as possible and just code in an Objective-C kind of style.

The above question's answer doesn't represent a valid case for where non-optionals are good. It's mute about that. As for choosing optionals: I'm guessing for models which get populated by network calls, optionals are the right choice, because you don't know whether it's nil or not.

回答1:

The choice depends on what you model.

If a property of the object that you model may be absent completely, e.g. a middle name, a name suffix, an alternative phone number, etc., it should be modeled with an optional. A nil optional tells you that the property is not there - i.e. a person does not have a middle name or an alternative phone number. You should also use optional when you must distinguish between an empty object and a missing object.

If a property of the object must be set, and has a meaningful default, use an non-optional with a default:

class AddressList {
    var addresses : [Address]
    var separator : String = ";"
    ...
}

If users of your class need to change the separator, they have a way to do that. However, if they do not care about the separator, they can continue using the default without mentioning it in their own code.

回答2:

Well you should use optionals if you think that the variable might not have a value. But if you're really sure that it's gonna have a value then you don't need to use them.

So only use non-optionals if you're sure that the variable will have a value else use optionals.

来源：https://stackoverflow.com/questions/44288488/when-should-i-use-optionals-and-when-should-i-use-non-optionals-with-default-val