Swift string vs. string! vs. string?

Question

I have read this question and some other questions. But they are somewhat unrelated to my question

For UILabel if you don\'t specify

Answer 1

Thanks to Andrew Madsen's answer and all other answers, I learnt a bit myself:

struct pairOfOptionalAndNonOptionalAndImplicitUnwrapped{
    var word1 :String?
    var word2 :String
    var word3: String!

    init (a:String, b: String, c: String){
        self.word1 = a // Line A
        self.word2 = b // Line B
        self.word3 = c // Line C
    } //Line BBBB


    func equal() ->Bool{
        return word1 == word2 + word3
    }
}

let wordParts = pairOfOptionalAndNonOptionalAndImplicitUnwrapped (a:"partTwo", b: "part", c:"Two")
wordParts.equal() // Line CCCC

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• If I comment out Line A only, I will get no errors, because it is optional, and optionals can be set to nil.
• If I comment out Line B only, I will get a compilation error on Line BBBB, saying: Return from initializer without initializing all stored properties, I get this because I have declared property word2 as non-optional. I have told the compiler I have guaranteed to have you set...
• If I comment out Line C only, I will not get an error, unless I actually run a method on my object. The error you could get is: Execution was interrupted, reason: EXC_BAD_INSTRUCTION (code=EXC_1386_INVOP, subcode=0x0) if and only if I run wordParts.equal(), because I told my code that it's an optional meaning that it will be set from elsewhere after instantiation/compilation. Meaning that lldb can through me a runtime error if you it didn't get set. ( the error would happen on line CCCC)