little confuse about optionals in swift

Question

Maybe I'm very confused idk yet. I thought i understood the basic concept but when playing around in xcode I'm not getting what I thought I should be Take the following code

var title: String? = "hello world"
println("\(title)")

I thought that this would give me an error because I thought that title should have to be unwrapped since I;m accessing it, but it seems to compile just fine. Can someone shed some light on this.

Answer 1

All an optional is is that if it is nil, the app won't crash. If you put the elimination mark, and did not assign a value, it would crash.

In an if statement, you can check if the variable is nil:

if (title == nil) {

//this works

}

Answer 2

var title: String? = "hello world"
println("\(title)") // This would print out Optional(Title)
var title2: String! = "hello world"
println("\(title2)") // This would print in Title

Optional do not cause your program to crash when they are executed without a value but !(variable) will cause your code to crash if they have no value.

Swift documentation:

As described above, optionals indicate that a constant or variable is allowed to have “no value”. Optionals can be checked with an if statement to see if a value exists, and can be conditionally unwrapped with optional binding to access the optional’s value if it does exist.

Answer 3

Optionals are Printables, meaning you can print them to an output stream (here the console). You should see something like Optional("hello world") in your console and not hello world

Answer 4

As per documentation, your statement will be converted to

var title: Optional<String> = Some("hello world").

From Apple documentation, Optional is

enum Optional<A> {
case Nil
case Some(A)
}

So when you are passing a value to an optional, at the time of declaration, compiler understands, it has got a value and that's why it is not showing error.

But if you do the below code you may get different result.

var title: String? = "hello world"
title = nil
println("\(title)")

It will print "nil" in playground.

Answer 5

Based on apple doc

The concept of optionals doesn’t exist in C or Objective-C. The nearest thing in Objective-C is the ability to return nil from a method that would otherwise return an object, with nil meaning “the absence of a valid object.” However, this only works for objects—it doesn’t work for structures, basic C types, or enumeration values. For these types, Objective-C methods typically return a special value (such as NSNotFound) to indicate the absence of a value. This approach assumes that the method’s caller knows there is a special value to test against and remembers to check for it. Swift’s optionals let you indicate the absence of a value for any type at all, without the need for special constants.

Example

let possibleString: String? = "An optional string."
let forcedString: String = possibleString! // requires an exclamation mark

let assumedString: String! = "An implicitly unwrapped optional string."
let implicitString: String = assumedString // no need for an exclamation mark

Reference: https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/TheBasics.html