What are they?
Optionals are variables that can have 2 states: No value and some value.
Here’s an example:
That ‘?’ is equivalent to saying:
Notice something about the above type declaration? It means that the variable is an optional of type
Int. So you have to ‘unwrap’ them to get their actual value.
How do you ‘unwrap’ an Optional value? By doing this:
The ‘!’ is functionally equivalent to casting the Optional
Implicitly Unwrapped Optional
They are variables that can never be nil once you use them. Their value by default if none is assigned is nil. So once you use them you don’t have to unwrap them with ‘!’.
If let, Guard and Defer
When working with optionals, your code can become error-prone. Conside this example from the documentation:
If you create a new Person instance, its residence property is default initialized to nil, by virtue of being optional. In the code below, john has a residence property value of nil:
If you try to access the
numberOfRooms property of this person’s residence, by placing an exclamation mark after residence to force the unwrapping of its value, you trigger a runtime error, because there is no residence value to unwrap:
Here’s where if let comes in.
Say you don’t want the above example to crash. What do you do? Well, you want to check ‘if’ the residence is not nil, if true then do something otherwise do something else that doesn’t crash because residence is nil.
The if-let statement lets you do this in a much more compact way:
Wanna make this even more compact? Use the nil coalescing operator ‘??’ and the code above becomes equivalent to this:
Notice that now we didn’t have to force unwrap john.residence.
A problem with the if-let statement is that you cannot use that the scope of
roomCount is limited to the if statement. What do you do if you want to use it outside the if statement? This is where the guard statement comes in.
The guard statement is used to exit out of a scope if a certain condition is met which means that the code inside the guard statement is only executed if a certain condition is not met. Here’s an example: