Why create “Implicitly Unwrapped Optionals”, since that implies you know there's a value?

Question

Why would you create a \"Implicitly Unwrapped Optional\" vs creating just a regular variable or constant? If you know that it can be successfully unwrapped then why create a

Answer 1

Implicitly unwrapped optionals are pragmatic compromise to make the work in hybrid environment that has to interoperate with existing Cocoa frameworks and their conventions more pleasant, while also allowing for stepwise migration into safer programing paradigm — without null pointers — enforced by the Swift compiler.

Swift book, in The Basics chapter, section Implicitly Unwrapped Optionals says:

Implicitly unwrapped optionals are useful when an optional’s value is confirmed to exist immediately after the optional is first defined and can definitely be assumed to exist at every point thereafter. The primary use of implicitly unwrapped optionals in Swift is during class initialization, as described in Unowned References and Implicitly Unwrapped Optional Properties.
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You can think of an implicitly unwrapped optional as giving permission for the optional to be unwrapped automatically whenever it is used. Rather than placing an exclamation mark after the optional’s name each time you use it, you place an exclamation mark after the optional’s type when you declare it.

This comes down to use cases where the non-nil-ness of properties is established via usage convention, and can not be enforced by compiler during the class initialization. For example, the UIViewController properties that are initialized from NIBs or Storyboards, where the initialization is split into separate phases, but after the viewDidLoad() you can assume that properties generally exist. Otherwise, in order to satisfy the compiler, you had to be using the forced unwrapping, optional binding or optional chaining only to obscure the main purpose of the code.

Above part from the Swift book refers also to the Automatic Reference Counting chapter:

However, there is a third scenario, in which both properties should always have a value, and neither property should ever be nil once initialization is complete. In this scenario, it is useful to combine an unowned property on one class with an implicitly unwrapped optional property on the other class.

This enables both properties to be accessed directly (without optional unwrapping) once initialization is complete, while still avoiding a reference cycle.

This comes down to the quirks of not being a garbage collected language, therefore the breaking of retain cycles is on you as a programmer and implicitly unwrapped optionals are a tool to hide this quirk.

That covers the “When to use implicitly unwrapped optionals in your code?” question. As an application developer, you’ll mostly encounter them in method signatures of libraries written in Objective-C, which doesn’t have the ability to express optional types.

From Using Swift with Cocoa and Objective-C, section Working with nil:

Because Objective-C does not make any guarantees that an object is non-nil, Swift makes all classes in argument types and return types optional in imported Objective-C APIs. Before you use an Objective-C object, you should check to ensure that it is not missing.

In some cases, you might be absolutely certain that an Objective-C method or property never returns a nil object reference. To make objects in this special scenario more convenient to work with, Swift imports object types as implicitly unwrapped optionals. Implicitly unwrapped optional types include all of the safety features of optional types. In addition, you can access the value directly without checking for nil or unwrapping it yourself. When you access the value in this kind of optional type without safely unwrapping it first, the implicitly unwrapped optional checks whether the value is missing. If the value is missing, a runtime error occurs. As a result, you should always check and unwrap an implicitly unwrapped optional yourself, unless you are sure that the value cannot be missing.

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