The force-unwrap operator ! returns the value inside an optional if it contains a value, or panics and aborts the execution if the optional has no value, i.e., the optional value is nil.
// Declare a constant which has an optional integer type
//
let a: Int? = nil
// Declare a constant with a non-optional integer type,
// which is initialized to `a` if `a` is non-nil.
// If `a` is nil, the program aborts.
//
let b: Int = a!
// The program aborts because `a` is nil.
// Declare another optional integer constant
let c: Int? = 3
// Declare a non-optional integer
// which is initialized to `c` if `a` is non-nil.
// If `c` is nil, the program aborts.
let d: Int = c!
// `d` is initialized to 3 because c isn't nil.
The force-unwrap operator can only be applied to values which have an optional type.
// Declare a constant with a non-optional integer type.
//
let a = 1
// Invalid: force-unwrap operator is applied to a value which has a
// non-optional type (`a` has the non-optional type `Int`).
//
let b = a!
// Invalid: The force-unwrap operator is applied
// to a value which has a non-optional type
// (the integer literal is of type `Int`).
//
let c = 1!
