The nil-coalescing operator ?? returns the value inside an optional if it contains a value, or returns an alternative value if the optional has no value, i.e., the optional value is nil.
If the left-hand side is non-nil, the right-hand side is not evaluated.
// 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 it is non-nil, or 42 otherwise.
//
let b: Int = a ?? 42
// `b` is 42, as `a` is nil
The nil-coalescing 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: nil-coalescing operator is applied to a value which has a non-optional type
// (a has the non-optional type `Int`).
//
let b = a ?? 2
// Invalid: nil-coalescing operator is applied to a value which has a non-optional type
// (the integer literal is of type `Int`).
//
let c = 1 ?? 2
The type of the right-hand side of the operator (the alternative value) must be a subtype of the type of left-hand side, i.e. the right-hand side of the operator must be the non-optional or optional type matching the type of the left-hand side.
// Declare a constant with an optional integer type.
//
let a: Int? = nil
let b: Int? = 1
let c = a ?? b
// `c` is `1` and has type `Int?`
// Invalid: nil-coalescing operator is applied to a value of type `Int?`,
// but the alternative has type `Bool`.
//
let d = a ?? false
