swift-guide/TypeCasting.md

Type Casting

In general, follow the official guidelines.

Convention - Static casting

Use keyword as for static casting. Only use static casting when automatic type casting is not available or when the compiler's inferred type does not match the intended type.

Rationale

Compiler can infer most types correctly so static casting is not necessary in most cases.

Examples

// Bad: unnecessary type casting
let padding = 42 as CGFloat

// Good: use type annotations instead
let padding: CGFloat = 42

Necessary type casting

// setTelemetryID is an Objective-C function that takes an NSNumber as the argument
// id is of type Int, the casting to NSNumber is not automatic
button.setTelemetryID(id as NSNumber)

let x = 10
switch x {
// Without the type cast, 1...Int.max would be interpreted as Range type
case 1...Int.max as ClosedInterval:
    ...
case Int.min..<0 as HalfOpenInterval:
    ...
case 0:
    ...
}