Swift Error Handling

Swift Error Handling

Throw and catch errors explicitly, or convert failures to optionals with try? when appropriate.

Throw, Try, Catch

Functions can throw errors with throw.

Callers must use try and handle failures with do/catch, or use try? to get an optional result.

Syntax:

• func f() throws -> T
• try f()
• try? f()
• do { ... } catch { ... }

Example

enum InputError: Error { case negative }

func validate(_ n: Int) throws -> String {
  if n < 0 { throw InputError.negative }
  return "ok: \(n)"
}

do {
  let result = try validate(-1)
  print(result)
} catch {
  print("error")
}

let maybe = try? validate(1)
print(maybe ?? "nil")

Tip: Use defer to run cleanup code before exiting the current scope, even when errors are thrown.

Defer (Cleanup)

Use defer to ensure cleanup code always runs when a scope exits, even if an error is thrown.

Example

enum FileError: Error { case fail }

func work(_ ok: Bool) throws {
  print("start")
  defer { print("cleanup") }
  if !ok { throw FileError.fail }
  print("done")
}

do { try work(false) } catch { print("error") }

This example prints "cleanup" even when an error is thrown, guaranteeing resource cleanup.