How does error handling work in Swift?

Swift uses throws functions and try for recoverable errors. Errors conform to the Error protocol, usually as enums.

enum NetworkError: Error {
    case badStatus(Int)
    case noData
}

func load() throws -> Data {
    guard status == 200 else { throw NetworkError.badStatus(status) }
    guard let data else { throw NetworkError.noData }
    return data
}

do {
    let data = try load()
} catch NetworkError.badStatus(let code) {
    print(code)
} catch {
    print(error)
}

try? converts a thrown error into an optional and discards the error; try! crashes on error and should be avoided. Functions that call throwing functions must be throws or handle errors with do/catch.

Use defer for cleanup that must run on both success and failure. Since Swift 5.5 you also get async throws. Prefer typed errors and exhaustive catch for recovery, and use Result when you need to store or pass the outcome.