Simplest way to throw an error exception with a custom message in Swift 2

Question

I want to do something in Swift 2 that I m used to doing in multiple other languages   throw a runtime exception with a custom message   For example  in Java    throw new RuntimeException  A custom message here     I understand that I can throw enum types that conform to the ErrorType protocol  but I don t want to have to define enums for every type of error I throw   Ideally  I d like to be able mimic the example above as closely as possible   I looked into creating a custom class that implements the ErrorType protocol  but I can t even figure out that what that protocol requires  see documentation    Ideas

User · Accepted Answer

The simplest approach is probably to define one custom enum with just one case that has a String attached to it:

enum MyError: ErrorType {
    case runtimeError(String)
}

Or, as of Swift 4:

enum MyError: Error {
    case runtimeError(String)
}

Example usage would be something like:

func someFunction() throws {
    throw MyError.runtimeError("some message")
}
do {
    try someFunction()
} catch MyError.runtimeError(let errorMessage) {
    print(errorMessage)
}

If you wish to use existing Error types, the most general one would be an NSError, and you could make a factory method to create and throw one with a custom message.

User · Answer

Check this cool version out  The idea is to implement both String and ErrorType protocols and use the error s rawValue   enum UserValidationError  String  Error     case noFirstNameProvided    Please insert your first name     case noLastNameProvided    Please insert your last name     case noAgeProvided    Please insert your age     case noEmailProvided    Please insert your email       Usage   do     try User define firstName                    lastName  lastName                    age  age                    email  email                    gender  gender                    location  location                    phone  phone    catch let error as User UserValidationError     print error rawValue    return

User · Answer

In case you don t need to catch the error and you want to immediately stop the application you can use a fatalError      fatalError   Custom message here

User · Answer

The simplest way is to make String conform to Error   extension String  Error      Then you can just throw a string   throw  Some Error    To make the string itself be the localizedString of the error you can instead extend LocalizedError   extension String  LocalizedError       public var errorDescription  String    return self

User · Answer

Throwing code should make clear whether the error message is appropriate for display to end users or is only intended for developer debugging  To indicate a description is displayable to the user  I use a struct DisplayableError that implements the LocalizedError protocol  struct DisplayableError  Error  LocalizedError       let errorDescription  String       init   description  String            errorDescription   description          Usage for throwing  throw DisplayableError  quot Out of pixie dust  quot    Usage for display  let messageToDisplay   error localizedDescription

User · Answer

Based on  Nick keets answer  here is a more complete example    extension String  Error       Enables you to throw a string  extension String  LocalizedError      Adds error localizedDescription to Error instances     public var errorDescription  String    return self      func test color  NSColor  throws      if color     red           throw  I don t like red       else if color     green           throw  I m not into green       else           throw  I like all other colors           do       try test color   green    catch let error where error localizedDescription     I don t like red       Swift print   Error    error        I don t like red   catch let error       Swift print   Other cases  Error    error localizedDescription       I like all other colors     Originally published on my swift blog  http   eon codes blog 2017 09 01 throwing-simple-errors

User · Answer

I like  Alexander-Borisenko s answer  but the localized description was not returned when caught as an Error  It seems that you need to use LocalizedError instead   struct RuntimeError  LocalizedError       let message  String      init   message  String                self message   message            public var errorDescription  String                return message           See this answer for more details

User · Answer

Simplest solution without extra extensions  enums  classes and etc    NSException name NSExceptionName rawValue   name    reason  reason   userInfo nil  raise

User · Answer

Swift 4  As per  https   developer apple com documentation foundation nserror if you don t want to define a custom exception  you could use a standard NSError object as follows  import Foundation  do     throw NSError domain   quot my error domain quot   code  42  userInfo    quot ui1 quot  12   quot ui2 quot   quot val2 quot        catch let error as NSError     print  quot Caught NSError    error localizedDescription     error domain     error code  quot     let uis   error userInfo    print  quot  tUser info  quot     for  key value  in uis       print  quot  t tkey   key   value   value  quot          Prints  Caught NSError  The operation could not be completed  my error domain  42     User info          key ui1  value 12         key ui2  value val2  This allows you to provide a custom string  the error domain   plus a numeric code and a dictionary with all the additional data you need  of any type  N B   this was tested on OS Linux  Ubuntu 16 04 LTS

User · Answer

nick-keets s solution is most elegant  but it did break down for me in test target with the following compile time error   Redundant conformance of  String  to protocol  Error   Here s another approach   struct RuntimeError  Error       let message  String      init   message  String            self message   message            public var localizedDescription  String           return message           And to use   throw RuntimeError  Error message

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