How to use try-catch in Swift?

Question

I get how I can create my own errors and trigger them in Swift with the throws keyword. What I don't get is how to replicate the regular try - catch found in other languages (or the Ruby rescue) for unhandled exceptions.

Example (In Swift):

func divideStuff(a: Int, by: Int) -> Int {
  return a / by
}

let num = divideStuff(4, by: 0)  //divide by 0 exception

Here's how I'd deal with it in C#, for example:

int DivideStuff(int a, int b) {
  int result;
  try {
    result = a / b;  
  }
  catch {
    result = 0;
  }
  return result;
}

How can I achieve the same with Swift?

It's `do`.. `catch` in the doc: https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/ErrorHandling.html — Jonas, Jun 15 '16 at 12:10
You *cannot* catch arbitrary runtime exceptions (such as division by zero) with try/catch, compare http://stackoverflow.com/questions/34628999/swift-force-unwrapping-exception-not-propagated for a similar question and links to the documentation. — Martin R, Jun 15 '16 at 12:11

score 2 · Accepted Answer · answered Jun 15 '16 at 12:11

2

In Swift there is no functionality to catch arbitrary runtime errors.
The developer is responsible to handle the errors properly.

For example

func divideStuff(a : Int, b : Int) -> Int {
  if b == 0 { return 0 }
  return a / b
}

answered Jun 15 '16 at 12:11

vadian

274,689
30
353
361

2

Thank you. I didn't know Swift is incapable of catching arbitrary runtime errors. Somehow, this feels odd. – ilovebigmacs Jun 15 '16 at 12:24
This does not really explain a try/catch - this is good, but it's defensive programming. – geekyaleks Jan 18 '17 at 21:26

score 2 · Answer 2 · answered Jun 15 '16 at 12:28

2

You can also handle it like this:

enum MyErrors: ErrorType {
  case DivisionByZero
}

func divideStuff(a: Int, by: Int) throws -> Int {
  if by == 0 {
    throw MyErrors.DivisionByZero
  }
  return a / by
}

let result: Int

do {
  result = try divideStuff(10, by: 0)
} catch {
  // ...
}

answered Jun 15 '16 at 12:28

Viktor Simkó

2,607
16
22

You may want to add a catch let error as NSError to the do/catch statement so that the error can be identified in the catch portion – Ike10 Jun 15 '16 at 15:05

How to use try-catch in Swift?

2 Answers2