Handling errors in swift with try/catch

Question

I come from a .NET background, where error handling can be as simple as wrapping a set of statements in a try-catch. For example:

try
{
   statement1
   statement2
}
catch (ex)
{
   log(ex.Message)
}

I'm trying to add error handling in my Swift project, and all the articles I have read so far seem to indicate that error handling requires more work in Swift. In the example above, it seems I would need to know exactly which statement throws an error, and then add a "try" before it. Is it not possible to simply wrap a block of code in a try-catch, and inspect the error that gets thrown?

Wow, that seems like a lot of work. My app is crashing, and all I wanted to do is add a simple try/catch around the suspected code and do some logging, but it seems like I need to do a lot of exception design work before I can do that! — Prabhu
– Prabhu, Commented Dec 11, 2016 at 18:13
To inspect the error that gets thrown, set an exception breakpoint. — shallowThought
– shallowThought, Commented Dec 11, 2016 at 18:17
Is it really that unreasonable to require devs to know what functions they're trying to catch errors from? — Alexander
– Alexander, Commented Dec 11, 2016 at 19:28
@AlexanderMomchliov Sometimes, in service oriented architecture, there is so much dependency on 3rd party software, and we can't always design for all error possibilities up front--it's an incremental process. Having the ability to quickly detect an arbitrary exception can be a great boon for developers. C# does this really well. Swift chose not to give us this power, possibly for good reasons. But that doesn't mean that it's not a good feature to have in a programming language. — Prabhu
– Prabhu, Commented Dec 11, 2016 at 19:50

Simbos · Accepted Answer · 2016-12-11 18:15:57Z

3

Nope, you can't just wrap a block of code with try-catch.

First of all, cause not every line of code could produce exceptions. Only functions marked as "throws" could produce exceptions.

For example, you have some function:

deserialise(json: JSON) -> MyObjectType throws

And let this functions throws exceptions of type MyErrorType

That's how you should use it:

....
do {
  let deserialisedObject = try deserialise(jsonObject)
  ... // do whatever you want with deserialised object here
} catch let error as MyErrorType {
  ... // do whatever you want with error here
}
...

So, as you see, exceptions in swift is not the same thing as exceptions in C++(or other regular language) for .NET

answered Dec 11, 2016 at 18:15

Simbos

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Harshad Madaye · Accepted Answer · 2017-03-01 07:25:32Z

-1

You can use try catch in swift just like you do in .net with this little change in structure, before that create a enum of exceptions which you want to catch like follows

//exceptions to catch, change as per your need
enum someActionError: Error {
    case invalidSelection
    case outOfStock
}

//use
do {
    try statement1
} catch someActionError.invalidSelection {
    print("Invalid Selection.")
} catch someActionError.outOfStock {
    print("Out of Stock.")
}

edited Mar 1, 2017 at 7:25

answered Dec 12, 2016 at 13:23

Harshad Madaye

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