How to pass an error pointer in the Swift language?

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You just pass a reference like so:

var error: NSError?
var results = context.executeFetchRequest(request, error: &error)

if error != nil {
    println("Error executing request for entity \(entity)")
}

Two important points here:

  1. NSError? is an optional (and initialized to nil)
  2. you pass by reference using the & operator (e.g., &error)

See: Using swift with cocoa and objective-c

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