Today I Learned: Obj-C NSError** Methods Become Swift Throwing Functions

I was brushing up on my Obj-C knowledge today when I learned this cool interop feature: Obj-C methods that use NSError** for error handling can be called from Swift as throwing functions.

Objective-C Pattern

- (BOOL)performOperationWithError:(NSError **)error;

Swift Transformation

When imported into Swift, this method automatically becomes a throwing function:

func performOperation() throws

Example

// Instead of the old Objective-C style:
// var error: NSError?
// let success = someObject.performOperation(&error)
// if !success { /* handle error */ }

// You can now use Swift's try-catch:
do {
    try someObject.performOperation()
} catch {
    print("Operation failed: \(error)")
}

Key Points

  1. Automatic conversion: No need for manual bridging code
  2. Return value handling: The BOOL return value is automatically discarded since errors are now handled via exceptions
  3. Seamless interop: Existing Objective-C libraries work naturally with Swift error handling patterns

This is just one example of how Apple has made the transition between Objective-C and Swift as smooth as possible. The automatic bridging of error handling patterns means legacy Objective-C code feels natural when used from Swift.

Real-World Example

Here’s how this might look with Core Data:

// Objective-C
- (BOOL)save:(NSError **)error;

// Swift usage
do {
    try managedObjectContext.save()
    print("Successfully saved changes")
} catch {
    print("Save failed: \(error.localizedDescription)")
}

Much cleaner than manual error checking!

Part of my ongoing learning series. Small discoveries that make development more enjoyable.