Swift doesn't convert Objective-C NSError** to throws

Question

I have some Objective-C legacy code, that declares method like

- (void)doSomethingWithArgument:(ArgType)argument error:(NSError **)error

As

Answer 1

You need to make your method return a BOOL, to tell the runtime that an error should or should not be thrown. Also you should add __autoreleasing to the error parameter, to make sure ARC doesn't accidentally release the error before you have a chance to use it:

- (BOOL)doSomethingWithArgument:(ArgType)argument error:(NSError * __autoreleasing *)error

You can then call it from Swift like this:

do {
    object.doSomethingWithArgument(someArgument)
} catch let err as NSError {
    print("Error: \(err)")
}

Answer 2

Only Objective-C methods which return a BOOL or a (nullable) object are translated to throwing methods in Swift.

The reason is that Cocoa methods always use a return value NO or nil to indicate the failure of a method, and not just set an error object. This is documented in Using and Creating Error Objects:

Important: Success or failure is indicated by the return value of the method. Although Cocoa methods that indirectly return error objects in the Cocoa error domain are guaranteed to return such objects if the method indicates failure by directly returning nil or NO, you should always check that the return value is nil or NO before attempting to do anything with the NSError object.

For example, the Objective-C interface

@interface OClass : NSObject

NS_ASSUME_NONNULL_BEGIN

-(void)doSomethingWithArgument1:(int) x error:(NSError **)error;
-(BOOL)doSomethingWithArgument2:(int) x error:(NSError **)error;
-(NSString *)doSomethingWithArgument3:(int) x error:(NSError **)error;
-(NSString * _Nullable)doSomethingWithArgument4:(int) x error:(NSError **)error;

NS_ASSUME_NONNULL_END

@end

is mapped to Swift as

public class OClass : NSObject {

    public func doSomethingWithArgument1(x: Int32, error: NSErrorPointer)
    public func doSomethingWithArgument2(x: Int32) throws
    public func doSomethingWithArgument3(x: Int32, error: NSErrorPointer) -> String
    public func doSomethingWithArgument4(x: Int32) throws -> String
}

If you can change the interface of your method then you should add a boolean return value to indicate success or failure.

Otherwise you would call it from Swift as

var error : NSError?
object.doSomethingWithArgument(argumentValue, error: &error)
if let theError = error {
    print(theError)
}

Remark: At

• https://github.com/apple/swift/blob/master/test/Inputs/clang-importer-sdk/usr/include/errors.h

I found that Clang has an attribute which forces a function to throw an error in Swift:

-(void)doSomethingWithArgument5:(int) x error:(NSError **)error
  __attribute__((swift_error(nonnull_error)));

is mapped to Swift as

public func doSomethingWithArgument5(x: Int32) throws

and seems to work "as expected". However, I could not find any official documentation about this attribute, so it might not be a good idea to rely on it.