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

Only Objective-C methods are translated to throwing Swift methods, which do return a BOOL (not lower-cased bool), or a nullable-object.(Tested with Xcode 11.7, and Swift 5 language.)

The reason is that Cocoa methods always use a return value NO or nilto indicate the failure of a method, and not just set an error object.This is documented inUsing 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 errordomain are guaranteed to return such objects if the method indicates failureby directly returning nil or NO, you should always check that the returnvalue is nil or NO before attempting to do anything with the NSError object.

For example, the Objective-C interface

@interface OClass : NSObjectNS_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;-(BOOL)doSomething:(NSError **)error;NS_ASSUME_NONNULL_END@end

is mapped to Swift as

open class OClass : NSObject {    open func doSomethingWithArgument1(x: Int32, error: NSErrorPointer)    open func doSomethingWithArgument2(x: Int32) throws    open func doSomethingWithArgument3(x: Int32, error: NSErrorPointer) -> String    open func doSomethingWithArgument4(x: Int32) throws -> String    open func doSomething() throws}

If you can change the interface of your method then you should add a booleanreturn 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 documentationabout this attribute, so it might not be a good idea to rely on it.

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)")}