throwing an exception in objective-c/cocoa

Question

What\'s the best way to throw an exception in objective-c/cocoa?

Answer 1

I use [NSException raise:format:] as follows:

[NSException raise:@"Invalid foo value" format:@"foo of %d is invalid", foo];

Answer 2

Use NSError to communicate failures rather than exceptions.

Quick points about NSError:

• NSError allows for C style error codes (integers) to clearly identify the root cause and hopefully allow the error handler to overcome the error. You can wrap error codes from C libraries like SQLite in NSError instances very easily.

• NSError also has the benefit of being an object and offers a way to describe the error in more detail with its userInfo dictionary member.

• But best of all, NSError CANNOT be thrown so it encourages a more proactive approach to error handling, in contrast to other languages which simply throw the hot potato further and further up the call stack at which point it can only be reported to the user and not handled in any meaningful way (not if you believe in following OOP's biggest tenet of information hiding that is).

Reference Link: Reference

Answer 3

You can use two methods for raising exception in the try catch block

@throw[NSException exceptionWithName];

or the second method

NSException e;
[e raise];

Answer 4

Sample code for case: @throw([NSException exceptionWithName:...

- (void)parseError:(NSError *)error
       completionBlock:(void (^)(NSString *error))completionBlock {


    NSString *resultString = [NSString new];

    @try {

    NSData *errorData = [NSData dataWithData:error.userInfo[@"SomeKeyForData"]];

    if(!errorData.bytes) {

        @throw([NSException exceptionWithName:@"<Set Yours exc. name: > Test Exc" reason:@"<Describe reason: > Doesn't contain data" userInfo:nil]);
    }


    NSDictionary *dictFromData = [NSJSONSerialization JSONObjectWithData:errorData
                                                                 options:NSJSONReadingAllowFragments
                                                                   error:&error];

    resultString = dictFromData[@"someKey"];
    ...


} @catch (NSException *exception) {

      NSLog( @"Caught Exception Name: %@", exception.name);
      NSLog( @"Caught Exception Reason: %@", exception.reason );

    resultString = exception.reason;

} @finally {

    completionBlock(resultString);
}

}

Using:

[self parseError:error completionBlock:^(NSString *error) {
            NSLog(@"%@", error);
        }];

Another more advanced use-case:

- (void)parseError:(NSError *)error completionBlock:(void (^)(NSString *error))completionBlock {

NSString *resultString = [NSString new];

NSException* customNilException = [NSException exceptionWithName:@"NilException"
                                                          reason:@"object is nil"
                                                        userInfo:nil];

NSException* customNotNumberException = [NSException exceptionWithName:@"NotNumberException"
                                                                reason:@"object is not a NSNumber"
                                                              userInfo:nil];

@try {

    NSData *errorData = [NSData dataWithData:error.userInfo[@"SomeKeyForData"]];

    if(!errorData.bytes) {

        @throw([NSException exceptionWithName:@"<Set Yours exc. name: > Test Exc" reason:@"<Describe reason: > Doesn't contain data" userInfo:nil]);
    }


    NSDictionary *dictFromData = [NSJSONSerialization JSONObjectWithData:errorData
                                                                 options:NSJSONReadingAllowFragments
                                                                   error:&error];

    NSArray * array = dictFromData[@"someArrayKey"];

    for (NSInteger i=0; i < array.count; i++) {

        id resultString = array[i];

        if (![resultString isKindOfClass:NSNumber.class]) {

            [customNotNumberException raise]; // <====== HERE is just the same as: @throw customNotNumberException;

            break;

        } else if (!resultString){

            @throw customNilException;        // <======

            break;
        }

    }

} @catch (SomeCustomException * sce) {
    // most specific type
    // handle exception ce
    //...
} @catch (CustomException * ce) {
    // most specific type
    // handle exception ce
    //...
} @catch (NSException *exception) {
    // less specific type

    // do whatever recovery is necessary at his level
    //...
    // rethrow the exception so it's handled at a higher level

    @throw (SomeCustomException * customException);

} @finally {
    // perform tasks necessary whether exception occurred or not

}

}

Answer 5

This is how I learned it from "The Big Nerd Ranch Guide (4th edition)":

@throw [NSException exceptionWithName:@"Something is not right exception"
                               reason:@"Can't perform this operation because of this or that"
                             userInfo:nil];

Answer 6

I believe you should never use Exceptions to control normal program flow. But exceptions should be thrown whenever some value doesn't match a desired value.

For example if some function accepts a value, and that value is never allowed to be nil, then it's fine to trow an exception rather then trying to do something 'smart'...

Ries