Question

I’m not sure if this is correct or not:

- (void)parseSomething:(id)targetObject error:(NSError **)error {
    NSError *parserError = nil;
    [myParser parse:targetObject error:&parserError];

    if (parserError != nil) {
        *error = parserError;
    }
}

the line:

*error = parserError;

I set error in parameter to be a local error, is it done correctly?

or should I do:

error = &parserError;

instead?

Answer 1

You need to make sure error isn’t nil before you try to dereference it. Also, there is no need for the local NSError. It’d write that code this way:

- (void)parseSomething:(id)targetObject error:(NSError **)error {
    [myParser parse:targetObject error:error];
}

But if you really wanted the locale variable (or for demonstration purposes). then this:

- (void)parseSomething:(id)targetObject error:(NSError **)error {
    NSError *parserError = nil;
    [myParser parse:targetObject error:&parserError];
    if (error && parserError) {
        *error = parserError;
    }
}

Also, most methods that have an NSError out parameter like this usually have a BOOL return value or some other return value to indicate success or not. You shouldn’t rely in the error parameter to indicate whether there was an error or not.

- (BOOL)parseSomething:(id)targetObject error:(NSError **)error {
    NSError *parserError = nil;
    BOOL ok = [myParser parse:targetObject error:&parserError];
    if (error && parserError) {
        *error = parserError;
    }

    return ok;
}