Objective-C pass by reference vs pass by pointer. What's the difference? And why do we need it at all? [duplicate]

Question

In Objective-C I saw two common patterns for passing objects to functions. Basically all objects to functions are passed by reference like this: -(void) someFunc:(UIImage*)image; this is pass by reference, isn't it?

But then what is this: -(void) someFunc2:(UIImage**)image?? Is it also passing by reference? Or passing by pointer to pointer? Or what? I don't understand what is an actual difference (but I saw this code a lot). And the main question: why do we need this pointer to pointer passing : -(void) someFunc2:(UIImage**)image? Thanks.

Answer 1

Passing a double pointer allows the function to swap out the object you pass in. A good example is all of the many Cocoa APIs that pass a double pointer to an NSError. Take a look at this:

NSError *error = nil;
Result *result = [self someMethodWithPossibleError:&error];
if (![result isValid]) {
    //handle the error
    NSLog(@"Error occurred: %@", error);
}

In this case, we aren't passing an actual error instance, however since we are passing a pointer to our error, this allows them to create an NSError instance in this method, and after exiting, we will be pointing at that new instance.