Question

Say I have a method that takes a CLLocationCoordinate2D. I can’t pass nil directly to that in code; the compiler complains. However, the following compiles. So what happens at runtime?

CLLocation* loc = nil;
[self passMeCoordinates:loc.coordinate];

Answer 1

This is an interesting question. I assume your earlier code was:

[self passMeCoordinates:nil];

The compiler complains about this because CLLocationCoordinate2D is not an object. It’s a C-style struct. So, you can’t pass an object/pointer (nil) where a struct is expected. (They’re not the same size, etc.)

If I slightly paraphrase your code to:

CLLocation* loc = nil;
CLLocationCoordinate2D coord = loc.coordinate;
[self passMeCoordinates:coord];

The question comes down to “what value does coord have”. As you may know, the rule in Objective-C is that if you send a message to nil (as we do here — loc.coordinate is equivalent to [loc coordinate]), then you get back 0. But what if we’re expecting a struct? As I just mentioned, it’s not the same size as an integer. Well, it turns out that the result depends on what compiler you’re using:

LLVM 3.0+ (Xcode 4.2+): returns all zeros, so it’s equivalent to a coordinate of (0,0): Can I rely on nil UIView returning CGRectZero for its frame?

LLVM Earlier/GCC: a struct can be filled with garbage/undefined contents, so it could be anything.