Note about distanceFromLocation: of CLLocation

CLLocation
- (CLLocationDistance)distanceFromLocation:(const CLLocation *)location;

When argument is nil, this method returns -1.
There is no document about this behavior and no definition about -1.
(Maybe it means invalid distance. ex. CLLocationInvalidDistance = -1 in internal)

CLLocation *location = [[CLLocation alloc] initWithLatitude:37.785834 longitude:-122.406417];

CLLocation *nilLocation = nil;

CLLocationDistance distance;

// distance from nil to location

distance = [location distanceFromLocation:nilLocation];

NSLog(@"distance %f", distance);

// -1.000000

// message send to nil object, results is nil

distance = [nilLocation distanceFromLocation:location];

NSLog(@"distance %f", distance);

// -0.000000

CLLocation の distanceFromLocation: メソッドの引数に nil を渡すと-1が返ってくる。
この挙動はドキュメントには載っていなくて、-1 が何を意味するかも定義されてない。
定義は無いけどたぶん CLLocationInvalidDistance = -1 とか内部的にはなってるんじゃないかな。