On Fri, May 15, 2015, at 11:31 AM, Jens Alfke wrote: > Yes, but when you evaluate (cdr nil) doesn’t the interpreter trigger an > exception? (Sorry, it’s been decades since I used LISP.) Similarly, in > Smalltalk-80 ‘null’ is a real object, a singleton of class > UndefinedObject, but trying to message it is going to raise a > message-not-handled exception. The difference from a nil pointer is > mostly that you get a language-level rather than an OS-level exception.
Just as a point of esoterica, nothing about the C spec requires the null pointer to map to a hardware-level trap. The only requirement is that there exist a value called the null pointer in every pointer type, that all null pointers compare equal to each other, and that they do not compare equal to any non-null pointers. It's not valid to dereference a null pointer, but what happens when you do is undefined. The assumption that dereferencing a null pointer would always lead to a hardware fault led to a pretty serious vulnerability in the Linux kernel: http://lwn.net/Articles/342330/ Here's some more elaboration: https://software.intel.com/en-us/blogs/2015/04/20/null-pointer-dereferencing-causes-undefined-behavior And all of this is why Swift and every other modern programming languages goes the route of Optionals. --Kyle Sluder _______________________________________________ Cocoa-dev mailing list (Cocoa-dev@lists.apple.com) Please do not post admin requests or moderator comments to the list. Contact the moderators at cocoa-dev-admins(at)lists.apple.com Help/Unsubscribe/Update your Subscription: https://lists.apple.com/mailman/options/cocoa-dev/archive%40mail-archive.com This email sent to arch...@mail-archive.com
