I was using +[NSDictionary dictionaryWithObjectsAndKeys:] to make a new dictionary, but one of the objects in the dictionary was the result of a call to a method that was returning nil, so the dictionary was incomplete.
This got me thinking about NULL/nil terminated argument lists. I don’t think they are a great idea (the compiler should be able to handle the list-termination for you!), but I think they are an especially bad idea in Objective-C.
The problem that it’s very common to have a nil object in Objective-C, relative to, say C++. Many Cocoa methods return nil on error. Since doing stuff with nil (generally) won’t cause an exception, these nils stick around much longer then in other languages. As you can see, nil is a pretty poor choice of a sentinel value.
It’s the 21st century! The compiler could tell an Obj-C method using a variable-argument-list how many arguments are in the list. This is trivial when all arguments are of type id. Since Obj-C methods use a radically different syntax from C functions, it shouldn’t effect existing C-code. Unfortunately, I don’t see this being added, because Objective-C is already so mature.
In the meantime. Be a little more suspicious of any objective-C methods taking a NULL-terminated list. I wish I had a perfect solution to avoid them, but I don’t! Sometimes they are the best way to do something. If you have a great work-around for constructing, say an NSDictionary with a variable number of key/values please let me know!
I don’t see what’s wrong with them, except that I didn’t know about this until I started getting a weird run-time error. Then someone on IRC revealed that UIAlert uses a nil-terminated list for button labels. That fixed my problem. So beyond throwing off newbies like myself, I don’t see it as a big deal.
Comment by Elliot Lee — August 15, 2008 @ 4:15 am