Wednesday, March 21, 2012 [Tweets] [Favorites]

Objective-C Literals

The updated Clang documentation:

Using array and dictionary literals is safer than the variadic creation forms commonly in use today. Array literal expressions expand to calls to +[NSArray arrayWithObjects:count:], which validates that all objects are non-nil. The variadic form, +[NSArray arrayWithObjects:] uses nil as an argument list terminator, which can lead to malformed array objects. Dictionary literals are similarly created with +[NSDictionary dictionaryWithObjects:forKeys:count:] which validates all objects and keys, unlike +[NSDictionary dictionaryWithObjectsAndKeys:] which also uses a nil parameter as an argument list terminator.

There are lots of carrots for going 64-bit-only and adopting the modern Objective-C runtime.

9 Comments

I don't see the issue with having nil as a terminator. You just have to know your code, as always,

For instance, it's not unusual for me to set the item before nil in arrayWithObjects: to be an object that can probably be nil itself.

So in this case, the convenience would become an inconvenience,

@bob This issue has bit me a couple times over the years, when some system class that should never have returned nil did exactly that, and so half my dictionary was wiped out—silently. I think failing fast is a good thing.

The old way still works for those cases where you expect a possible nil at the end. Although, maybe it would be a good idea to use a different method name to document your intentions.

@michael: "when some system class that should never have returned nil did exactly that, "

I usually check for nil in most cases except when I don't care if it is nil.

@bob You check things like +[NSFileManager defaultManager] every time?

Nope but:

- I mostly use it to check for the existence of files. So if it's nil, fileExistsBlahBlah will return NO and the error will be handled.

- The day I start trusting NSFileManager for complex operations such as copying, renaming, getting most attributes, symbolic link has yet to come.

@bob I didn’t have much user for NSFileManager when it wasn’t threadsafe, so I wrote replacements for most of its methods. Lately, it seems pretty good except that it still doesn’t copy Finder comments.

@Michael: I never had to deal with thread-safe issue in my usage of NSFileManager as it is only invoked on the main thread. For copy operations, I still rely on the FS API that is based on the old FSCopy sample code.
Probably will have to move to NSFileManager in the not so distant future though.

@bob Hmm, I found the FS API to be crashy, and it would sometimes say that it had deleted a file without doing so. I wish NSFileManager would learn the preflighting and progress stuff, though.

[…] Objective-C literals are described as an easy sugar feature that Apple added while much of the team was busy with Swift. I have never understood why it took so long for this feature to arrive. It could have been added to Objective-C 1.x but instead came about 6 years after Objective-C 2.0. Or, to put a positive spin on this, the rate of progress with Swift is incredible compared with what we saw with Objective-C. […]

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