a glob of nerdishness

December 10, 2008

Cocoa NSNumber “constant” macro for Objective-C

written by natevw @ 1:22 pm

The following tlnum() macro (which I dedicate to the public domain), will allow you to easily create NSNumbers from constant floats and integers in your Objective-C code:

#define TL_FLOATARG(x) ( (x) ? ((x) / (2 * (x))) : (((x) + 1) / 2) )
#define tlnum(x) (TL_FLOATARG(x) ? [NSNumber numberWithDouble:(x)] : [NSNumber numberWithLong:(x)])

You could even rename tlnum() to simply N() if it won’t conflict with anything in your code. Update: Mike Ash pointed out that the TL_FLOATARG trick will not work if 2 * (x) overflows. So until a better macro can be developed, use this with care.

Why would you use this? Well, Objective-C lets you make a constant NSString-like object at compile-time @”like this”. This is extremely handy, as the next easiest way to create a string object is [NSString stringWithUTF8String:"like this"]. Since string objects are used all over the place — as format specifiers, when querying for localized strings or application resources, in any sort of situation where a key is needed… — the @”" syntax saves a lot of typing! Constant number objects aren’t needed quite as often, but almost every Cocoa programmer has at some point wanted to create constant NSNumber-like objects in a similarly succinct and easy way. Using this macro lets you do just that.

While it would be impractical (if not impossible) to create truly constant NSNumber-like objects at compile time using preprocessor macros, I still wanted something that would allow me to replace code like [NSNumber numberWithInt:42] and [NSNumber numberWithDouble:3.14] with something shorter. Furthermore, I wanted *it* to worry about the number’s type for me. I didn’t want to have to call a different macro depending on the number, as this would be require more thought for each use and maybe even more typing.

My first thought was to do something like this, following similar tricks that Dave Dribin used for his to-NSString converter:

#define tlnum(x) ({ typeof(x) _x = (x); [NSNumber value:&_x withObjCType:@encode(typeof(_x))]; })

This has two problems. For one, the NSNumber subclass does not override -[NSValue value:withObjCType:] and so the object created does not work as an NSNumber at all! I worked around this with my own function that checked the encoded type, but that seemed like overkill. The second problem is that even with a helper function, the code depends on GCC extensions for the block macro and the typeof() operator. I wanted to keep my code within the C99 standard.

Since I was intending this for constant numbers only, and since NSNumber (or at least the CFNumber implementation in the most recent CFLite distribution) uses at least 8 bytes of storage for even chars and bools, the only important distinction is between floats and integers. Thus the TL_FLOATARG, which takes advantage of the fact that 1 / 2 yields the integer 0, while 1.0 / 2 yields a floating-point 0.5. The one gotcha with the final version is that it evaluates its arguments multiple times. My compile settings give a warning, but be careful not to do something like tlnum(i++), as you’ll end up incrementing i multiple times. Jens Ayton kindly provided another macro trick to turn this into an error, but in the end I opted to leave it out for simplicity.

Please enjoy, and I’d be glad to know if this is helpful!

December 2, 2008

Using fully synthesized instance variables in Objective-C 2.0

written by natevw @ 7:12 pm

One of the great new features added to Objective-C in version 2.0 was property declarations. While I’m not a big fan of the dot syntax, the ability to clarify the semantics of setter methods is a big win in my opion. With old-style setters, one was rarely sure if the value passed would be copied, retained, or simply referenced. The other neat feature of properties is that the compiler can provide instance variable storage automatically when compiling for the “modern” ObjC runtime. So you don’t have to declare an instance variable matching (or matched to) a fully-synthesized property if you’d prefer not to.

If you intend to take full advantage* of synthesized variables in the future, you must never use the instance variable directly. Unfortunately, if you’re also compiling an application that needs to run on 32-bit Leopard (as is pretty likely these days whether you’re supporting a PowerPC desktop app or testing an iPhone app in the simulator) you will need to declare these storage for your properties for the sake of those builds — Leopard needed to be backwards compatible with the “legacy” ObjC runtime, and the compiler relies on runtime support to implement this ObjC 2 magic. This conflicts with best practice for init/awake and dealloc methods, which should prefer to use the instance variable directly to avoid unwanted side effects of custom setters (undo, notifications, redisplay, etc.). If you’re in the habit of using instance variables directly in other places, you may need a little help from the compiler to remind you that the ivar may not exist.

What we might want is a set of macros that does the right thing in these special circumstances, but yet will cause compiler errors during universal builds for any other direct use of the instance variables:

/* The following macros allow one to mark uses of declared instance variables
 that could be replaced by automatic property storage when only the modern ObjC
 runtime needs to be supported.

 TL_SYNTHESIZABLE_IVAR_DECL is used to declare the instance variable.
 TL_SYNTHESIZABLE_IVAR_SET should be used only in init/awake implementations.
 TL_SYNTHESIZABLE_IVAR_RELEASE should be used only in dealloc implementation. */
#ifdef __OBJC2__
#define TL_SYNTHESIZABLE_IVAR_DECL(type, name)
#define TL_SYNTHESIZABLE_IVAR_SET(name, value) (self.name = value)
#define TL_SYNTHESIZABLE_IVAR_RELEASE(name) (self.name = nil)
#define TL_SYNTHESIZABLE_IVAR_DECL(type, name) type name
#define TL_SYNTHESIZABLE_IVAR_SET(name, value) (name = value)
#define TL_SYNTHESIZABLE_IVAR_RELEASE(name) ([name release])
#endif /* __OBJC2__ */

You would use them like this:

@interface MyClass : NSObject {
@property (nonatomic, retain) id myVar;

@implementation MyClass
- (id)init {
	self = [super init];
	if (self) {
		id defaultValue;
		TL_SYNTHESIZABLE_IVAR_SET(myVar, defaultValue);
	return self;
- (void)dealloc {
	[super dealloc];
@synthesize myVar;

*Now the real question is, what advantage is there to synthesized instance variable storage? In short, you’re able to declare the property in one place only, which results in less duplicate code. So why would you use these macros, when they require you to declare the property twice anyway? I’ll tell you: I don’t know. If you like to mark “deprecatable” code, TL_SYNTHESIZABLE_IVAR_DECL will be handy. The other two could be replaced by their __OBJC2__ setter equivalents with no real harm. Regardless, here they are, and I dedicate them to the public domain. If you think they’d be helpful, feel free use them at your own risk.