When would you NOT want to use @synthesized instance variables?

Question

In the Modern Objective-C runtime, you can do something like this:

@interface MyClass : NSObject {
}
@property NSString *stringProperty;
@end

@implementation MyClass
@synthesize stringProperty;
@end

It is my understanding with the modern runtime this will not only synthesize the accessors for my property, but also the instance variable itself, so I could then say in one of this class's methods [stringProperty length]; and it would work just as if I'd declared an instance variable.

I've started using this in all my code now, because, well it's one less thing I have to write over and over again. And I've heard with the clang 2.0 compiler, I'll even be able to skip the @synthesize (but that's another matter). But I've been wondering, what are some of the downsides to doing this? When might I truly need an instance variable in addition to my properties?

I know there are sometimes when I want to keep a variable private and not give access to it externally (but then I usually just declare the property in my private class extension, or I don't create a property for it at all, if I don't need accessors for it).

Are there any times when I wouldn't want to do this?

Answer 1

One possible reason why it might be advisable to not use synthesized instance variables is they are a bit more of a pain to debug in the current version of Xcode (3.2.5). They don't seem to show up in the live debugger view when running code through GDB, the only way to get at them is through the gdb console like po [0xOBJ_ADDRESS propertyName]. Not exactly as nice as a standard, non-synthesized ivar.

Maybe Xcode 4 fixes this but I don't have enough experience with it to say (and it's still under NDA).

More info on this SO question: Seeing the value of a synthesized property in the Xcode debugger when there is no backing variable

Answer 2

You may want to provide many properties for a single instance variable, e.g., :

• for accessing an angle value both in degrees and radians
• for accessing coordinates both in rectangular and in polar systems

In these cases, you don't want to synthesize instance variables (it already exist) nor accessor methods (you want to provide them for doing conversions).