What is the underlying mechanism for ivar synthesis in the modern Objective C runtime

Question

One of the features of the modern (64 bit OS X and iPhone OS) Objective C runtime is the ability for properties to dynamically synthesize ivars without explicitly declaring them in the class:

@interface MyClass : NSObject {
//  NSString *name; unnecessary on modern runtimes
}

@property (retain) NSStrng *name;

@end

@implementation MyClass

@synthesize name;

@end

In quite a bit of my code I use custom getter implementations in order to initialize the properties:

- (NSString *) name {
  if (!name) {
    name = @"Louis";
  }

  return name;
}

The above is incompatible with synthesized ivars since it needs to access a an ivar that is not declared in the header. For various reasons I would like to update a number of my personal frameworks to use synthesized ivars when built on the modern runtimes, the above code needs to be modified to work with synthesized ivars in order to achieve that goal.

While the Objective C 2.0 documentation states that the synthesized accessors on the modern runtime will synthesize the ivar on first use. It does not specify what low level mechanism is used to do this. Is it done by class_getInstanceVariable(), are the restrictions on class_addIvar() loosened, is it an undocumented function int he objective C 2.0 runtime? While I could implement my own side storage for the data backing my properties, I would much rather use the mechanism that synthesized accessors are using.

Answer 1

I went and looked at the documentation again just now, and I think you're misreading it. Synthesized ivars are created at compile time, not at run time.

According to the Objective-C 2.0 documentation:

There are differences in the behavior that depend on the runtime (see also “Runtime Differences”):

For the legacy runtimes, instance variables must already be declared in the @interface block. If an instance variable of the same name and compatible type as the property exists, it is used—otherwise, you get a compiler error.

For the modern runtimes, instance variables are synthesized as needed. If an instance variable of the same name already exists, it is used.

So all you need to do is declare the instance variable you need, and the same code will work on both runtimes...

Answer 2

Ultimately this question is best answered by those bearing wands, staffs, and pointy hats - but I don't believe there is any way to access the variable from an accessor without use the properties accessor and...well... it gives Infinite Loop a new name.