Should get 20 errors... but get 0... when compiling without declaring an instance variable.

Question

In my iPhone apps I regularly do this in xCode v3.2.3:

1. Declare a BOOL variable in the *.h file
2. Use @property in the same *.h file.
3. Use @sythesize in the matching *.m file.

I accidentally forgot to do #1... but it still complied fine. 0 warnings. 0 errors. 0 analyzer errors.

How can that be? Shouldn't my code to loaded with compiler-errors everywhere that variable is trying to be used?

Answer 1

This is a feature of the new runtime. See this question for more details.

Answer 2

Automatic synthesis of instance variables (ivars) is a feature of the Objective-C 2.0 runtime on OS X and of the new iOS Objective-C runtime. The @synthesize directive will automatically create the necessary ivar at runtime unless you have declared it yourself. This is made possible by Objective-C 2.0's non-fragile ivar support. Thus, there is no error and you should not receive them.

Ultimately, it's a good thing that you don't have to declare ivars in the interface of a class. They are (probably) implementation-specific details which you don't want to have visible as part of the public interface of the class. Note that using class categories you can also automatically synthesize ivars for "private" properties as well.