Question: If you have a property in your public interface like the following

```
@interface MyClass : NSObject
@property(strong) NSString *myProp;
@end

```

And then synthesize it, in effect synthesizing the variable:

```
@implementation MyClass
@synthesize myProp = _myProp; // or just leave it at the default name..
@end

```

What is the visibility of the instance variable `_myProp`? That is, is this considered `@public`, `@protected` or `@private`? I'm guessing since `MySubClass` could inherit from `MyClass` then it would also get the properties (naturally), but would it also inherit the instance variable visibility?

What difference does it make if I put the property in a class extension? That would hide the property from subclasses, and I'm guessing the instance variable, too. Is this documented anywhere?

Answer: 
A synthesized ivar is *completely invisible* to all code that cannot see the `@synthesize` line (which basically means anything outside of the .m file). It's not `@protected`, it's not `@private`, it's simply unknown. With a `@private` ivar, other code trying to access it will be told that it's private, but with a synthesized ivar, other code trying to access it will be told that the field simply doesn't exist.

As a thought experiment, try imagining a situation where the ivar acted like it was `@protected`. You make a subclass, and you muck about with the ivar there. Now you go back to the superclass and change `@synthesize myProp` to `@synthesize myProp=foo`. What happens in the subclass? When the compiler processes the subclass, it *cannot see* the `@synthesize` line, so it would have no idea that you just changed the name of the ivar. In fact, it cannot even tell if the property is backed by an ivar at all, or if it's implemented with custom-written accessor methods. I hope it's obvious why this means that the subclass cannot possibly access the ivar, and neither can any other class.

That said, I'm not quite sure what the compiler does if you write code in the same .m file that tries to access the ivar. I expect it will treat the ivar as `@private` (since the compiler can, in fact, see that the ivar exists).

Also, none of this has any bearing on the runtime methods. Other classes can still use the obj-c runtime methods to dynamically look up your class's ivar list and muck about with it.
Other classes have access to everything that they `#include`. In other words, to everything that is inside your header.

If something appears only in your implementation file, other classes (including subclasses) don't know it exists. A synthesized property is like that. Other classes know only about the property (a property means a getter and a setter method) but they don't know anything about the inner implementation of its methods.

Note, that the access specifiers (public/private/protected) in obj-c are only a hint to the compiler that even if something appears in the header file, it can't be accessed. The runtime does not check it in any way.

What happens if you put it into a class extension? Note that a property is a set of two methods. You just hide the methods from every class which includes your class main header but not the class extension header.

We use this for example to declare a property as readonly and in class continuation we declare it as readwrite. Then, we can use the setter only from inside of the class.