How a class extension works as a means of implementing private methods

Question

I believe a popular way to declare "private methods" in Objective-C is to create its class extension and declare methods that you would like to make as private.

I would like to know more in detail on how an class extension makes the methods work as private.

• Update: I asked this question with the term empty category which is incorrect. I now changed it as class extension

Obidiah

They’re not called empty categories: they’re called class extensions. Also, note that there’s no such thing as private methods in Objective-C — the runtime will happily invoke a class extension method if the corresponding message is sent in an arbitrary implementation file. – Ferbam

Right, so to make sure, I doubled quoted private methods. Thanks for the info. – Obidiah

Answer 1

That's not an "empty category", it's a class extension. Read Bbum's explanation of them at the link I provided.

Suffragette

Answer 2

That's because you create your empty category in your implementation file, not your header file so other classes can't access it.

//TestClass.h

@interface TestClass : NSObject 
{
}

-(void)publicMethod;

@end

//TestClass.m

@interface TestClass()

-(void)privateMethod;

@end

@implementation TestClass

-(void)publicMethod
{
NSLog (@"public");
}

-(void)privateMethod
{
NSLog (@"private");
}

@end

Gizela