What is the Objective-C equivalent of a public get/protected set property in C#

Question

Is there any way to create a property like this C# property in Objective-C?

public int prop { get; protected set;}

Essentially, I want to make it possible to get the value from outside the class, but to only be able to set the value from within the class.

Answer 1

The easiest way to accomplish this is to declare the property as readonly in the public interface for MyClass (i.e. the .h file):

@property (readonly) NSInteger prop;

Then, in .m file for that class, declare a class extension (a category with an empty name). In class extensions, you may redeclare a @property to change its writeability to readwrite:

@interface MyClass ()
@property (readwrite) NSInteger prop;
@end

@implementation MyClass
@synthesize prop;
...
@end

Of course, Objective-C does not enforce access restrictions, so there's nothing stopping some rogue code from calling -[MyClass setProp:]. The compiler will flag this as a warning, however, which is as good as you can get in Objective-C. Unfortunately, there is no standard way to document this "protected" writeable property to subclasses; you'll have to settle on a convention for your team and or put it in the public documentation for the class if it's in a framework that you're going to release.

Answer 2

I believe you are looking for the readonly attribute on the @property

@property (readonly) NSInteger prop;

Read more here.

Answer 3

There's no way to make methods @private/@protected in ObjC, these compiler directoves apply only on instance variables. You can, however, declare a static C function in the same file to get a "@private" method.

keno's answer seems like most correct way of achieving this.