Question

I’ve started learning Objective-C a few weeks ago and I still don’t understand how to manage the encapsulation of a class correctly. What is the best way to declare a private member variable in a class?

It seems that setting the right getter/setter for your member variable with “@property” is the right way to go, more than just declaring it “@private” in the interface. But it seems to me that this still gives other classes an access to these variables. Even if you declare the property “readonly”, an outside class can access the reference to the member variable and modify it!

So I’m guessing the best way to declare a private member variable is to not include any guetter/setter by not declaring a property. Am i right? Or is there a better way?

Thanks

Answer 1

if you don’t want it accessible to other classes, declare the @property on your implementation, creating an anonymous category for your class.

Header file:

// MyClass.h
@interface MyClass : NSObject {
    NSObject *_privateObject;
    NSObject *_readonlyObject;
    NSObject *_publicObject;
}

@property (nonatomic, retain, readonly) NSObject *readonlyObject;
@property (nonatomic, retain) NSObject *publicObject;

@end

Implementation:

// MyClass.m
@interface MyClass ()
    @property (nonatomic, retain) NSObject *privateObject;
    // Make it writable on the implementation
    @property (nonatomic, retain, readwrite) NSObject *readonlyObject;
@end

@implementation MyClass

@synthesize privateObject = _privateObject;
@synthesize readonlyObject = _readonlyObject;
@synthesize publicObject = _publicObject;

These are examples of three different properties.

• privateObject is not visible on other classes;
• readonlyObject is visible but is read only;
• publicObject is visible and can be get and set;