Hide instance variable from header file in Objective C

Question

I came across a library written in Objective C (I only have the header file and the .a binary). In the header file, it is like this:

@interface MyClass : MySuperClass 
{ 
    //nothing here
}

@property (nonatomic, retain) MyObject anObject;
- (void)someMethod;

How can I achieve the same thing? If I try to declare a property without its corresponding ivar inside the interface's {}, the compiler will give me an error. Ultimately, I want to hide the internal structure of my class inside the .a, and just expose the necessary methods to the header file. How do I declare instance variables inside the .m? Categories don't allow me to add ivar, just methods.

Answer 1

For 64 bit applications and iPhone applications (though not in the simulator), property synthesis is also capable of synthesizing the storage for an instance variable.

I.e. this works:

@interface MyClass : MySuperClass 
{ 
    //nothing here
}

@property (nonatomic, retain) MyObject *anObject;
@end

@implementation MyClass
@synthesize anObject;
@end

If you compile for 32 bit Mac OS X or the iPhone Simulator, the compiler will give an error.

Answer 2

You may use of the same idiom used in Cocoa classes. If you have a look to NSString class interface in NSString.h you'll see that there is no instance variable declared. Going deeper in GNUstep source code you'll find the trick.

Consider the following code.

MyClass.h

@interface MyClass : NSObject

// Your methods here
- (void) doSomething;

@end

MyClass.m

@interface MyClassImpl : MyClass {
   // Your private and hidden instance variables here
}
@end

@implementation MyClass

+ (id) allocWithZone:(NSZone *)zone
{
   return NSAllocateObject([MyClassImpl class], 0, zone);
}

// Your methods here
- (void) doSomething {
  // This method is considered as pure virtual and cannot be invoked
  [self doesNotRecognizeSelector: _cmd];          
}

@end

@implementation MyClassImpl

// Your methods here
- (void) doSomething {
  // A real implementation of doSomething
}

@end

As you can see, the trick consist in overloading allocWithZone: in your class. This code is invoked by default alloc provided by NSObject, so you don't have to worry about which allocating method should be used (both are valid). In such allocWithZone:, you may use the Foundation function NSAllocateObject() to allocate memory and initialize isa for a MyClassImpl object instead of MyClass. After that, the user is dealing with a MyClassImpl object transparently.

Of course, the real implementation of your class shall be provided by MyClassImpl. The methods for MyClass shall be implemented in a way that considers a message receiving as an error.

Answer 3

You can use a class extension. A class extension is similar as category but without any name. On the Apple documentation they just define private methods but in fact you can also declare your internal variables.

MyClass.h

@class PublicClass;

// Public interface 
@interface MyClass : NSObject

@property (nonatomic, retain) PublicClass *publicVar;
@property (nonatomic, retain) PublicClass *publicVarDiffInternal;

- (void)publicMethod;

@end

MyClass.m

#import "PublicClass.h"
#import "InternalClass.h"

// Private interface
@interface MyClass ( /* class extension */ ) 
{
@private
    // Internal variable only used internally
    NSInteger defaultSize;

    // Internal variable only used internally as private property
    InternalClass *internalVar;  

@private 
    // Internal variable exposed as public property 
    PublicClass *publicVar;

    // Internal variable exposed as public property with an other name
    PublicClass *myFooVar;
}

@property (nonatomic, retain) InternalClass *internalVar;

- (void)privateMethod;

@end

// Full implementation of MyClass
@implementation MyClass

@synthesize internalVar;
@synthesize publicVar;
@synthesize publicVarDiffInternal = myFooVar

- (void)privateMethod 
{
}

- (void)publicMethod 
{
}

- (id)init
{
   if ((self = [super init]))
     {
       defaultSize = 512;
       self.internalVar = nil;
       self.publicVar = nil;
       self.publicVarDiffInternal = nil; // initialize myFooVar
     }

   return self;
}
@end

You can give MyClass.h to anyone with just your public API and public properties. On MyClass.m you declare your member variable private and public, and your private methods, on your class extension.

Like this it's easy to expose public interfaces and hide detail implementation. I used on my project without any troubles.