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

No you can't. But you can do this if you're not using @property:

.h

@interface X : Y {
  struct X_Impl* impl;
}
-(int)getValue;
@end

.m

struct X_Impl {
  int value;
};
...
@implementation X
-(void)getValue {
  return impl->value * impl->value;
}
@end

Answer 2

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 3

Two possibilities:

1. It could be taking advantage of the modern runtime's ability to synthesize instance variables, as bbum suggested.
2. The property might not have an underlying instance variable in that class. Properties do not necessarily have a one-to-one mapping with instance variables.

Answer 4

How about a macro trick?

Have tested code below

• have tested with dylibs - worked fine
• have tested subclassing - Warning! will break, I agree this makes the trick not that useful, but still I think it tells some about how ObjC works...

MyClass.h

@interface MyClass : NSObject {
#ifdef MYCLASS_CONTENT
    MYCLASS_CONTENT // Nothing revealed here
#endif
}
@property (nonatomic, retain) NSString *name;
@property (nonatomic, assign) int extra;
- (id)initWithString:(NSString*)str;
@end

MyClass.m

// Define the required Class content here before the #import "MyClass.h"
#define MYCLASS_CONTENT \
  NSString *_name; \
  int _extra; \
  int _hiddenThing; 

#import "MyClass.h"

@implementation MyClass
@synthesize name=_name;
@synthesize extra=_extra;

- (id)initWithString:(NSString*)str
{
    self = [super init];
    if (self) {
        self.name = str;
        self.extra = 17;
        _hiddenThing = 19;
    }
    return self;
}

- (void)dealloc
{
    [_name release];
    [super dealloc];
}

@end

Answer 5

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 6

DON'T do this, but I feel it should be noted that the runtime has the ability to add ivars whenever you want with class_addIvar