Question

I’m writing a media player framework for a project I’m working on. This depends on VLC. One of my classes’ header file looks like this

#import <vlc/vlc.h>

@interface MediaPlayerVLC : MediaPlayer
{
    libvlc_media_player_t *player;
    libvlc_media_t *media;
}

@end

I need the instance variables in the class, and I need the #import <vlc/vlc.h>, because they’re defined in there. But I don’t want users of this framework to have to import all of VLC’s headers just for these two types. I’ve seen a few solutions to this problem around…

• Forward declaration, such as @class. Unfortunately, these types are typedef struct types, I can’t seem to find any way to forward declare them
• declare the ivars as void *, then cast them whenever I want to use them. I’d like to avoid this if possible, as we lose type-safety and implementation files become full of ugly casts.

• I’ve seen this in Apple’s frameworks…

  @interface CAAnimation : NSObject <NSCoding, NSCopying, CAMediaTiming, CAAction>
  {
  @private
    void *_attr;
    uint32_t _flags;
  }
  

  What does _attr point to? I guess it would be a struct of ivars, I’m curious what advantages this has…

• Two header files for the class, one public and one private. The private one would look like the above, and the public would just have void * pointers. This is pretty ugly, as I’d have to be very careful to keep them both in sync.

What’s considered best practise? Is there an approach I’ve missed?

Answer 1

You can use class extensions. You should try doing this:

MediaPlayerVLC.h:

@interface MediaPlayerVLC : MediaPlayer
{
}

@end

MediaPlayerVLC.m:

#import "MediaPlayerVLC.h"
#import <vlc/vlc.h>

@interface MediaPlayerVLC ()
{
    libvlc_media_player_t *player;
    libvlc_media_t *media;
}

@end

@implementation MediaPlayerVLC

// The implementation

@end

From Apple’s docs:

Class extensions are like anonymous categories, except that the
methods they declare must be implemented in the main @implementation
block for the corresponding class.
Using the Clang/LLVM 2.0 compiler,
you can also declare properties and instance variables in a class
extension.

That’s using a class extension category to declare extra ivars in the implementation file.