Objective-C and C++ mixing together.

// Header:

#import <Cocoa/Cocoa.h>

@class NativeInterfaceExtensions_;

namespace nim {
namespace interface {

struct NativeInterfaceExtensions
{
  NativeInterfaceExtensions(id window);
  ~NativeInterfaceExtensions();
private:
  NativeInterfaceExtensions_* nativeWindow;
};

}
}

// Source:

#import "NativeMainWindow.h"

@interface NativeInterfaceExtensions_ : NSObject

@property(copy) NSWindow* WindowHandler;

- (id) initWithWindowHandler:(id)window;

@end

@implementation NativeInterfaceExtensions_

@synthesize WindowHandler;

- (id) initWithWindowHandler:(id)window {
  self = [super self];
  if (self) {
    NSLog(@"%d", (int)[window isKindOfClass:[NSWindow class]]);
    WindowHandler = (NSWindow*)window;
    return self;
  }

  return nil;
}

@end

using namespace nim::interface;

NativeInterfaceExtensions::NativeInterfaceExtensions(id window) {
    nativeWindow = [[NativeInterfaceExtensions_ alloc] initWithWindowHandler:window];
}

NativeInterfaceExtensions::~NativeInterfaceExtensions() {
  [nativeWindow release];
}