Adding C++ Object to Objective-C Class

Question

I'm trying to mix C++ and Objective-C, I've made it most of the way but would like to have a single interface class between the Objective-C and C++ code. Therefore I would like to have a persistent C++ object in the ViewController interface.

This fails by forbidding the declaration of 'myCppFile' with no type:

#import <UIKit/UIKit.h>
#import "GLView.h"
#import "myCppFile.h"

@interface GLViewController : UIViewController <GLViewDelegate>
{
    myCppFile cppobject;
}

@end

However this works just fine in the .mm implementation file (It doesn't work because I want cppobject to persist between calls)

#import "myCppFile.h"
@implementation GLViewController
- (void)drawView:(UIView *)theView
{
    myCppFile cppobject;
    cppobject.draw();
}

Georg Fritzsche · Accepted Answer · 2010-03-20 00:12:35Z

29

You should use opaque pointers and only include C++ headers in the file that implements your Objective-C class. That way you don't force other files that include the header to use Objective-C++:

// header:
#import <UIKit/UIKit.h>
#import "GLView.h"

struct Opaque;

@interface GLViewController : UIViewController <GLViewDelegate>
{
    struct Opaque* opaque;
}
// ...
@end

// source file:
#import "myCppFile.h"

struct Opaque {
    myCppFile cppobject;
};

@implementation GLViewController
// ... create opaque member on initialization

- (void)foo
{
    opaque->cppobject.doSomething();
}
@end

edited Mar 20, 2010 at 0:12

answered Feb 14, 2010 at 19:17

Georg Fritzsche

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7 Comments

Winder Over a year ago

Thanks, the other suggestions didn't seem to work and I ended up declaring my object as an "id*" then casting it to the correct type everywhere I wanted to use it. This seems like the same idea, but is much cleaner since I don't have to do all the extra casts.

Georg Fritzsche Over a year ago

I'd always prefer opaque pointers over casts - it might be a bit more work, but you get full type safety back.

Rob Napier Over a year ago

I've been playing with opaque pointers for this, which I use extensively for C/C++ wrapping, but so far I haven't seen the real benefit. Can you compare this style (along with some code showing how "opaque" is allocated and released) to the void* based style here: robnapier.net/blog/wrapping-c-objc-20

Georg Fritzsche Over a year ago

Note: Rob Napier posted a nice update on wrapping C++.

dubbeat Over a year ago

thanks a million for this code example. I was 2 steps shy of throwing my mac out the window.

|

oefe · Accepted Answer · 2010-02-14 18:45:36Z

3

Make sure that all files that include GLViewController.h are Objective-C++ sources (*.mm).

When you include C++ code in the header of your view controller, all sources that import this header must be able to understand it, so they must be in Objective-C++

answered Feb 14, 2010 at 18:45

oefe

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Comments

pizzamonster · Accepted Answer · 2013-08-11 15:52:27Z

2

You need to declare the C++ objects in an interface block in your .mm file.

In .mm:

#include "SomeCPPclass.h"

@interface SomeDetailViewController () {
    SomeCPPclass*    _ipcamera;
}
@property (strong, nonatomic) UIPopoverController *masterPopoverController;
- (void)blabla;
@end

answered Aug 11, 2013 at 15:52

pizzamonster

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1 Comment

goelectric Over a year ago

Take close note of the () - that's the black(et) magic that makes this work :)

JanX2 · Accepted Answer · 2012-05-09 11:51:46Z

0

I think you need to set the following flag to true in your project settings:

GCC_OBJC_CALL_CXX_CDTORS = YES

This should allow you to instantiate C++ objects in your Objective-C classes.

edited May 9, 2012 at 11:51

JanX2

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answered Feb 14, 2010 at 17:41

Martin Cote

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1 Comment

Winder Over a year ago

This didn't seem to work. I'm using Xcode 3.2.1 and the 3.0 iphone SDK.

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