how do I make an objective-c delegate for a cpp class?

Question

I'm stuck trying to combine openGL-es (xcode openGL game template with the ogles2tools library from powervr 3.0 sdk. My problem is the line of code where I load the effect file:

/* 
 Load the effect.
 We pass 'this' as an argument as we wish to receive callbacks as the PFX is loaded.
 This is optional and supplying NULL implies that the developer will take care
 of all texture loading and binding to to the Effect instead.
*/
if(m_pEffect->Load(*m_pEffectParser, "Effect", c_szPfxFile, NULL, uiUnknownUniforms, &error) != PVR_SUCCESS)
{
    NSLog(@"%s",error.c_str());
    return;
}

I'm supposed to pass a "this" pointer so I can receive the callbacks. The delegate method I need to implement is:

EPVRTError OGLES2IntroducingPFX::PVRTPFXOnLoadTexture(const CPVRTStringHash& TextureName, GLuint& uiHandle, unsigned int& uiFlags)
{
    /*
     This is an optional callback function for PVRTPFXEffect and can be used to automate 
     the texture loading process.
     If multiple effects are to be loaded and they share textures it would be
     prudent to have a caching system in place so texture memory is not wasted.
     Please see OGLES2MagicLantern for an example of this.
    */
    if(PVRTTextureLoadFromPVR(TextureName.String().c_str(), &uiHandle) != PVR_SUCCESS)
    return PVR_FAIL;

    return PVR_SUCCESS;
}

I guess the big issue for me is how do I go about providing a cpp delegate method in objective-c? I did some reading on this issue, but it seemed what I was reading was going the other way. That is, an objective-c delegate in cpp. It's pretty confusing, but here's my thought...

I create a cpp class the implements the method I need. I add that to my viewController class and pass the pointer to this cpp class in the m_pEffect->Load call. Does this seem correct?

Thanks.

P.S. Sorry if my code formatting is bad. I'm still learning.

Edit: Here's the example I found regarding mixing the objective-c and cpp. It seems really similar to what I want to do.

Update: Here's some additional info (requested by user1118321)

The CPP class which needs a delegate is CPVRTPFXEffect (PVRTPFXParserAPI.h - from powerVR SDK 3.0). I would add a link, but I'm not sure if this is allowed. Here's a link to the class header, but this version (and others on the web) did not include the pDelegate attribute for the load method. I'm assuming they are examples of a previous version. Let me know if it's okay to post this class file and I will do so.

I found a good example of what I think I'm supposed to do from reading this thread. So here's what I have so far:

My CPP delegate class...

    class myCppDelegate : public PVRTPFXEffectDelegate {
public:
    myCppDelegate() {};
    EPVRTError PVRTPFXOnLoadTexture(const CPVRTStringHash& TextureName, GLuint& uiHandle, unsigned int& uiFlags) { 
        return PVR_FAIL; 
    };

};

My Obj-C wrapper class (just modified from the example link above)...

struct RNWrapOpaque;

@interface RNWrap : NSObject {
struct RNWrapOpaque *_cpp;
}

- (id)init;
@end

implementation...

#import "RNWrap.h"
#import "Wrap.h"

@interface RNWrap ()
@property (nonatomic, readwrite, assign) RNWrapOpaque *cpp;
@end

@implementation RNWrap
@synthesize cpp = _cpp;


struct RNWrapOpaque
{
public:
    RNWrapOpaque() : wrap() {};
    myCppDelegate wrap;
};

- (id)init
{
    self = [super init];
    if (self != nil)
    {
        self.cpp = new RNWrapOpaque();
    }
    return self;
}

- (void)dealloc
{
    delete _cpp;
    _cpp = NULL;

//  [super dealloc];
}

@end

Basically I am able to compile the code and debug, but when the the CPVRTPFEffect class makes this call:

        if(pDelegate->PVRTPFXOnLoadTexture(pTexDesc->FileName, uiHandle, uiFlags) != PVR_SUCCESS)

I get EXC_BAD_ACCESS. I'm assuming it's not finding my callback method, because I set a breakpoint and the line never gets called.

Here's my updated code which calls CPVRTPFXEffect::Load using a bridge command for the delegate parameter.

    if(m_pEffect->Load(*m_pEffectParser, "Effect", c_szPfxFile,(__bridge myCppDelegate*)opaqueCppWrap, uiUnknownUniforms, &error) != PVR_SUCCESS)

Thanks for your help!

Update 2: The project uses ARC. Here's what my viewController interface looks like:

@interface ViewController : GLKViewController {
    ...
    RNWrap* opaqueCppWrap;
    ...
}

@property (strong) RNWrap *opaqueCppWrap;

Adding the @property didn't help with the EXC_BAD_ACCESS. I'm not sure how to "see" the value of pDelegate when I'm tracing the CPP code. Xcode doesn't reveal anything when I hover over the variable.

I added the following line of code to the CPVRTPFXEffect::Load method (just prior to the line where it crashes):

        *pReturnError += PVRTStringFromFormattedStr("Here is your class typeid: %s.\n", typeid(pDelegate).name());
        return PVR_FAIL;

This is what displayed in the debug output window:

Here is your class typeid: P21PVRTPFXEffectDelegate.

I'm not sure what the "P21" means (if anything), but it looks like I'm close to getting this working. I dunno, maybe this is as close as it gets. Still crashing and not finding my method.

Answer 1

First, you may want to look at the last article in the series on wrapping C++. Most of it has gotten much simpler in the latest versions of clang. You probably don't need half this code anymore. ObjC++ objects can now have private C++ properties without any tricks, while maintaining a pure-ObjC interface.

Here is how you want to think about this problem:

• Build a C++ object that is the delegate. Write all the code involved in setting up the delegation, etc, in C++. So when it says "pass a this pointer" you should really be passing a this pointer (because you should be doing this in the C++ code). The fact that you're doing a _bridge cast in a C++ call is a real hint something is going wrong.
• Let an ObjC own the C++ object as a property.
• Write the delegate callbacks in C++ inside the C++ object. If useful, you can let the C++ object then make calls into the ObjC object as needed, but it may be easier if the C++ object does all the delegate work.

Answer 2

I finally got this working, but had to remove the obj-c wrapper class from my viewController in order to do so. Here's what the code looks like:

ViewController.h

struct Opaque;

@interface ViewController : GLKViewController {

    ...
    //RNWrap* opaqueCppWrap;  // this didn't work 
    struct Opaque *opaqueCpp; // try this
    ...
}

ViewController.mm

// declare the Opaque structure
struct Opaque {
public:
    Opaque() : cppobject() {};

    myCppDelegate cppobject;
};

viewDidLoad

    // ... create opaque member on initialization
    opaqueCpp = new Opaque();

    //opaqueCppWrap = [[RNWrap alloc] init];  // old way of doing things using wrapper

pass the delegate to the Load method

// old way using bridge cast and wrapper 
//if(m_pEffect->Load(*m_pEffectParser, "Effect", c_szPfxFile,(__bridge myCppDelegate*)opaqueCppWrap, uiUnknownUniforms, &error) != PVR_SUCCESS)

// this works...
if(m_pEffect->Load(*m_pEffectParser, "Effect", c_szPfxFile, (myCppDelegate*)opaqueCpp, uiUnknownUniforms, &error) != PVR_SUCCESS)

Not sure why the wrapper class doesn't work, but I'm happy that my callback is working (appy no crashy!)

Phew, that was rough. Any thoughts/comments?