[c++] Calling Objective-C method from C++ member function?

I have a class (EAGLView) which calls a member function of a C++ class without problems. Now, the problem is that I need to call in that C++ class a objective-C function [context renderbufferStorage:GL_RENDERBUFFER fromDrawable:(CAEAGLLayer*)self.layer]; which I cannot do in C++ syntax.

I could wrap this Objective-C call to the same Objective-C class which in the first place called the C++ class, but then I need to somehow call that method from C++, and I cannot figure out how to do it.

I tried to give a pointer to EAGLView object to the C++ member function and include the "EAGLView.h" in my C++ class header but I got 3999 errors..

So.. how should I do this? An example would be nice.. I only found pure C examples of doing this.

You can compile your code as Objective-C++ - the simplest way is to rename your .cpp as .mm. It will then compile properly if you include EAGLView.h (you were getting so many errors because the C++ compiler didn't understand any of the Objective-C specific keywords), and you can (for the most part) mix Objective-C and C++ however you like.

You need to make your C++ file be treated as Objective-C++. You can do this in xcode by renaming foo.cpp to foo.mm (.mm is the obj-c++ extension). Then as others have said standard obj-c messaging syntax will work.

@DawidDrozd's answer above is excellent.

I would add one point. Recent versions of the Clang compiler complain about requiring a "bridging cast" if attempting to use his code.

This seems reasonable: using a trampoline creates a potential bug: since Objective-C classes are reference counted, if we pass their address around as a void *, we risk having a hanging pointer if the class is garbage collected up while the callback is still active.

Solution 1) Cocoa provides CFBridgingRetain and CFBridgingRelease macro functions which presumably add and subtract one from the reference count of the Objective-C object. We should therefore be careful with multiple callbacks, to release the same number of times as we retain.

// C++ Module
#include <functional>

void cppFnRequiringCallback(std::function<void(void)> callback) {
        callback();
}

//Objective-C Module
#import "CppFnRequiringCallback.h"

@interface MyObj : NSObject
- (void) callCppFunction;
- (void) myCallbackFn;
@end

void cppTrampoline(const void *caller) {
        id callerObjC = CFBridgingRelease(caller);
        [callerObjC myCallbackFn];
}

@implementation MyObj
- (void) callCppFunction {
        auto callback = [self]() {
                const void *caller = CFBridgingRetain(self);
                cppTrampoline(caller);
        };
        cppFnRequiringCallback(callback);
}

- (void) myCallbackFn {
    NSLog(@"Received callback.");
}
@end

Solution 2) The alternative is to use the equivalent of a weak reference (ie. no change to the retain count), without any additional safety.

The Objective-C language provides the __bridge cast qualifier to do this (CFBridgingRetain and CFBridgingRelease seem to be thin Cocoa wrappers over the Objective-C language constructs __bridge_retained and release respectively, but Cocoa does not appear to have an equivalent for __bridge).

The required changes are:

void cppTrampoline(void *caller) {
        id callerObjC = (__bridge id)caller;
        [callerObjC myCallbackFn];
}

- (void) callCppFunction {
        auto callback = [self]() {
                void *caller = (__bridge void *)self;
                cppTrampoline(caller);
        };
        cppFunctionRequiringCallback(callback);
}

Step 1

Create a objective c file(.m file) and it's corresponding header file.

// Header file (We call it "ObjCFunc.h")

#ifndef test2_ObjCFunc_h
#define test2_ObjCFunc_h
@interface myClass :NSObject
-(void)hello:(int)num1;
@end
#endif

// Corresponding Objective C file(We call it "ObjCFunc.m")

#import <Foundation/Foundation.h>
#include "ObjCFunc.h"
@implementation myClass
//Your objective c code here....
-(void)hello:(int)num1
{
NSLog(@"Hello!!!!!!");
}
@end

Step 2

Now we will implement a c++ function to call the objective c function that we just created! So for that we will define a .mm file and its corresponding header file(".mm" file is to be used here because we will be able to use both Objective C and C++ coding in the file)

//Header file(We call it "ObjCCall.h")

#ifndef __test2__ObjCCall__
#define __test2__ObjCCall__
#include <stdio.h>
class ObjCCall
{
public:
static void objectiveC_Call(); //We define a static method to call the function directly using the class_name
};
#endif /* defined(__test2__ObjCCall__) */

//Corresponding Objective C++ file(We call it "ObjCCall.mm")

#include "ObjCCall.h"
#include "ObjCFunc.h"
void ObjCCall::objectiveC_Call()
{
//Objective C code calling.....
myClass *obj=[[myClass alloc]init]; //Allocating the new object for the objective C   class we created
[obj hello:(100)];   //Calling the function we defined
}

Step 3

Calling the c++ function(which actually calls the objective c method)

#ifndef __HELLOWORLD_SCENE_H__
#define __HELLOWORLD_SCENE_H__
#include "cocos2d.h"
#include "ObjCCall.h"
class HelloWorld : public cocos2d::Layer
{
public:
// there's no 'id' in cpp, so we recommend returning the class instance pointer
static cocos2d::Scene* createScene();
// Here's a difference. Method 'init' in cocos2d-x returns bool, instead of returning  'id' in cocos2d-iphone
virtual bool init();
// a selector callback
void menuCloseCallback(cocos2d::Ref* pSender);
void ObCCall();  //definition
// implement the "static create()" method manually
CREATE_FUNC(HelloWorld);
};
#endif // __HELLOWORLD_SCENE_H__

//Final call

#include "HelloWorldScene.h"
#include "ObjCCall.h"
USING_NS_CC;
Scene* HelloWorld::createScene()
{
// 'scene' is an autorelease object
auto scene = Scene::create();
// 'layer' is an autorelease object
auto layer = HelloWorld::create();
// add layer as a child to scene
scene->addChild(layer);
// return the scene
return scene;
}
// on "init" you need to initialize your instance
bool HelloWorld::init()
{
//////////////////////////////
// 1. super init first
if ( !Layer::init() )
{
    return false;
}
Size visibleSize = Director::getInstance()->getVisibleSize();
Vec2 origin = Director::getInstance()->getVisibleOrigin();

/////////////////////////////
// 2. add a menu item with "X" image, which is clicked to quit the program
//    you may modify it.

// add a "close" icon to exit the progress. it's an autorelease object
auto closeItem = MenuItemImage::create(
                                       "CloseNormal.png",
                                       "CloseSelected.png",
                                       CC_CALLBACK_1(HelloWorld::menuCloseCallback,  this));

closeItem->setPosition(Vec2(origin.x + visibleSize.width - closeItem->getContentSize().width/2 ,
                            origin.y + closeItem->getContentSize().height/2));

// create menu, it's an autorelease object
auto menu = Menu::create(closeItem, NULL);
menu->setPosition(Vec2::ZERO);
this->addChild(menu, 1);

/////////////////////////////
// 3. add your codes below...

// add a label shows "Hello World"
// create and initialize a label

auto label = Label::createWithTTF("Hello World", "fonts/Marker Felt.ttf", 24);

// position the label on the center of the screen
label->setPosition(Vec2(origin.x + visibleSize.width/2,
                        origin.y + visibleSize.height - label- >getContentSize().height));
// add the label as a child to this layer
this->addChild(label, 1);
// add "HelloWorld" splash screen"
auto sprite = Sprite::create("HelloWorld.png");
// position the sprite on the center of the screen
sprite->setPosition(Vec2(visibleSize.width/2 + origin.x, visibleSize.height/2 +     origin.y));
// add the sprite as a child to this layer
this->addChild(sprite, 0);
this->ObCCall();   //first call
return true;
}
void HelloWorld::ObCCall()  //Definition
{
ObjCCall::objectiveC_Call();  //Final Call  
}
void HelloWorld::menuCloseCallback(Ref* pSender)
{
#if (CC_TARGET_PLATFORM == CC_PLATFORM_WP8) || (CC_TARGET_PLATFORM ==   CC_PLATFORM_WINRT)
MessageBox("You pressed the close button. Windows Store Apps do not implement a close    button.","Alert");
return;
#endif
Director::getInstance()->end();
#if (CC_TARGET_PLATFORM == CC_PLATFORM_IOS)
exit(0);
#endif
}

Hope this works!

Also, you can call into Objective-C runtime to call the method.

Sometimes renaming .cpp to .mm is not good idea, especially when project is crossplatform. In this case for xcode project I open xcode project file throught TextEdit, found string which contents interest file, it should be like:

/* OnlineManager.cpp */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.cpp; path = OnlineManager.cpp; sourceTree = "<group>"; };

and then change file type from sourcecode.cpp.cpp to sourcecode.cpp.objcpp

/* OnlineManager.cpp */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = **sourcecode.cpp.objcpp**; path = OnlineManager.cpp; sourceTree = "<group>"; };

It is equivalent to rename .cpp to .mm

The easiest solution is to simply tell Xcode to compile everything as Objective C++.

Set your project or target settings for Compile Sources As to Objective C++ and recompile.

Then you can use C++ or Objective C everywhere, for example:

void CPPObject::Function( ObjectiveCObject* context, NSView* view )
{
   [context renderbufferStorage:GL_RENDERBUFFER fromDrawable:(CAEAGLLayer*)view.layer]
}

This has the same affect as renaming all your source files from .cpp or .m to .mm.

There are two minor downsides to this: clang cannot analyse C++ source code; some relatively weird C code does not compile under C++.

You can mix C++ in with Objectiv-C (Objective C++). Write a C++ method in your Objective C++ class that simply calls [context renderbufferStorage:GL_RENDERBUFFER fromDrawable:(CAEAGLLayer*)self.layer]; and call it from your C++.

I haven't tried it before my self, but give it a shot, and share the results with us.