Communication between C++ and Objective-C code

This article pretends to explain how to, in an easy way, communicate your C++ code with Objective-C in iOS projects. I will talk about Cocos2d-x, but it is also valid for just plain C++ and Objective-C.

Cocos2D-X is a cross platform game development framework that runs on iOS, Android and other devices. But even we can do anything related to a game with cocos, we will sometimes need to get access to platform specific APIs, like Facebook SDK, Admob or iAds.

In those cases we will need to implement those features using the device native language (Objective-C for iPhone or Java for Android). And while Cocos2D-X is a great engine, it lacks of tutorials... so today I am going to talk on how to communicate Objective-C with C++.

The process should be easy. As an example, I'll show a native iOS alert message from Cocos2D-X. In order to do this, follow these steps:

• Create a cpp file and a .h file. Call them iOsHelper.cpp and iOsHelper.h. These will be our bridge to Objective-C. 
• An important detail: Click the iOsHelper.cpp file and in the "Identity and type" window on Xcode (normally at the top right of the screen), choose Objective C++ source as the type.
• Define the body of the h and the cpp files. It will be:

#import "iOSHelper.h"

#import "iOSHelper_objc.h"

namespace iOSBridge{

    void iOSHelper::ShowAlert()

    {

        [iOSHelper_objc  ShowAlert];

    }   

    

}

and

#ifndef FightNow_iOSHelper_h

#define FightNow_iOSHelper_h

namespace iOSBridge{

    

    class  iOSHelper {

    

        public:

            void ShowAlert();

        

    };

    

}

#endif

• Now create an Objective-C class called iOSHelper_objc

@interface iOSHelper_objc : NSObject

    

    +(void) ShowAlert;

@end

and

#import "iOSHelper_objc.h"

@implementation iOSHelper_objc

+(void) ShowAlert{

    UIAlertView *alertDialog;

    

    alertDialog = [[UIAlertView alloc]

                   initWithTitle:@"You!" 

                   message:@"I know what you did!" 

                   delegate:nil

                   cancelButtonTitle: @"OK" 

                   otherButtonTitles: nil];

    

    [alertDialog show];

    [alertDialog release];

}

@end

Note that what we are doing is call the Objective-C function from a C++ class. This C++ class can be called from any of our Cocos2DX scenes for instance, so we are done :-)

As commented below (thanks guys for the comments) we need to instance the iOs "bridge" from our C++ class. One way to do it is the following:

Type* instanceName = [Type new];
[instanceName methodName:parameter];
[instanceName release];

Now you would probably want to handle what was the option chosen by the user right? In order to do this you might use callbacks, you can find a post about them in my colleague Xavier's blog post :-)

Hey, feel free to follow me on twitter if you are interested on discussing about Cocos2DX and game development :-) @jboschaiguade