Thread: using c++ objects from C

I'm trying to use c++ objects from my c code. I need to create the c++ object, and make multiple calls from the c code to the methods in the class.

Here's what I've implemented:

In the C file I created a structure to be the pointer to the c++ class:

Code:

/* create a pointer that will actually be a pointer to the c++ object */
struct myClass* pMyStruct;

In that same file, I made a call to a c function in the c++ file passing that pointer:

Code:

call_class_init(pMyStruct);

In the cpp file is the c++ wrapper function as well as the myClass class. Is this right? Is this how the object is created and the reference to it is set?:

Code:

extern "C"  void call_class_init(myClass* p1)
 {
                 myClass fred;
                p1= &fred;
                p1->init_class_obj();
}

And then in the init_class_obj() method I do a bunch of class implementation stuff of the c++ class.

So then in the C file I can make other calls to the c++ class by having more extern C methods to call using the same pointer:

Code:

set_count(pMyStruct, 4);

and in the c++ file:

Code:

extern "C" void set_count(myClass* p1, int count)
{
         p1->set_count(count);
}

Does this all make sense? Is there an easier way? With my implementation the 1st call works, and then when I make another call to the c++ object, it fails.

any help, sample code, or links would be great. I did a search for "mix c c++" and found a few results via google.

Thanks!

I found that example before.

But my scenario is a little different. In that example, the c++ class creates and instance of Fred, and calls the c function passing the instance of the c++ object.

I need the opposite with the c function to call the c++ class, and the it returns a reference to the c++ object.

Salem,

Thanks for your reply. Sorry, when I said reference I meant pointer.
The main is in c++. And I have to use C since it's pre-existing code that has to be re-used.

So do I just create a function in the c++ file that returns a pointer to a new c++ object? But if I create that object in that function, it'll be destroyed once the function completes, right? And I don't want to use global objects.

I suppose the code I posted at the start isn't much help. I've changed the code somewhat so the c++ main() does all the work, and passes the object pointers to the c code.

here's the main:

Code:

int main(int argc, char* argv[])
{
        TToiVideoMixerCaller* videoMixer;
......
......        	
  	videoMixer = new TToiVideoMixerCaller(&ipcClient, videoMixerAddress);
	videoMixer->SetFullVideoWindow();               
	video_driver_class kvd;
	//set the videoMixer of the new object
	kvd.set_video_mixer(videoMixer);
	//call the init method
	kvd.init_video_driver_obj();
	//pass the pointer to the c function 
	set_kvd_pointer(&kvd);
       //call the c code
	the_main (argc, argv);
}

So the main creates a videoMixer object as well as a video_driver_class. It then passes a pointer of the videoMixer to the video_driver_class.

Here's the c++ file:

Code:

	class video_driver_class
	{
	    TToiVideoMixerCaller* videoMixer;
	  public:
	  	virtual void init_video_driver_obj();
	    virtual int get_colour_key();
	    virtual void set_video_mixer(TToiVideoMixerCaller *pVmx);
	    video_driver_class();
            video_driver_class(const video_driver_class&);
	  private:
          protected:
	};
    void video_driver_class::set_video_mixer(TToiVideoMixerCaller *pVmx)
    {
    	videoMixer = pVmx;
    }
    int video_driver_class::get_colour_key()
    {
    	fprintf(stderr, "entered get_colour_key\n");
    	int colourKey;
    	videoMixer->GetColorKey(colourKey);	
    	return colourKey;
    	fprintf(stderr, "leaving get_colour_key\n");
    }
    void video_driver_class::init_video_driver_obj() 
    {
        videoMixer->SetFullVideoWindow();
	int colourKey;
	videoMixer->GetColorKey(colourKey);
	fprintf(stderr,"videoMixer colourKey = %d\n",colourKey);
	fprintf(stderr, "leaving init_kreatel_video_driver_obj()\n");
  }

extern "C" 
{
	int get_colourkey_kvd_video_window(video_driver_class* kvd)
	{
	 	return kvd->get_colour_key();
	}
}

And here's the c file:

Code:

#include "video_driver_obj_mapping.h"
/* create a pointer that will actually be a pointer to the c++ object */
     struct video_driver_class* pVideoStruct;

void set_kvd_pointer(video_driver_class* kvd)
{
	pVideoStruct = kvd;
}

BOOLEAN
driver_PlatformPorts_device_video_init ()
{
    	fprintf(stderr, "video window colour key = %d\n" + get_colourkey_kvd_video_window(pVideoStruct));
    }

    return TRUE;
}

So, when the main() calls: kvd.init_video_driver_obj(), I see the colourkey. But then, when the c function calls: get_colourkey_kvd_video_window(pVideoStruct) The application dies.

I'd appreciate any and all help. Thanks!

looks like I found the problem. I guess I was coding with my Java hat on:
fprintf(stderr, "video window colour key = %d\n" + get_colourkey_kvd_video_window(pVideoStruct));

Duhh... notice the '+' symbol instead of the comma?

Arrgg, I hate those ones that the compiler doesn't catch.