Callback function in freeglut from object

Question

I'm using MSVC++ and freeglut in order to use openGL. Now, I have a class called Camera, which is pretty simple, but it also holds the function for the reshaping of my window.

My question is: how can I set glutReshapeFunc(void (*callback)(int,int)) to my function in my camera?

I have the following code, which won't work, because of a compiler error:

int main(int argc, char **argv)
{
    Camera *camera = new Camera();
    glutReshapeFunc(camera->ReshapeCamera);
}

and my Camera class looks like this in Camera.h:

class Camera
{
public:
    Camera(void);
    ~Camera(void);
    void ReshapeCamera(int width, int height);
};

Maybe this is just a more general callback question, but the only thing I found on the internet was creating some wrapper class around the callback. But it doesn't look like this should be so hard. Thanks in advance.

Answer 1

There's no clean way to do this. C does not know about objects, so pointers-to-members don't work. C does not do templates, so functors don't work. The API does not even allow you to pass an arbitrary void *userData to your callback, so you can't pass the object around in that way either.

So, indeed, you'll have to create a wrapper function that is not a member function, and you'll have to give it access to the Camera object instance somehow, through a static and/or global variable.

If there can be multiple objects listening for this event, you could create a singleton class that allows you to register arbitrary listeners.