Question

I have an class with instance functions (or methods?). From within an instance, I try to pass pointers to those functions to a library. The library expects static functions.

When I pass my pointers to the callback functions, the compiler complains that my functions are not static. I tried to put make them static, but if I do so, then I can’t access the instance fields from within the functions.

How could I go around this?

Similar question is : Using a C++ class member function as a C callback function where they suggest to put the method static. However I can’t do that, or I don’t see how I could.

Code

GlutController::GlutController (int argc, char **argv) {

   // stuff ..

   // Register callbacks
   glutSpecialFunc( OnSpecialKeys );  // Error, need static functions
   glutReshapeFunc( OnChangeSize );   // Error...
   glutDisplayFunc( OnRenderScene );  // Error...

   // stuff ..
}

GlutController::~GlutController() {

}

void GlutController::OnChangeSize(int aNewWidth, int aNewHeight){

   glViewport(0,0,aNewWidth, aNewHeight);
   mViewFrustrum.SetPerspective( APP_CAMERA_FOV,             // If this function is 
            float( aNewWidth ) / float( aNewHeight ),        // static, this won't 
            APP_CAMERA_NEAR,                                 // work
            APP_CAMERA_FAR );
   mProjectionMatrixStack.LoadMatrix(                        // Same here
            mViewFrustrum.GetProjectionMatrix() );
   mTransformPipeline.SetMatrixStacks(mModelViewMatrixStack, // Same here  
            mProjectionMatrixStack);

}

void GlutController::OnRenderScene(void){
   mGeometryContainer.draw();                                // Won't work if static
}

void GlutController::OnSpecialKeys(int key, int x, int y){
   mGeometryContainer.updateKeys(key);                       // Won't work if static
}

Disclaimer : I just begun C++. I read all of Accelerated C++ and this is my first project to try out the language. My background is in Java.

Answer 1

What you are trying to do is not possible. This is glut‘s fault actually.

The thing is this:

• glut wants to call a function, without giving it data,
• You want your function to use some data,

which are conflicting needs. I believe glut decided you can safely use global variables.

So, one solution is to use static functions, with static data. Or a better solution would be to switch to SDL.