OpenGL pointing glutSpecialFunc to member function - c++

Question

My OpenGL function glutSpecialFunc requires a void function pointer with 3 int parameters. This is easy to do with global functions by simply
glutSpecialFunc(processArrowKeys);
But i want to make it point to a member-function in a struct in another file like so:
inputs.h
struct Keyboard {
bool leftMouseDown;
bool keyRight, keyLeft, keyUp, keyDown;
Keyboard() : leftMouseDown(false), keyRight(false), keyLeft(false), keyUp(false), keyDown(false) {
}
void Keyboard::processArrowKeys(int key, int x, int y) {
// process key strokes
if (key == GLUT_KEY_RIGHT) {
keyRight = true;
keyLeft = false;
keyUp = false;
keyDown = false;
}
else if (key == GLUT_KEY_LEFT) {
keyRight = false;
keyLeft = true;
keyUp = false;
keyDown = false;
}
else if (key == GLUT_KEY_UP) {
keyRight = false;
keyLeft = false;
keyUp = true;
keyDown = false;
}
else if (key == GLUT_KEY_DOWN) {
keyRight = false;
keyLeft = false;
keyUp = false;
keyDown = true;
}
}
};
main.cpp
#include "inputs.h"
Keyboard keyboard;
...
int main(int argc, char **argv) {
...
// callbacks
glutDisplayFunc(displayWindow);
glutReshapeFunc(reshapeWindow);
glutIdleFunc(updateScene);
glutSpecialFunc(&keyboard.processArrowKeys); // compiler error: '&': illegal operation on bound member function expression
glutMouseFunc(mouseButton);
glutMotionFunc(mouseMove);
glutMainLoop();
return 0;
}
Any idea how to solve this compiler error?

Answer 1

You can't do that directly because member functions have an implicit this pointer which has to be passed somehow through the call-chain. Instead you create an intermediary function that will forward the call to the right place:
void processArrowKeys(int key, int x, int y) {
keyboard.processArrowKeys(key, x, y);
}
int main() {
// ...
glutSpecialFunc(processArrowKeys);
// ...
}
keyboard in your code appears to be global so it is going to work. If you ever want to have a non-global state, then you will have to use a user-data pointer that some GLUT implementations support as an extension (including FreeGLUT and OpenGLUT):
void processArrowKeys(int key, int x, int y) {
Keyboard *k = (Keyboard*)glutGetWindowData();
k->processArrowKeys(key, x, y);
}
int main() {
// ...
glutSpecialFunc(processArrowKeys);
glutSetWindowData(&keyboard);
// ...
}