freeglutのコールバック関数でメンバ変数を呼び出す

#include <Windows.h>
#include <gl/GL.h>
#include <gl/freeglut.h>
#include <cstdio>

#pragma comment(lib,"opengl32.lib")
#pragma comment(lib,"freeglut.lib")


class CfreeglutWindow {

  // コールバック関数から呼ばれるメンバ関数群 //////////////////////////
  static void mouse_callback(int button, int state, int x, int y) {
    CfreeglutWindow* pwin = (CfreeglutWindow*)glutGetWindowData();
    pwin->mouse(button, state, x, y);
  }
  static void motion_callback(int x, int y) {
    CfreeglutWindow* pwin = (CfreeglutWindow*)glutGetWindowData();
    pwin->motion(x, y);
  }
  static void mouseWheel_callback(int button, int dir, int x, int y) {
    CfreeglutWindow* pwin = (CfreeglutWindow*)glutGetWindowData();
    pwin->mouse(button, dir, x, y);
  }

  static void disp_callback() {
    CfreeglutWindow* pwin = (CfreeglutWindow*)glutGetWindowData();
    pwin->disp();
  }
  static void key_callback(unsigned char key, int x, int y) {
    CfreeglutWindow* pwin = (CfreeglutWindow*)glutGetWindowData();
    pwin->key(key,x,y);
  }

  //普通のメンバ変数
  int width;
  int height;
  float xpos,ypos;
public:

  CfreeglutWindow() {
    xpos = 0.0;
    ypos = 0.0;
  }

  void init(int argc, char** argv, const char* title) {
    glutInit(&argc, argv);
    glutInitWindowPosition(100, 50);
    glutInitWindowSize(400, 300);
    glutInitDisplayMode(GLUT_SINGLE | GLUT_RGBA);

    width = 400;
    height = 300;

    glutCreateWindow(title);
    glutDisplayFunc(disp_callback);
    glutMouseFunc(mouse_callback);
    glutMotionFunc(motion_callback);
    glutMouseWheelFunc(mouseWheel_callback);
    glutKeyboardFunc(key_callback);

    glutSetWindowData(this);


  }
  void loop() {
    glutMainLoop();
  }

  // 描画
  void disp(void) {
    glClearColor(0, 0, 0, 0);
    glClear(GL_COLOR_BUFFER_BIT);
    glViewport(0, 0, width, height);

    float v = 0.7;

    glPushMatrix();
    glLoadIdentity();
    glTranslatef(xpos, ypos, 0.0);
    glBegin(GL_TRIANGLES);
    glColor3d(1, 1, 1);
    glVertex2f(-v, -v);
    glColor3d(0, 1, 1);
    glVertex2f( v, -v);
    glColor3d(0, 0, 1);
    glVertex2f( v,  v);
    glEnd();
    glPopMatrix();

    glFlush();
    glutSwapBuffers();
  }

  // 普通キーの入力
  void key(unsigned char key, int x, int y) {
    if (key == 'a') {
      xpos -= 0.2;
    }
    if (key == 's') {
      xpos += 0.2;
    }
    glutPostRedisplay();
  }

  // クリック
  void mouse(int button, int state, int x, int y) {

    if (button == GLUT_LEFT_BUTTON && state == GLUT_DOWN) {
      ypos += 0.2;
    }
    if (button == GLUT_RIGHT_BUTTON && state == GLUT_DOWN) {
      ypos -= 0.2;
    }
    glutPostRedisplay();

  }

  //ドラッグ
  void motion(int x, int y){}

  // ホイール
  void mouseWheel(int button, int dir, int x, int y)
  {
    if (dir > 0){}
    else{}
    return;
  }

};

int main(int argc, char** argv)
{
  CfreeglutWindow cfw;

  cfw.init(argc, argv, "freeglut");

  cfw.loop();

  return 0;
}