[Linker error] undefined reference to `HGLDevice::hClassName'

/**************************
 * Includes
 *
 **************************/
 
#include <windows.h>
#include <gl/gl.h>
 
class HGLDevice
{
      public: 
              HGLDevice(void);
              virtual ~HGLDevice(void){};
              
              
              float theta;
              HDC hDC;
              HGLRC hRC;
              HWND hWnd;
              static char hClassName[];
              
              void EnableOpenGL (HWND hWnd, HDC *hDC, HGLRC *hRC);
              void DisableOpenGL (HWND hWnd, HDC hDC, HGLRC hRC);
              void DrawGraph(void);
              static LRESULT CALLBACK WndProc (HWND hWnd, UINT message,WPARAM wParam, LPARAM lParam);
      
      private:
              
              
              
};
HGLDevice::HGLDevice()
{
      theta = 0.0f;
      strcpy(hClassName, "GLSample");
}
 
 
                      
/**************************
 * Function Declarations
 *
 **************************/
 
 
void HGLDevice::DrawGraph(void)
{
    glClearColor (0.0f, 0.0f, 0.0f, 0.0f);
    glClear (GL_COLOR_BUFFER_BIT);
    
    glPushMatrix ();
    glRotatef (theta, 0.0f, 0.0f, 1.0f);
    glBegin (GL_TRIANGLES);
    glColor3f (1.0f, 0.0f, 0.0f);   glVertex2f (0.0f, 1.0f);
    glColor3f (0.0f, 1.0f, 0.0f);   glVertex2f (0.87f, -0.5f);
    glColor3f (0.0f, 0.0f, 1.0f);   glVertex2f (-0.87f, -0.5f);
    glEnd ();
    glPopMatrix ();
    
    SwapBuffers (hDC);
    
    theta += 1.0f;
    Sleep (1);
}
 
/**************************
 * WinMain
 *
 **************************/
 
int WINAPI WinMain (HINSTANCE hInstance,
                    HINSTANCE hPrevInstance,
                    LPSTR lpCmdLine,
                    int iCmdShow)
{
    WNDCLASS wc;
            
    MSG msg;
    BOOL bQuit = FALSE;
    HGLDevice hglDev;
    
    
 
    /* register window class */
    wc.style = CS_OWNDC;
    wc.lpfnWndProc = hglDev.WndProc;
    wc.cbClsExtra = 0;
    wc.cbWndExtra = 0;
    wc.hInstance = hInstance;
    wc.hIcon = LoadIcon (NULL, IDI_APPLICATION);
    wc.hCursor = LoadCursor (NULL, IDC_ARROW);
    wc.hbrBackground = (HBRUSH) GetStockObject (BLACK_BRUSH);
    wc.lpszMenuName = NULL;
    wc.lpszClassName = hglDev.hClassName;
    RegisterClass (&wc);
 
    /* create main window */
    hglDev.hWnd = CreateWindow (
      "GLSample", "OpenGL Sample", 
      WS_CAPTION | WS_POPUPWINDOW | WS_VISIBLE,
      0, 0, 256, 256,
      NULL, NULL, hInstance, NULL);
 
    /* enable OpenGL for the window */
    hglDev.EnableOpenGL (hglDev.hWnd, &hglDev.hDC, &hglDev.hRC);
 
    /* program main loop */
    while (!bQuit)
    {
        /* check for messages */
        if (PeekMessage (&msg, NULL, 0, 0, PM_REMOVE))
        {
            /* handle or dispatch messages */
            if (msg.message == WM_QUIT)
            {
                bQuit = TRUE;
            }
            else
            {
                TranslateMessage (&msg);
                DispatchMessage (&msg);
            }
        }
        else
        {
            /* OpenGL animation code goes here */
 
            
            hglDev.DrawGraph();
            
        }
    }
 
    /* shutdown OpenGL */
    hglDev.DisableOpenGL (hglDev.hWnd, hglDev.hDC, hglDev.hRC);
 
    /* destroy the window explicitly */
    DestroyWindow (hglDev.hWnd);
 
    return msg.wParam;
}
 
 
/********************
 * Window Procedure
 *
 ********************/
 
LRESULT CALLBACK HGLDevice::WndProc (HWND hWnd, UINT message,
                          WPARAM wParam, LPARAM lParam)
{
    char hgl[10];
    strcpy (hgl, hClassName);
    switch (message)
    {
    case WM_CREATE:
        return 0;
    case WM_CLOSE:
        PostQuitMessage (0);
        return 0;
 
    case WM_DESTROY:
        return 0;
 
    case WM_KEYDOWN:
        switch (wParam)
        {
        case VK_ESCAPE:
            PostQuitMessage(0);
            return 0;
        }
        return 0;
 
    default:
        return DefWindowProc (hWnd, message, wParam, lParam);
    }
}
 
 
/*******************
 * Enable OpenGL
 *
 *******************/
void HGLDevice::EnableOpenGL (HWND hWnd, HDC *hDC, HGLRC *hRC)
{
    PIXELFORMATDESCRIPTOR pfd;
    int iFormat;
 
    /* get the device context (DC) */
    *hDC = GetDC (hWnd);
 
    /* set the pixel format for the DC */
    ZeroMemory (&pfd, sizeof (pfd));
    pfd.nSize = sizeof (pfd);
    pfd.nVersion = 1;
    pfd.dwFlags = PFD_DRAW_TO_WINDOW | 
      PFD_SUPPORT_OPENGL | PFD_DOUBLEBUFFER;
    pfd.iPixelType = PFD_TYPE_RGBA;
    pfd.cColorBits = 24;
    pfd.cDepthBits = 16;
    pfd.iLayerType = PFD_MAIN_PLANE;
    iFormat = ChoosePixelFormat (*hDC, &pfd);
    SetPixelFormat (*hDC, iFormat, &pfd);
 
    /* create and enable the render context (RC) */
    *hRC = wglCreateContext( *hDC );
    wglMakeCurrent( *hDC, *hRC );
 
}
 
 
 
/******************
 * Disable OpenGL
 *
 ******************/
 
void HGLDevice::DisableOpenGL (HWND hWnd, HDC hDC, HGLRC hRC)
{
    wglMakeCurrent (NULL, NULL);
    wglDeleteContext (hRC);
    ReleaseDC (hWnd, hDC);
}