C++ Callbacks - Unresolved external symbol - c++

Question

I'm trying to write a error handling system in c++, which calls a callback function whenever an error rises. However, the linker always complains about an unresolved external symbol whenever I try to run the project.
GEWindow.h:
#include <Windows.h>
#include <stdlib.h>
#include <string.h>
#include <tchar.h>
#include <functional>
#ifndef GE_WINDOW
#define GE_WINDOW
//class GEWindow;
//typedef void(*GE_ERROR_CALLBACK)(HWND, UINT, LPCWSTR);
#define GE_WINDOW_REGISTERATION_FAILED 1
#define GE_WINDOW_CREATION_FAILED 2
class GEWindow {
public:
typedef std::function<void(GEWindow *, UINT, LPCWSTR)> GE_ERROR_CALLBACK;
static int geCreateWindow(GEWindow *window);
static LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam);
static inline void setErrorCallback(GE_ERROR_CALLBACK fun) { errCallback = fun; };
inline HWND getHWnd() { return hWnd; };
inline int getWidth() { return _width; };
inline int getHeight() { return _height; };
inline LPCWSTR getTitle() { return _title; };
inline void setWidth(int width) { _width = width; };
inline void setHeight(int height) { _height = height; };
inline void setTitle(LPCWSTR title) { _title = title; };
private:
static GE_ERROR_CALLBACK errCallback;
HWND hWnd;
int _width = 400, _height = 400;
LPCWSTR _title;
};
#endif
GEWindow.cpp:
#include "GEWindow.h"
int GEWindow::geCreateWindow(GEWindow *window) {
HINSTANCE hInstance = GetModuleHandle(NULL);
TCHAR szWindowClass[] = _T("GEWindow");
WNDCLASSEX wcex;
wcex.cbSize = sizeof(WNDCLASSEX);
wcex.style = CS_HREDRAW | CS_VREDRAW;
wcex.lpfnWndProc = GEWindow::WndProc;
wcex.cbClsExtra = 0;
wcex.cbWndExtra = 0;
wcex.hInstance = hInstance;
wcex.hIcon = LoadIcon(hInstance, MAKEINTRESOURCE(IDI_APPLICATION));
wcex.hCursor = LoadCursor(NULL, IDC_ARROW);
wcex.hbrBackground = (HBRUSH)(COLOR_WINDOW + 1);
wcex.lpszMenuName = NULL;
wcex.lpszClassName = szWindowClass;
wcex.hIconSm = LoadIcon(wcex.hInstance, MAKEINTRESOURCE(IDI_APPLICATION));
if (!RegisterClassEx(&wcex))
{
//if (!errCallback)
// errCallback(window, GE_WINDOW_REGISTERATION_FAILED, _T("Failed to register the GE window"));
MessageBox(NULL, _T("Failed to register"), _T("Error"), NULL);
return NULL;
}
window->hWnd = CreateWindow(
szWindowClass,
window->_title,
WS_OVERLAPPEDWINDOW,
CW_USEDEFAULT, CW_USEDEFAULT,
window->_width, window->_height,
NULL,
NULL,
hInstance,
NULL);
if (!window->hWnd) {
//if (!errCallback)
// errCallback(window, GE_WINDOW_CREATION_FAILED, _T("Failed to create the GE window"));
MessageBox(NULL, _T("Failed to create"), _T("Error"), NULL);
return NULL;
}
ShowWindow(window->hWnd, SW_SHOW);
UpdateWindow(window->hWnd);
MSG msg;
while (GetMessage(&msg, NULL, 0, 0))
{
TranslateMessage(&msg);
DispatchMessage(&msg);
}
return 1;
}
LRESULT CALLBACK GEWindow::WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam) {
switch (message)
{
case WM_PAINT:
break;
case WM_DESTROY:
PostQuitMessage(0);
break;
default:
return DefWindowProc(hWnd, message, wParam, lParam);
break;
}
return 0;
}
main.cpp:
#include "GEWindow.h"
static void error_callback(GEWindow *window, UINT errorCode, LPCWSTR message) {
MessageBox(window->getHWnd(), message, _T("Error"), NULL);
}
int WINAPI WinMain(HINSTANCE hInstance,
HINSTANCE hPrevInstance,
LPSTR lpCmdLine,
int nCmdShow) {
GEWindow window;
window.setWidth(500);
window.setHeight(500);
window.setTitle(_T("lala lulu"));
/*
error LNK2001: unresolved external symbol "private: static class std::function<void __cdecl(class GEWindow *, unsigned int, wchar_t const *)> GEWindow::errCallback" (?errCallback#GEWindow##0V?$function#$$A^AXPAVGEWindow##IPB_W#Z#std##A)
*/
window.setErrorCallback(error_callback);
GEWindow::geCreateWindow(&window);
}

Answer 1

When using static member variables, you only declare them in the class, you need to define them as well. That definition has to be done in a source file, like this:
GEWindow::GE_ERROR_CALLBACK GEWindow::errCallback;

Answer 2

static void GeWindows::error_callback() will fix this problem..I found it after days by days by the help of "Tony The Lion" (credit...)