Custom window proc in a separate file - c++

Question

I'm currently learning winapi, and would like to get some advice on how to set up a custom control using different window procedures.
Let's say I have 20 buttons. I want each button to respond differently when the mouse hovers over it. Say, an "exit" button draws a red rectangle when hovered, or blue when some other button is hovered.
So, I have set up a custom control procedure that handles mouse clicks, mouse hover, and such, and is stored in custom.cpp. In main.cpp, there is a MainProc() that "links/assigns" hwndButton to ButtonProc() using
CustomProcHandler = (WNDPROC)SetWindowLong(hwndButton, GWL_WNDPROC, (long)CustomProc)
main.h:
#include <windows.h>
#include <iostream>
using namespace std;
const char g_szClassName[] = "Applicaton";
static HWND hwnd, hwndButton;
static HINSTANCE hInst;
static WNDPROC buttonProcHandler;
LRESULT CALLBACK MainProc(HWND, UINT, WPARAM, LPARAM);
LRESULT CALLBACK ButtonProc(HWND, UINT, WPARAM, LPARAM);
main.cpp:
#include "main.h"
int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR nCmdLine, int nCmdShow){
WNDCLASSEX wc;
MSG msg;
hInst = hInstance;
wc.cbSize = sizeof(WNDCLASSEX);
wc.style = 0;
wc.lpfnWndProc = WndProc;
wc.cbClsExtra = 0;
wc.cbWndExtra = 0;
wc.hInstance = hInst;
wc.hIcon = LoadIcon(hInst, 0);
wc.hCursor = LoadCursor(NULL, IDC_ARROW);
wc.hbrBackground = CreateSolidBrush(RGB(20, 20, 20));
wc.lpszMenuName = 0;
wc.lpszClassName = g_szClassName;
wc.hIconSm = LoadIcon(hInst, 0);
RegisterClassEx(&wc);
hwnd = CreateWindowEx(
WS_EX_CLIENTEDGE,
g_szClassName,
"The title of my window",
WS_OVERLAPPEDWINDOW,
CW_USEDEFAULT, CW_USEDEFAULT, 600, 400,
NULL, NULL, hInst, NULL);
ShowWindow(hwnd, 1);
UpdateWindow(hwnd);
while (GetMessage(&msg, NULL, 0, 0)) {
TranslateMessage(&msg);
DispatchMessage(&msg);
}
return msg.wParam;
}
LRESULT CALLBACK WndProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam){
switch(msg){
case WM_CREATE:
button = CreateWindow("button", 0, WS_CHILD | WS_VISIBLE | BS_OWNERDRAW, 10, 10, 32, 32, hwnd, 0, hInst, 0);
buttonProcHandler = (WNDPROC)SetWindowLong(hwndButton, GWL_WNDPROC, (long)ButtonProc);
break;
case WM_MOUSEMOVE:
break;
case WM_CLOSE:
DestroyWindow(hwnd);
break;
case WM_DESTROY:
PostQuitMessage(0);
break;
default:
return DefWindowProc(hwnd, msg, wParam, lParam);
}
return 0;
}
custom.cpp:
#include "main.h"
LRESULT CALLBACK ButtonProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam){
switch(msg){
case WM_CREATE:
break;
case WM_MOUSEMOVE:
break;
case WM_LBUTTONDOWN:
break;
default:
return CallWindowProc(buttonProcHandler, hwnd, msg, wParam, lParam);
}
return 0;
}
The problem is the button disappears... but when I move buttonProc() inside of main.cpp then everything works fine.
So, I am guessing I did something wrong when declaring global variables like static WNDPROC buttonProcHandler.
I know what I am doing is entirely wrong, and that there is a better way of doing it. I just don't know what that is.
Can anyone can help/teach me the standard way of creating custom procedures?