Null handle error - c++

Question

I am working on a Win32++ application that has a listview in the main window. This is my code:
HWND CarsListView = NULL;
switch (message)
{
case WM_SHOWWINDOW:
CarsListView = CreateListView(hWnd);
ShowWindow(CarsListView, SW_SHOW);
break;
case WM_SIZING:
{
if(!CarsListView)
MessageBox(hWnd, _T("Null handle."), _T("Error"), MB_ICONERROR | MB_OK);
RECT WindowRect;
GetWindowRect( hWnd, &WindowRect);
SetWindowPos(CarsListView, NULL, 0, 0, WindowRect.right - WindowRect.left, WindowRect.bottom - WindowRect.top, SWP_SHOWWINDOW);
}
break;
// ...
}
and the CreateListView definition is this:
HWND CreateListView (HWND hwndParent)
{
INITCOMMONCONTROLSEX icex; // Structure for control initialization.
icex.dwICC = ICC_LISTVIEW_CLASSES;
InitCommonControlsEx(&icex);
RECT rcClient; // The parent window's client area.
GetClientRect (hwndParent, &rcClient);
// Create the list-view window in report view with label editing enabled.
HWND hWndListView = CreateWindow(WC_LISTVIEW,
L"",
WS_CHILD | LVS_REPORT | LVS_EDITLABELS,
0, 0,
rcClient.right - rcClient.left,
rcClient.bottom - rcClient.top,
hwndParent,
/*(HMENU)*/NULL,
hInst,
NULL);
return (hWndListView);
}
When the window receives WM_SIZING, I get that CarsListView = NULL
What can I do to have that handle pointing to my listview?

Answer 1

Three way to do that kind of stuff.
The Ugly
Store your CarsListView HWND in a static. You can't have 2 instance of the parent windows.
The Bad
Use SetWindowsLongPtr(parentHWND,GWLP_USERDATA,CarsListViewHWND) in your init and GetWindowLongPtr when needed. It's fast you can have as many instance as you want, but if you need more than one information I recommend storing a struct with your HWND inside instead of single HWND for future extensibility.
The Good ?
Use SetProp(parentHWND,"Your Unique String",hDataHandle); its by far the more code but with that usage you can use it on every windows without caring if the USERDATA is already used or not. It's the best approach when you need to add personal property to a windows/code you can't be sure how it will be used or change over time

Answer 2

There are two ways to do this: The "good" way and the "bad" way.
The "bad" way is to simply declare the local variable as static, however it means you can't create two windows of this type in the same process.
The "good" way is to memorize it in a heap-allocated structure, and store a pointer to this structure in the Window information using SetWindowLongPtr(). You can then retrieve this structure with GetWindowLongPtr().

Answer 3

I would create the listview in WM_CREATE and not in WM_SHOWWINDOW. Also make the handle either global or static.
Or you can also create the list view globally and keep it hidden and just make it visible and set its position whenever you want it to do.