Best method for storing this pointer for use in WndProc

Question

I\'m interested to know the best / common way of storing a this pointer for use in the WndProc. I know of several approaches, but each as I underst

Answer 1

ATL's thunk is the most efficent. the thunk executes once and replaces the callback function for the WINPROC to the classes own message processing member function. subsiquent messages are passed by a direct call to the classes member function by windows. it doesnt get any faster than that.

Answer 2

With regard to SetWindowLong() / GetWindowLong() security, according to Microsoft:

The SetWindowLong function fails if the window specified by the hWnd parameter does not belong to the same process as the calling thread.

Unfortunately, until the release of a Security Update on October 12, 2004, Windows would not enforce this rule, allowing an application to set any other application's GWL_USERDATA. Therefore, applications running on unpatched systems are vulnerable to attack through calls to SetWindowLong().

Answer 3

In your constructor, call CreateWindowEx with "this" as the lpParam argument.

Then, on WM_NCCREATE, call the following code:

SetWindowLongPtr(hwnd, GWLP_USERDATA, (LONG_PTR) ((CREATESTRUCT*)lParam)->lpCreateParams);
SetWindowPos(hwnd, 0, 0, 0, 0, 0, SWP_NOMOVE | SWP_NOSIZE | SWP_NOZORDER);

Then, at the top of your window procedure you could do the following:

MyWindowClass *wndptr = (MyWindowClass*) GetWindowLongPtr(hwnd, GWL_USERDATA);

Which allows you to do this:

wndptr->DoSomething();

Of course, you could use the same technique to call something like your function above:

wndptr->WndProc(msg, wparam, lparam);

... which can then use its "this" pointer as expected.

Answer 4

In order to prevent the problem that occurred in the Zeus editor, simply specify the window in the GetMessage function:

BOOL GetMessage(
LPMSG lpMsg,
HWND  hWnd, /*A handle to the window whose messages are to be retrieved.*/
UINT  wMsgFilterMin,
UINT  wMsgFilterMax
);

NOTE The window must belong to the current thread.

Easy to read Documentation of the function

Answer 5

I've used SetProp/GetProp to store a pointer to data with the window itself. I'm not sure how it stacks up to the other items you mentioned.

Answer 6

You can use GetWindowLongPtr and SetWindowLongPtr; use GWLP_USERDATA to attach the pointer to the window. However, if you are writing a custom control I would suggest to use extra window bytes to get the job done. While registering the window class set the WNDCLASS::cbWndExtra to the size of the data like this, wc.cbWndExtra = sizeof(Ctrl*);.

You can get and set the value using GetWindowLongPtr and SetWindowLongPtr with nIndex parameter set to 0. This method can save GWLP_USERDATA for other purposes.

The disadvantage with GetProp and SetProp, there will be a string comparison to get/set a property.