WindowProc in a class - function and pointer problem

[D-sturbed]

On Wednesday, 22 May 2013 at 21:22:52 UTC, Diggory wrote:
> On Wednesday, 22 May 2013 at 20:25:40 UTC, Simen Kjaeraas wrote:
>> On 2013-05-22, 21:30, D-sturbed wrote:
>>
>>> Hello, is there a way to wrap a WindowProc (so "LRESULT 
>>> WindowProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM 
>>> lParam) nothrow") in a class and to link it to a WindowClass 
>>> without puting it as "static" ?
>>>
>>> Because defacto every datum used in the WindowProc must also 
>>> be static.
>>> The problem technically is that if "static" is not specified, 
>>> the compiler won't allow this: "MyWinClass.lpfnWndProc = 
>>> &TheWindowProcInMyClass".
>>>
>>> I've also tried this: "MyWinClass.lpfnWndProc = 
>>> (&TheWindowProcInMyClass).funcptr" but, while it compiles, it 
>>> drastically fails at the run-time...
>>
>> Not possible, no. What you *can* do is have some way to 
>> translate from
>> hwnd to class instance, and fetch the right instance inside 
>> the static
>> wndProc to call a member function on that.
>
> If you are only going to have one window you can store the 
> "this" pointer in a global variable.
>
> If you want to have multiple windows each with messages going 
> to an instance of a class, you need to do the following:
> - Specify the "this" pointer as the lpParam argument to 
> CreateWindow
> - Hook up a static WndProc function
> - Have the static function handle a WM_NCCREATE message as 
> follows:
>     - Cast the "lParam" parameter to a CREATESTRUCT* and 
> retrieve the "this" pointer from the "lpCreateParams" member.
>     - Use "SetWindowLongPtr" to set the GWLP_USERDATA property 
> of the window "hwnd" to the "this" pointer
> - Have the static function handle all messages as follows:
>     - Use "GetWindowLongPtr" to get the GWLP_USERDATA property 
> of the window "hwnd" to get the "this" pointer
>     - Pass the message on to a non-static WndProc in the class 
> using the discovered "this" pointer
>
> You also need to make sure that there is a separate reference 
> to the class instance for as long as the window exists, because 
> the garbage collector will not scan the window properties and 
> so may think the object is garbage otherwise.

Yes I'm in the "multiple Window case", every window is wraped in 
a class and has its own message handler. I know that Win, in its 
callback system, often lets you retrieve a pointer to something, 
and I haven't get it was possible in this case...(which is you 
seem to describe). I will try this tomorrow.