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Assingning a WinProc

Started by Cyphoid Oct 31, 2005 at 3:24 PM 2 replies 1.2k views

Original Post

Oct 31, 2005 at 3:24 PM

This is a rather silly question to most people. While I have written a 3d rendering engine and have done lots of things in C++, I am not all that knowledgeable in C++ at times (unfortuantely). Which leads to my question: I am setting up a window to render to. This, of course, requires me to define a WNDCLASSEX structure. In that structure is the lpfnWndProc value which points to the message handling function of my engine. Previously, I did not wrap my windows function up into a CWindow type class. I did this time. And I seem to be having a hard time assigning my event handler method to the WNDCLASSEX structure. This is how it is defined in my Window class. LRESULT WndProc(HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam); I attempt to assign it in my CreateD3DWindow method: windowClass.lpfnWndProc = cWndProc; And I end up with the error message: "error C2440: '=' : cannot convert from 'LRESULT (__thiscall D3Dwindow::* )(HWND,UINT,WPARAM,LPARAM)' to 'WNDPROC'" I am sure that this is really stupid on my part. I just want to give all of the details for you all. Thanks in advance.

templewulf

Oct 31, 2005 at 3:30 PM

I had a similar problem, and it was the difference between a "free" function and an "object" function.

When you declare a function inside an object, the function call implicitly passes a "this" pointer to it. So, there's actually one more parameter than you've typed in.

"Free" functions (I've forgotten the real word for them, but they're declared with _stdcall, I believe), don't have an implicit "this" pointer because they're not declared inside the object, so it essentially has a different function signature.

The solution is to declare your WinProc function as static, which makes it a class level function, removing the implicit pointer. Instead of saying object->method, you'll have to do Class::method.

Oluseyi has an excellent article on Win32 Wrapper object in the Platform Specific section. I'd give you a link, but GameDev no worky for me at the moment!

SirLuthor

Nov 1, 2005 at 1:48 AM

I believe you are referring to this article?

Quote:

"Free" functions (I've forgotten the real word for them, but they're declared with _stdcall, I believe), don't have an implicit "this" pointer because they're not declared inside the object, so it essentially has a different function signature.

Actually, the default calling convention, at least in MSVC for non-member functions is __cdecl, although not for functions with the 'dread ellipses construct', which have to use __stdcall, because __cdecl leaves the cleaning up of the stack to the called function, which in the case of a function with an unknown number of arguments can only be done by the calling function, because only it knows how many arguments were passed. __fastcall is also a valid calling convention. Member functions, however, are implicitly __thiscall, and I'm fairly sure there is no way at all to change that.

Cyphoid

Nov 2, 2005 at 6:58 AM

Thanks very much. I'll be looking into what you have said and will be working on it.

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