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OpenGL Windows message function in openGL

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I am trying to learn openGL and am trying to learn and use native windows code rather than GLUT handling everything for me. Thus, I thought i'll write a general window wrapper around the openGL stuff, without really mixing the two functionalities. Thus, I made two modules: -myWin -Application For starting, I needed a window in which I could render my openGL stuff. Thus, I tried learning setting up my window in the myWin class. I followed some tutorials online, and have been able to get a basic framework done. "But", there are some bugs which I still dont understand in their entirety and that is what is messing me up :( Following are the errors to which I could not even understand the solutions posted online (found them via searching on the web): 1.
LRESULT myWin::WndMsgProc (HWND hWnd, UINT uMsg, WPARAM  wParam, LPARAM  lParam)
I have this as a function member definition in my header file and defined in my code .cpp file. When I try to assign this method to the window :
wc.lpfnWndProc = (WNDPROC)WndMsgProc;
I get an error:
error C2440: 'type cast' : cannot convert from 'overloaded-function' to 'WNDPROC'
None of the functions with this name in scope match the target type
I dont understand since the method signature is exactly the same as it expects. After a lot of searching, I found that making [B]WndMsgProc[/B] static would solve my problem and the reason for that has got something to do with function pointers wrt classes. I don't understand how that works, but i'd like to know.. Also, if I make it static, I end up making all my other members static since it calls and accesses other members. I am really interested in finding another way of doing this. I think making things static is a bad thing to do and it breaches the OOPs idea. Its like making everything global.. Anyways, I would really appreciate if someone could tell me the right way to do things.. 2. There are more issues, like toggling between unicode and multiset character base, but i want to solve one problem at a time here.

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Making it Static isn't like making it Global. You can have local, private static variables. The only reason I know it has to be static is because you're putting it in a class.

These may help because I'm still learning , too:

Teach Yourself C++ in 21 days:
http://newdata.box.sk/bx/c/


And here: http://www.zeuscmd.com/tutorials/opengl/03-OpenGLWindow.php

Quote:

A static create method needs to be created to return a pointer to the singleton. This is done as we only ever want one OpenGL window. Most variables and methods will be declared as static because of this.


I hope that helps get you started.

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Lol, I used the zeuscmd site as thee tutorial. There everything is static and that is what I wanted to avoid.

I also wanted to learn if there is some other approach to do this ..

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Have you tried Apron Tutorials? http://www.morrowland.com/apron/tut_gl.php I think they're approach uses C-style, but some of the stuff was broken down into classes.

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Quote:
Original post by openglJunkie
Have you tried Apron Tutorials? http://www.morrowland.com/apron/tut_gl.php I think they're approach uses C-style, but some of the stuff was broken down into classes.


Thanks for the link :) I had not tried it earlier, but there are a lot of tutorials in there on openGL which I definitely see myself using in the coming times (book-marked) :D.

Also, I saw the code just now, but its not broken down into classes. They all stuff it in one class. :(

I wish I could upload my .h and .cpp files here..

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If it's in a class, then it's not exactly the same as the standard WNDPROC. Don't forget that member functions also silently pass a pointer to the correct class (the 'this' pointer) so it knows what instance it's dealing with. You need to make the member function static, because this removes the hidden requirement for the 'this' pointer.

If you want different applications to not use the same method ( as they would with a static function), then you can store a pointer to your C++ class when you create the window class (WNDCLASS/WNDCLASSEX) and have that call a function which then deals with the functions on an instance-based level. I'll update this in a moment, when I dig out an example I coded recently.
EDIT: here you go;

frame.h

namespace Engine
{
struct point2i
{
int point[2];
};

class Window
{
...

static LRESULT CALLBACK WndProc(HWND,UINT,WPARAM,LPARAM);
LRESULT CALLBACK Process(UINT,WPARAM,LPARAM);

Window(){}
~Window();

public:

...
};
}

#endif // H_ENGINE_WINDOW










frame.cpp
#include "frame.h"

namespace Engine
{

/// Window class function implementation

LRESULT CALLBACK Window::WndProc(HWND h_wnd,UINT message,WPARAM w_param,LPARAM l_param)
{
// We're going to be getting the pointer from where we store it
Window *wnd;
// This message is one of the first a window receives, so it's good
// to check for, for this bit.
if(message == WM_NCCREATE)
{
// This retrieves the window pointer. We send it by specifying the
// size of Window* in WNDCLASSEX, and sending the actual pointer in
// CreateWindowEx()
wnd = reinterpret_cast<Window*>(((LPCREATESTRUCT)l_param)->lpCreateParams);
// the pointer isn't in the create params.
if(wnd == NULL)
{
// my own error class, use whatever error logging you like here
throw ErrorClass("Unable to retrieve window pointer from WM_NCCREATE");
}
SetLastError(0);
DWORD err;
// we have the pointer from the create params, so we set it
// in the GWLP_USERDATA area.
if(SetWindowLongPtr(h_wnd,GWLP_USERDATA,reinterpret_cast<long>(wnd))==0)
{
// error checking
err=GetLastError();
if((err))
{
throw ErrorClass("SetWindowLongPtr failed in WndProc");
}
}
// give the Window instance its win32 handle (HWND)
wnd->window_handle = h_wnd;
}
// We're not creating the window, so we should be able to send to the
// correct Window instance.
else
{
// retrieve the Window* from GWLP_USERDATA
wnd = reinterpret_cast<Window*>(GetWindowLongPtr(h_wnd,GWLP_USERDATA));
// There are a couple of messages that can be called before
// WM_NCCREATE, but they are pretty standard, so we leave them be
if(wnd == NULL)
{
return DefWindowProc(h_wnd,message,w_param,l_param);
}
}
// call the Window class's message handling function
return wnd->Process(message,w_param,l_param);
}

LRESULT CALLBACK Window::Process(UINT message,WPARAM w_param,LPARAM l_param)
{
switch(message)
{
// deal with class-specific messages here

default: return DefWindowProc(window_handle,message,w_param,l_param);
}
}
}










For setting this up, I set the cbWndExtra member of the WNDCLASSEX struct to sizeof(Window*), and to send the class pointer, I call CreateWindowEx like so;

window_handle = CreateWindowEx(0,class_name.c_str(),"Generic MGL ver 0.1 window",
WS_CAPTION|WS_BORDER,CW_USEDEFAULT,CW_USEDEFAULT,
800,600,NULL,NULL,GetModuleHandle(NULL),reinterpret_cast<LPVOID>(this));

note the 'reinterpret_cast' in the last parameter.

What happens in the static function is that I check the message sent. If the message is WM_NCCREATE, I add the class's pointer to the GWLP_USERDATA section of the windows information (NOT the window class's). If any other message is called, I check to see if the pointer has been stored. If it has, I call the correct member function, otherwise, I call DefWindowProc().

I hope this helps. Also, please note that this just sets up the window. I've taken out all of the stuff that isn't relevant, but it should be pretty easy to build upon this.

[Edited by - webwraith on December 30, 2008 7:46:04 PM]

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