Hi a have a problem.

//BUTTON CONTROLS - Copyright © 2000,2005 Ken Fitlike
//=============================================================================
//API functions used: CreateWindowEx,DefWindowProc,DispatchMessage,GetMessage,
//GetSystemMetrics,LoadImage,MessageBox,PostQuitMessage,RegisterClassEx,
//SendMessage,ShowWindow,UpdateWindow,TranslateMessage,WinMain.
//=============================================================================
//This demonstrates the creation of the following button types: default push 
//button,push button,check box,radio button and groupbox. The image style of 
//'bitmap' buttons (BS_BITMAP) and 'icon' buttons (BS_ICON) can be used in 
//conjunction with other button styles to create image checkboxes, radio 
//buttons etc. The default is a push button.
//=============================================================================
#if defined _MSC_VER || defined __BORLANDC__
#define OEMRESOURCE
#endif

#include <windows.h>  //include all the basics
#include <tchar.h>    //string and other mapping macros
#include <string>

//define an unicode string type alias
typedef std::basic_string<TCHAR> ustring;
//=============================================================================
//message processing function declarations
LRESULT CALLBACK WndProc(HWND,UINT,WPARAM,LPARAM);
int OnCreate(const HWND,CREATESTRUCT*);

//non-message function declarations
HWND CreateButton(const HWND,const HINSTANCE,DWORD,const RECT&,const int,
                  const ustring&);
inline int ErrMsg(const ustring&);

//setup some button id's
enum {
  IDBC_DEFPUSHBUTTON=200,
  IDBC_PUSHBUTTON,
  IDBC_AUTOCHECKBOX,
  IDBC_AUTORADIOBUTTON,
  IDBC_GROUPBOX,
  IDBC_ICON,
  IDBC_BITMAP
};
//=============================================================================
int WINAPI WinMain(HINSTANCE hInst,HINSTANCE,LPSTR pStr,int nCmd)
{
ustring classname=_T("SIMPLEWND");
WNDCLASSEX wcx={0};  //used for storing information about the wnd 'class'

wcx.cbSize         = sizeof(WNDCLASSEX);           
wcx.lpfnWndProc    = WndProc;             //wnd Procedure pointer
wcx.hInstance      = hInst;               //app instance
//use 'LoadImage' to load wnd class icon and cursor as it supersedes the 
//obsolete functions 'LoadIcon' and 'LoadCursor', although these functions will 
//still work. Because the icon and cursor are loaded from system resources ie 
//they are shared, it is not necessary to free the image resources with either 
//'DestroyIcon' or 'DestroyCursor'.
wcx.hIcon         = reinterpret_cast<HICON>(LoadImage(0,IDI_APPLICATION,
                                            IMAGE_ICON,0,0,LR_SHARED));
wcx.hCursor       = reinterpret_cast<HCURSOR>(LoadImage(0,IDC_ARROW,
                                              IMAGE_CURSOR,0,0,LR_SHARED));
wcx.hbrBackground = reinterpret_cast<HBRUSH>(COLOR_BTNFACE+1);   
wcx.lpszClassName = classname.c_str(); 
//the window 'class' (not c++ class) has to be registered with the system
//before windows of that 'class' can be created
if (!RegisterClassEx(&wcx))
  {
  ErrMsg(_T("Failed to register wnd class"));
  return -1;
  }

int desktopwidth=GetSystemMetrics(SM_CXSCREEN);
int desktopheight=GetSystemMetrics(SM_CYSCREEN);

HWND hwnd=CreateWindowEx(0,                     //extended styles
                         classname.c_str(),     //name: wnd 'class'
                         _T("Button Controls"), //wnd title
                         WS_OVERLAPPEDWINDOW,   //wnd style
                         desktopwidth/4,        //position:left
                         desktopheight/4,       //position: top
                         desktopwidth/2,        //width
                         desktopheight/2,       //height
                         0,                     //parent wnd handle
                         0,                     //menu handle/wnd id
                         hInst,                 //app instance
                         0);                    //user defined info
if (!hwnd)
  {
  ErrMsg(_T("Failed to create wnd"));
  return -1;
  }

ShowWindow(hwnd,nCmd); 
UpdateWindow(hwnd);
//start message loop - windows applications are 'event driven' waiting on user,
//application or system signals to determine what action, if any, to take. Note 
//that an error may cause GetMessage to return a negative value so, ideally,  
//this result should be tested for and appropriate action taken to deal with 
//it(the approach taken here is to simply quit the application).
MSG msg;
while (GetMessage(&msg,0,0,0)>0)
  {
  TranslateMessage(&msg);
  DispatchMessage(&msg);
  }
return static_cast<int>(msg.wParam);
}
//=============================================================================
LRESULT CALLBACK WndProc(HWND hwnd,UINT uMsg,WPARAM wParam,LPARAM lParam)
{
switch (uMsg)
  {
  case WM_CREATE:
    return OnCreate(hwnd,reinterpret_cast<CREATESTRUCT*>(lParam));
  case WM_DESTROY:
    PostQuitMessage(0);    //signal end of application
    return 0;
  default:
    //let system deal with msg
    return DefWindowProc(hwnd,uMsg,wParam,lParam);  
  }
}
//=============================================================================
int OnCreate(const HWND hwnd,CREATESTRUCT *cs)
{
//handles the WM_CREATE message of the main, parent window; return -1 to fail
//window creation
RECT rc={10,10,200,40};
//the various button types are created by simply varying the style bits
CreateButton(hwnd,cs->hInstance,BS_DEFPUSHBUTTON,rc,IDBC_DEFPUSHBUTTON,
             _T("DEFAULT PUSH BUTTON"));
                 
rc.top+=50;
CreateButton(hwnd,cs->hInstance,BS_PUSHBUTTON,rc,IDBC_PUSHBUTTON,
             _T("PUSH BUTTON"));
                 
rc.top+=50;
CreateButton(hwnd,cs->hInstance,BS_AUTOCHECKBOX,rc,IDBC_AUTOCHECKBOX,
             _T("CHECK BOX"));
                 
rc.top+=50;
CreateButton(hwnd,cs->hInstance,BS_AUTORADIOBUTTON,rc,IDBC_AUTOCHECKBOX,
             _T("RADIO BUTTON"));
    
rc.top=10;
rc.left=240;
CreateButton(hwnd,cs->hInstance,BS_GROUPBOX,rc,IDBC_GROUPBOX,
             _T("GROUP BOX"));
                 
rc.top+=50;
HWND hIconBtn=CreateButton(hwnd,cs->hInstance,BS_ICON,rc,IDBC_ICON,
                           _T("ICON BUTTON - NO TEXT"));
//get and assign the icon. Because LR_SHARED is used with a system 
//resource, there is no need to use DestroyIcon to free icon resources 
//when finshed with the icon.
HICON hIcon=reinterpret_cast<HICON>(LoadImage(0,IDI_EXCLAMATION,IMAGE_ICON,
                                              0,0,LR_SHARED));
SendMessage(hIconBtn,BM_SETIMAGE,IMAGE_ICON,reinterpret_cast<LPARAM>(hIcon));
                 
rc.top+=50;
HWND hBmpBtn=CreateButton(hwnd,cs->hInstance,BS_BITMAP,rc,IDBC_BITMAP,
                          _T("BITMAP BUTTON - NO TEXT"));
    
//get and assign the bitmap. Because LR_SHARED is used with a system 
//resource, there is no need to use DeleteObject to free bitmap
//resources when finshed with the bitmap.
HBITMAP hBmp=reinterpret_cast<HBITMAP>(LoadImage(0,MAKEINTRESOURCE(OBM_CHECK),
                                       IMAGE_BITMAP,0,0,LR_SHARED));
SendMessage(hBmpBtn,BM_SETIMAGE,IMAGE_BITMAP,reinterpret_cast<LPARAM>(hBmp));
return 0;
}
//=============================================================================
inline int ErrMsg(const ustring& s)
{
return MessageBox(0,s.c_str(),_T("ERROR"),MB_OK|MB_ICONEXCLAMATION);
}
//=============================================================================
HWND CreateButton(const HWND hParent,const HINSTANCE hInst,DWORD dwStyle,
                  const RECT& rc,const int id,const ustring& caption)
{
dwStyle|=WS_CHILD|WS_VISIBLE;
return CreateWindowEx(0,                            //extended styles
                      _T("button"),                 //control 'class' name
                      caption.c_str(),              //control caption
                      dwStyle,                      //control style 
                      rc.left,                      //position: left
                      rc.top,                       //position: top
                      rc.right,                     //width
                      rc.bottom,                    //height
                      hParent,                      //parent window handle
                      //control's ID
                      reinterpret_cast<HMENU>(static_cast<INT_PTR>(id)),
                      hInst,                        //application instance
                      0);                           //user defined info
}
//=============================================================================

Hi line26 113,123. If you can be so nice to write me explain how this function OnCreate have influence on code.
Thank you for your effort of reading.

OnCreate is a standard Borland idiom for a procedure/method executed whenever a new form is created (or other windows object).

So what is your problem?

I do not have any problem with code but with understanding the code . I do not understand how OnCreate reflect on entire code.
Why i can't just put one function who could execute all these buttons?
I am using Dev-C++ for Win32 API .This the GUI code.
With compiler g++.

Ah, I get it.

The OnCreate function is separated out just for some modularity. You could stick everything in one spot, but that would be confusing and large. By putting initialization stuff in a routine by itself it is both obvious and re-usable: the OnCreate is used when creating a window, and more than one window can be created easily.

This follows the philosophy that each routine should stick to doing one thing.

Hope this helps.

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