I am reading the Trick Of the Windows Game programming book nad in Chapter 4 where it draws some lines in a window when I try to do the same it nakes an error.Can you please see the code and tell me why? // DEMO4_2.CPP - Line drawing demo // INCLUDES /////////////////////////////////////////////// #define WIN32_LEAN_AND_MEAN // just say no to MFC #include <windows.h> // include all the windows headers #include <windowsx.h> // include useful macros #include <mmsystem.h> // very important and include WINMM.LIB too! #include <stdio.h> #include <stdlib.h> #include <math.h> // DEFINES //////////////////////////////////////////////// // defines for windows #define WINDOW_CLASS_NAME "WINCLASS1" #define WINDOW_WIDTH 400 #define WINDOW_HEIGHT 300 // MACROS ///////////////////////////////////////////////// #define KEYDOWN(vk_code) ((GetAsyncKeyState(vk_code) & 0x8000) ? 1 : 0) #define KEYUP(vk_code) ((GetAsyncKeyState(vk_code) & 0x8000) ? 0 : 1) // GLOBALS //////////////////////////////////////////////// HWND main_window_handle = NULL; // globally track main window HINSTANCE hinstance_app = NULL; // globally track hinstance char buffer[80]; // general printing buffer // FUNCTIONS ////////////////////////////////////////////// LRESULT CALLBACK WindowProc(HWND hwnd, UINT msg, WPARAM wparam, LPARAM lparam) { // this is the main message handler of the system PAINTSTRUCT ps; // used in WM_PAINT HDC hdc; // handle to a device context char buffer[80]; // used to print strings // what is the message switch(msg) { case WM_CREATE: { // do initialization stuff here // return success return(0); } break; case WM_PAINT: { // simply validate the window hdc = BeginPaint(hwnd,&ps); // end painting EndPaint(hwnd,&ps); // return success return(0); } break; case WM_DESTROY: { // kill the application, this sends a WM_QUIT message PostQuitMessage(0); // return success return(0); } break; default:break; } // end switch // process any messages that we didn''t take care of return (DefWindowProc(hwnd, msg, wparam, lparam)); } // end WinProc // WINMAIN //////////////////////////////////////////////// int WINAPI WinMain( HINSTANCE hinstance, HINSTANCE hprevinstance, LPSTR lpcmdline, int ncmdshow) { WNDCLASSEX winclass; // this will hold the class we create HWND hwnd; // generic window handle MSG msg; // generic message HDC hdc; // graphics device context // first fill in the window class stucture winclass.cbSize = sizeof(WNDCLASSEX); winclass.style = CS_DBLCLKS | CS_OWNDC | CS_HREDRAW | CS_VREDRAW; winclass.lpfnWndProc = WindowProc; winclass.cbClsExtra = 0; winclass.cbWndExtra = 0; winclass.hInstance = hinstance; winclass.hIcon = LoadIcon(NULL, IDI_APPLICATION); winclass.hCursor = LoadCursor(NULL, IDC_ARROW); winclass.hbrBackground = (HBRUSH)GetStockObject(BLACK_BRUSH); winclass.lpszMenuName = NULL; winclass.lpszClassName = WINDOW_CLASS_NAME; winclass.hIconSm = LoadIcon(NULL, IDI_APPLICATION); // save hinstance in global hinstance_app = hinstance; // register the window class if (!RegisterClassEx(&winclass)) return(0); // create the window if (!(hwnd = CreateWindowEx(NULL, // extended style WINDOW_CLASS_NAME, // class "Line Drawing Demo", // title WS_OVERLAPPEDWINDOW | WS_VISIBLE, 0,0, // initial x,y WINDOW_WIDTH, // initial width WINDOW_HEIGHT,// initial height NULL, // handle to parent NULL, // handle to menu hinstance,// instance of this application NULL))) // extra creation parms return(0); // save main window handle main_window_handle = hwnd; // enter main event loop, but this time we use PeekMessage() // instead of GetMessage() to retrieve messages while(TRUE) { // test if there is a message in queue, if so get it if (PeekMessage(&msg,NULL,0,0,PM_REMOVE)) { // test if this is a quit if (msg.message == WM_QUIT) break; // translate any accelerator keys TranslateMessage(&msg); // send the message to the window proc DispatchMessage(&msg); } // end if // draw a random line // get the graphics device context hdc = GetDC(hwnd); // create a random colored pen HPEN hpen = CreatePen(PS_SOLID,1,RGB(rand()%256,rand()%256,rand()%256)); // select the pen into context HPEN old_pen = SelectObject(hdc,hpen); // move to a random postion MoveToEx(hdc, rand()%WINDOW_WIDTH, rand()%WINDOW_HEIGHT, NULL); // draw a line LineTo(hdc,rand()%WINDOW_WIDTH, rand()%WINDOW_HEIGHT); // now delete the pen SelectObject(hdc,old_pen); DeleteObject(hpen); // release the device context ReleaseDC(hwnd,hdc); // main game processing goes here if (KEYDOWN(VK_ESCAPE)) SendMessage(hwnd, WM_CLOSE, 0,0); } // end while // return to Windows like this return(msg.wParam); } // end WinMain ///////////////////////////////////////////////////////////

Hey guys...no answer?

Find the bit in the code that says looks like this...

// select the pen into contextHPEN old_pen = SelectObject(hdc,hpen);// move to a random postion  

Change that line to look like this:

HPEN old_pen = (HPEN)SelectObject(hdc,hpen);  

You have to explicitly typecast the result since C++ is stricter than C (the above is valid C code). You're saying there "oi! This is an HPEN object!" The problem before was that the function returns a void* type, which is a very generic thing. You have to typecast when assigning something a void pointer if you're using C++.

A couple of hints too: next time, post the error message and the offending line - many of us here wouldn't even need to compile the program to figure out the problem with that info . Also, use the ['source]big code here['/source] tags (without the two extra ' of course) to get nice boxes for your code:




[edited by - Alimonster on September 3, 2002 5:55:17 PM]

Thanks guys I will try it now