Win32 API Dealing with windows Creating a window


#define UNICODE
#define _UNICODE
#include <windows.h>
#include <tchar.h>
const TCHAR CLSNAME[] = TEXT("helloworldWClass");

int WINAPI WinMain(HINSTANCE hInst, HINSTANCE hPrevInst, PTSTR cmdline,
                   int cmdshow)
    WNDCLASSEX wc = { };
    MSG msg;
    HWND hwnd;

    wc.cbSize        = sizeof (wc);         = 0;
    wc.lpfnWndProc   = winproc;
    wc.cbClsExtra    = 0;
    wc.cbWndExtra    = 0;
    wc.hInstance     = hInst;
    wc.hIcon         = LoadIcon (NULL, IDI_APPLICATION);
    wc.hCursor       = LoadCursor (NULL, IDC_ARROW);
    wc.hbrBackground = (HBRUSH) GetStockObject (WHITE_BRUSH);
    wc.lpszMenuName  = NULL;
    wc.lpszClassName = CLSNAME;
    wc.hIconSm       = LoadIcon (NULL, IDI_APPLICATION);

    if (!RegisterClassEx(&wc)) {
        MessageBox(NULL, TEXT("Could not register window class"), 
                  NULL, MB_ICONERROR);
        return 0;

    hwnd = CreateWindowEx(WS_EX_LEFT,
    if (!hwnd) {
        MessageBox(NULL, TEXT("Could not create window"), NULL, MB_ICONERROR);
        return 0;

    ShowWindow(hwnd, cmdshow);
    while (GetMessage(&msg, NULL, 0, 0)) {
    return msg.wParam;
    return DefWindowProc(hwnd, wm, wp, lp);

The first thing one sees are the two macro definitions, UNICODE and _UNICODE. These macros cause our program to understand wide character strings (wchar_t[n]), not plain narrow strings(char[n]). As a result, all string literals must be wrapped in a TEXT( macro. The generic character type for Win32 strings is TCHAR, whose definition depends on whether or not UNICODE is defined. A new header is included: <tchar.h> contains the declaration of TCHAR.

A window consists of what is known as a window class. This describes information about a window that is to be shared between instances of it, like the icon, the cursor, and others. A window class is identified by a window class name, which is given in the CLSNAME global variable in this example. The first act of WinMain is to fill in the window class structure, WNDCLASSEX wc. The members are:

  • cbSize: The size, in bytes, of the structure
  • style: The window class styles. This is 0 for now.
  • lpfnWndProc: This is one of the more important fields. It stores the address of the window procedure. The window procedure is a function that handles events for all windows that are instances of this window class.
  • cbClsExtra: The number of extra bytes to allocate for the window class. For most situations, this member is 0.
  • cbWndExtra: The number of extra bytes to allocate for each individual window. Do not confuse this with cbClsExtra, which is common to all instances. This is often 0.
  • hInstance: The instance handle. Just assign the hInst argument in WinMain to this field.
  • hIcon: The icon handle for the window class. LoadIcon(NULL, IDI_APPLICATION) loads the default application icon.
  • hCursor: The cursor handle for the window class. LoadCursor(NULL, IDC_ARROW) loads the default cursor.
  • hbrBackground: A handle to the background brush. GetStockObject (WHITE_BRUSH) gives a handle to a white brush. The return value must be cast because GetStockObject returns a generic object.
  • lpszMenuName: The resource name of the menu bar to use. If no menu bar is needed, this field can be NULL.
  • lpszClassName: The class name that identifies this window class structure. In this example, the CLSNAME global variable stores the window class name.
  • hIconSm: A handle to the small class icon.

After this structure is initialized, the RegisterClassEx function is called. This causes the window class to be registered with Windows, making it known to the application. It returns 0 on failure.

Now that the window class has been registered, we can display the window using CreateWindowEx. The arguments are:

  • stylesex: The extended window styles. The default value is WS_EX_LEFT.
  • clsname: The class name
  • cap: The window title, or caption. In this case, it is the caption that is displayed in a window's title bar.
  • styles: The window styles. If you want to create a top-level (parent) window like this one, the flag to pass in is WS_OVERLAPPEDWINDOW.
  • x: The x-coordinate of the upper-left corner of the window.
  • y: The y-coordinate of the upper-left corner of the window
  • cx: The width of the window
  • cy: The height of the window
  • hwndParent: The handle to the parent window. Since this window is in itself a parent window, this argument is NULL.
  • hMenuOrID: If the window being created is a parent window, then this argument is a handle to the window menu. Do not confuse this with the class menu, which is WNDCLASSEX::lpszClassName. The class menu is common to all instances of windows with the same class name. This argument, however, is specific for just this instance. If the window being created is a child window, then this is the ID of the child window. In this case, we are creating a parent window with no menu, so NULL is passed.
  • hInst: The handle to the instance of the application.
  • etc: The extra information that is passed to the window's window procedure. If no extra information is to be transmitted, pass NULL.

If x or y or cx or cy is CW_USEDEFAULT, then that argument's value will be determined by Windows. That is what is done in this example.

CreateWindowEx returns the handle to the newly created window. If window creation failed, it returned NULL.

We then show the window by calling ShowWindow. The first argument for this function is the handle to the window. The second argument is the show style, which indicates how the window is to be displayed. Most applications just pass the cmdshow argument passed in WinMain. After the window is shown, it must be updated by a call to UpdateWindow. It causes an update message to be sent to the window. We will learn what this means in another tutorial.

Now comes the heart of the application: The message pump. It pumps messages sent to this application by the operating system, and dispatches the messages to the window procedure. The GetMessage call returns non-zero until the application receieves a messages that causes it to quit, in which case it returns 0. The only argument that concerns us is the pointer to an MSG structure that will be filled in with information about the message. The other arguments are all 0.

Inside the message loop, TranslateMessage translates virtual-key messages into character messages. The meaning of this, again, is unimportant to us. It takes a pointer to an MSG structure. The call directly following it, DispatchMessage, dispatches the message pointed to by its argument to the window's window procedure. The last thing WinMain must do is return a status code. The wParam member of the MSG structure contains this return value, so it is returned.

But that's just for the WinMain function. The other function is winproc, the window procedure. It will handle messages for the window that are sent to it by Windows. The signature for winproc is:

  • hwnd: A handle to the window whose messages are being processed.
  • wm: The window message identifier
  • wp: One of the message information arguments. This depends on the wm argument
  • lp: One of the message information arguments. This depends on the wm argument. This argument is usually used to transmit pointers or handles

In this simple program, we do not handle any messages ourselves. But that doesn't mean Windows doesn't either. This is why one must call DefWindowProc, which contains default window handling code. This function must be called at the end of every window procedure.

What is a handle?

A handle is a data type that represents a unique object. They are pointers, but to secret data structures maintained by the operating system. The details of these structures need not concern us. All a user needs to do is simply create/retreive a handle using an API call, and pass it around to other API calls taking that type of handle. The only type of handle we used was the HWND returned by CreateWindowEx.


In this example, we encounter a handful of constants, which are in all-caps and begin with a 2 or 3 letter prefix. (The Windows types are also in all-caps)

  • IDI_APPLICATION: The resource name containing the default application icon. This is used with either LoadIcon or LoadImage (LoadIcon in this example).
  • IDC_ARROW: The resource name countaining the default application cursor. This is used with either LoadIcon or LoadImage (LoadIcon in this example).
  • WHITE_BRUSH: The name of a stock object. This stock object is the white brush.
  • MB_ICONERROR: A flag used with MessageBox to display an error icon.
  • WS_EX_LEFT: The default extended window style. This causes the window to have left-aligned properties.
  • WS_OVERLAPPEDWINDOW: A window style indicating that the window should be a parent window with a title bar, size box, and others elements typical of top-level windows.
  • CW_USEDEFAULT: Used with CreateWindowEx's x, y, cx, or cy arguments. Causes Windows to choose a valid value for the argument for which CW_USEDEFAULT was passed.

Windows Types

When programming for Windows, you will have to get used to the Win32 types, which are aliases for builtin types. These types are in all caps. The alias types used in this program are:

  • TCHAR: The generic character type. If UNICODE is defined, this is a wchar_t. Otheriwse, it is a char.
  • UINT: An unsigned integer. Used to represent the message identifier in window procedures, and other purposes.
  • WPARAM: In Win16, this was a WORD argument (hence the W prefix). With the introduction of Win32, however, this is now a UINT_PTR. This illustrates the point of these Windows aliases; they are there to protect programs from change.
  • LPARAM: This is a LONG argument (LONG_PTR in Win64).
  • PTSTR: The P means pointer. The T means generic character, and the STR means string. Thus, this is a pointer to a TCHAR string. Other string types include:
    • LPTSTR: Same as PTSTR
    • LPCTSTR: Means const TCHAR *
    • PCTSTR: Same as LPCTSTR
    • LPWSTR: Wide string (wchar_t *)
    • LPCWSTR: Means const wchar_t *
    • PWSTR: Same as LPWSTR
    • and much more As you can see, the Win32 types can be a hassle to understand, especially with so many synonymous types, which is an artifact of Win16.
  • LRESULT: This type is used to represent the return value of window procedures. It is usually a LONG (hence the L).