What is A Window?

A window is a programming construct that:

The usual idea of a window being a rectangle with a title bar and maximize/minimize/close buttons is misleading because a button that you see in a window, by definition, is a window. All UI controls in Windows (the OS) are. A basic difference of a window that is a UI control from an application window is that they don't exist by themselves, but positioned relative to the application window. Windows have parent/child or owner/owned relationships. A button is a child window, a modal dialog window is owned. The owned window's lifetime is the owner's and always appears in front of the owner and hidden when the owner is.

Window Handles

The OS maintains a table of windows and refers to them by a value called handle. The handle HWND (pronounced "aitch-wind") is an opaque type. The CreateWindow() and CreateWindowEx() functions return a HWND, and the handle is used to perform some operation on a window using functions like MoveWindow().

Screen and Window Coordinates

Coordinates are measured in device-independent pixels (DIP), relative to the screen, relative to the window or relative to the window's client area. The origin is the top left and windows are positioned relative to the screen and drawn into using client coordinates.

WinMain: The Application Entry Point

The function signature is:

int WINAPI wWinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, PWSTR pCmdLine, int nCmdShow);

wWinMain accepts command-line arguments as Unicode while WinMain reads it in as an ANSI string. pCmdLine contains the command-line arguments, nCmdShow determines whether the main application window will be minimized, maximized or normal. hPrevInstance is obsolete now, a remnant from 16-bit Windows. hInstance is a handle to an instance or handle to a module. The OS uses this value to identify the executable (EXE file) when it is loaded in memory. It is needed for certain Windows functions like loading icons or bitmaps.

The Microsoft C runtime library (CRT) provides an implementation of main that calls either WinMain or wWinMain. In addition the CRT calls any static initializers. Although the linker can be told to use a different entry point, such things should be avoided because that would amount to skipping CRT's initialization code leading to unpredictable results.

Window Classes

A window class defines a set of behaviors that several windows might have in common. Data that is unique for each window is called instance data. Every window must be associated with a window class. This is not a C++ class but a data structure used internally by the OS. Classes are registered with the system at runtime using the RegisterClass() function.

Of the many fields of the structure the three shown in the code must be filled in: a window procedure, the handle to the application instance and a string that identifies the class (lpfnWndProc, hInstance, lpszClassName). Class names are local to the current process, so the name only needs to be unique within the process.

Creating a Window

To create a new instance of a window we call CreateWindowEx() with a long list of parameters. A few things to note about them is that the first parameter lets you specify optional behaviors (like transparent windows), the window text is used in the title bar (or in different ways by different types of windows), the style is a set of bitwise-OR flags (WSOVERLAPPEDWINDOW has a title bar, border, system menu and minimize/maximize buttons; the most common style for a top-level application window), the position and size are set to default values (CWUSEDEFAULT), the NULLs are parent window, menu for the window and a void* pointer to additional data, and, the hInstance is the instance handle received by wWinMain().

The function call returns a handle tot he new window or zero if the function fails.

Subsequently, ShowWindow() is called with the nCmdShow parameter which is used to minimize or maximize a window. The OS passes this value to the program.

Window Messages

Events from the user and the OS are passed on to the window using messages. As events may occur at any time, in almost any order, the flow of execution can not be predicted. So Windows uses a message passing model. A message is simply a numeric code that designates a particular event. To pass a message to a window the OS calls its window procedure.

For each thread that creates a window, the OS creates a queue for windows messages. This queue holds messages for all the windows that are created on that thread. The GetMessage is used to pull messages out of the queue. If the queue is empty this function blocks until another message is queued. Even though GetMessage blocks, your program will not become unresponsive because there is nothing for it to do when there are no messages. Additional threads can be spawned to do background processing while GetMessage waits.

The MSG structure contains information about a message but you will almost never examine this structure directly. Instead messages are passed to TranslateMessage and DispatchMessage. TranslateMessage translates keystrokes into characters. DispatchMessage tells the OS to call the window procedure of the target window of the message.

To exit the application (or quit the message processing loop), call PostQuitMessage which puts a WM_QUIT message on the queue and this in turn causes GetMessage to return zero. One interesting result of this behavior is that your window procedure never receives a WM_QUIT message, and therefore you need not handle this message in the switch-case block in your window procedure.

Messages are of two kinds: Postedmessages are those that go on the message queue and is dispatched through the message loop. Sent messages skip the queue and the OS calls the window procedure directly.

The Window Procedure

LRESULT CALLBACK WindowProc(HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam);

We write this out as a switch-case structure that performs actions for each kind of message we want to handle. For a uMsg, wParam and lParam which are word-size integers, carry more information. DefWindowProc must be called at the end to take care of all other messages.

While a window procedure executes, it blocks any other messages for windows created on the same thread. Therefore, lengthy computations must be avoided in the window procedure. Something you wouldn't want to do is open a TCP connection and wait for the server response from inside the window procedure. In stead, you should move that to another thread using one of the multitasking facilities built into Windows:

Painting the Window and the WM_PAINT Message

Showing something in the client area of the window is termed painting the window. Sometimes your program will want to update a region of the window, or the OS will want you to. In either case your or the OS sends your window a WM_PAINT message which tells you the update region or the portion to be painted.

The painting has to happen between a call to BeginPaint and EndPaint. BeginPaint fills a structure called PAINTSTRUCT with information on the repaint request. One of which is a RECT structure specifying a rectangular update region (defined relative to the client area). At this point you have two options; paint the entire client area regardless of the size of the update region or, more efficiently, paint just the update region. It is less code to write in the former case, but more efficient in the latter case.

EndPaint, called last, after your painting logic, clears the update region, which in turn signals to Windows that the window has completed painting itself and that it doesn't need to send another WM_PAINT until something changes.

On the first WM_PAINT message the entire client area needs to be painted. The update region's size is the entire client rectangle. On subsequent WM_PAINT messages, PAINTSTRUCT's rcPaint field may contain a smaller rectangle.

The recommended pattern is to skip handling WM_CLOSE and leaving it to DefWindowProc to catch and call DestroyWindow, or, catch WM_CLOSE yourself, do something in there and call DestroyWindow. Then, in WM_DESTROY (which you handle either way), call PostQuitMessage. Then, as you know, WM_QUIT is sent to the window, which you don't need to handle.


Most of the content here has been picked from the Microsoft Docs section titled Get Started with Win32 and C++.