When registering a window class, it's necessary to fill in the WNDCLASS
struct, which has a hInstance
field:
typedef struct {
UINT style;
WNDPROC lpfnWndProc;
int cbClsExtra;
int cbWndExtra;
HINSTANCE hInstance;
HICON hIcon;
HCURSOR hCursor;
HBRUSH hbrBackground;
LPCTSTR lpszMenuName;
LPCTSTR lpszClassName;
} WNDCLASS
Then, the window is created using CreateWindow()
which takes in these parameters:
HWND CreateWindow(
LPCTSTR lpClassName,
LPCTSTR lpWindowName,
DWORD dwStyle,
int x,
int y,
int nWidth,
int nHeight,
HWND hWndParent,
HMENU hMenu,
HINSTANCE hInstance,
LPVOID lpParam
);
If the hInstance
was provided with WNDCLASS
when the class was registered, why is it necessary to provide a hInstance
to CreateWindow()
?
Is the hInstance
provided to WNDCLASS
supposed to be same as the hInstance
provided to CreateWindow()
?
If the WNDCLASS
hInstance
and the CreateWindow()
hInstance
are different, what do they mean?
Here is a sample program demonstrating class registration / window creation:
#include <windows.h>
LPSTR szClassName = "MyClass";
HINSTANCE hInstance;
LRESULT CALLBACK MyWndProc(HWND, UINT, WPARAM, LPARAM);
int WINAPI WinMain(HINSTANCE hInst, HINSTANCE hPrevInstance, LPSTR szCmdLine, int iCmdShow)
{
WNDCLASS wnd;
MSG msg;
HWND hwnd;
hInstance = hInst;
wnd.style = CS_HREDRAW | CS_VREDRAW; //we will explain this later
wnd.lpfnWndProc = MyWndProc;
wnd.cbClsExtra = 0;
wnd.cbWndExtra = 0;
wnd.hInstance = hInstance;
wnd.hIcon = LoadIcon(NULL, IDI_APPLICATION); //default icon
wnd.hCursor = LoadCursor(NULL, IDC_ARROW); //default arrow mouse cursor
wnd.hbrBackground = (HBRUSH)(COLOR_BACKGROUND+1);
wnd.lpszMenuName = NULL; //no menu
wnd.lpszClassName = szClassName;
if(!RegisterClass(&wnd)) //register the WNDCLASS
{
MessageBox(NULL, "This Program Requires Windows NT",
"Error", MB_OK);
return 0;
}
hwnd = CreateWindow(szClassName,
"Window Title",
WS_OVERLAPPEDWINDOW, //basic window style
CW_USEDEFAULT,
CW_USEDEFAULT, //set starting point to default value
CW_USEDEFAULT,
CW_USEDEFAULT, //set all the dimensions to default value
NULL, //no parent window
NULL, //no menu
hInstance,
NULL); //no parameters to pass
ShowWindow(hwnd, iCmdShow); //display the window on the screen
UpdateWindow(hwnd); //make sure the window is updated correctly
while(GetMessage(&msg, NULL, 0, 0)) //message loop
{
TranslateMessage(&msg);
DispatchMessage(&msg);
}
return msg.wParam;
}
LRESULT CALLBACK MyWndProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
{
switch(msg)
{
case WM_DESTROY:
PostQuitMessage(0);
return 0;
}
return DefWindowProc(hwnd, msg, wParam, lParam);
}
When a window class is registered it can be global (the CS_GLOBALCLASS
style is set) or local to the module that registers it. For global classes the class names must be unique but for module-local classes the class names can clash - there's nothing stopping two DLLs registering window classes of the same name, and an executable could load both of those DLLs.
You therefore you need to tell the CreateWindow
function which module's instance of the named class you want to use to create the window.
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