Question

what does it class style means actually ? it confused me. this is from MSDN: style
Specifies the class style(s). This member can be any combination of the class styles.

typedef struct _WNDCLASS { 
    UINT       style; 
    WNDPROC    lpfnWndProc; 
    int        cbClsExtra; 
    int        cbWndExtra; 
    HINSTANCE  hInstance; 
    HICON      hIcon; 
    HCURSOR    hCursor; 
    HBRUSH     hbrBackground; 
    LPCTSTR    lpszMenuName; 
    LPCTSTR    lpszClassName; 
} WNDCLASS, *PWNDCLASS; 

Answer 1

The class styles are properties that affect every instance of a window of that specific class. To clarify, let’s compare window instance properties vs class properties below. Supposes you create a new windows class called MyCoolControl, and you create several instances of this:

• Every instance will have its own location, window text, and enabled and visible state – these are window instance properties, and you can set these on one window independently of the others.

• However, all instances of this control will share the same WndProc, as specified in the WNDCLASS that you use to create the class. They’ll also have the same class properties, such as whether the windows receive double-click messages rather than two separate click messages (CS_DBLCLKS class style bit), or whether the window is completely redrawn when resized (CS_HREDRAW, CS_VREDRAW), or whether the windows have a drop-shadow (CS_DROPSHADOW). The full list of class styles are listed on MSDN here.

So, for example, if you want a window to have a border or not, that’s a window style bit (WS_BORDER), and you specify it as a window style value in CreateWindow (or can change it later per-window with SetWindowLongPtr(GWL_STYLE)), and only that window is affected. But if you want to create a window that has a drop shadow, you specify that in the style member of the WNDCLASS, and it affects all instances of that class.

(There are some exceptions to this – the WndProc specified in the WNDCLASS is really the default wndproc for that class of windows; you can actually override it per-instance if you want to. But the big picture is still mostly the same: WNDCLASS and CS_ styles are across-the-board settings, while the WS_ ones and the parameters to CreateWindow are specific to that one window.)