So what are stock objects? “Stock” as the name says means goods in hand, which a shop keeper always has as a backup. In the same way GDI also keep a set of backup of objects which it creates during initialization and these are known as stock objects. Let’s think of a case like this… you are creating a fancy brush but somehow the creation fails, so what will be your backup policy, mine would be to use a stock brush, for e.g. BLACK_BRUSH since these objects are always available. Think of a case where you have to quickly create a black brush, I would go for stock object BLACK_BRUSH.
Following are the stock objects that I know, taken straight out of MSDN…
|DKGRAY_BRUSH||Dark gray brush.|
|DC_BRUSH||Windows 2000/XP: Solid color brush. The default color is white. The color can be changed by using the SetDCBrushColor function. For more information, see the Remarks section.|
|HOLLOW_BRUSH||Hollow brush (equivalent to NULL_BRUSH).|
|LTGRAY_BRUSH||Light gray brush.|
|NULL_BRUSH||Null brush (equivalent to HOLLOW_BRUSH).|
|DC_PEN||Windows 2000/XP: Solid pen color. The default color is white. The color can be changed by using the SetDCPenColor function. For more information, see the Remarks section.|
|ANSI_FIXED_FONT||Windows fixed-pitch (monospace) system font.|
|ANSI_VAR_FONT||Windows variable-pitch (proportional space) system font.|
|DEVICE_DEFAULT_FONT||Windows NT/2000/XP: Device-dependent font.|
|DEFAULT_GUI_FONT||Default font for user interface objects such as menus and dialog boxes. This is MS Sans Serif. Compare this with SYSTEM_FONT.|
|OEM_FIXED_FONT||Original equipment manufacturer (OEM) dependent fixed-pitch (monospace) font.|
|SYSTEM_FONT||System font. By default, the system uses the system font to draw menus, dialog box controls, and text.Windows 95/98 and Windows NT: The system font is MS Sans Serif.Windows 2000/XP: The system font is Tahoma|
|SYSTEM_FIXED_FONT||Fixed-pitch (monospace) system font. This stock object is provided only for compatibility with 16-bit Windows versions earlier than 3.0.|
|DEFAULT_PALETTE||Default palette. This palette consists of the static colors in the system palette.|
If you look at above table, we can see that almost all GDI object classes (exceptions are regions and bitmaps) has a backup stock object, mainly to act as a fallback method just in case of a failure IMO.
To use retrieve a stock object we call function GetStockObject. The good thing about working with a stock object is that we don’t have to delete it, since we didn’t create it. Now think how easy it’s to create a black pen, or black brush. No overheads to delete/free them. It’s definitely faster too.
There is a brush called NULL_BRUSH, a pen called NULL_PEN which are quite useful to draw a hollow rectangle without any fill, instead of selecting a brush equivalent to background window color to simulate drawing a rectangle without any fill.
CDC class of MFC has a very easy to use function for stock object. It’s CDC::SelectStockObject. I’m going nuts for this function. 😉 Take a look at these calls….
[sourcecode language=’cpp’]// Select a black brush
ClientDC.Attach( ::GetDC( m_hWnd ));
const int RestorePoint = ClientDC.SaveDC();
// Select a black brush
ClientDC.SelectStockObject( BLACK_BRUSH );
// Select a black pen
ClientDC.SelectStockObject( BLACK_PEN );
// Select existing font
ClientDC.SelectStockObject( DEVICE_DEFAULT_FONT );
ClientDC.RestoreDC( RestorePoint );[/sourcecode]
See how easy it’s to use these stock objects. No overhead of creating or freeing them, but as I already said these are just for backup action purpose. For fancy stuff you of course have to go for other functions.
So bottom line is stock objects are just to avoid cases were you end up with no brush to paint or no pen to write. 🙂