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GDI Object Support in MFC

Many GDI drawing operations are accomplished using a series of GDI objects, such as pens, brushes, or fonts. The Microsoft Foundation Classes Library provides a series of wrapper classes that encapsulate the functionality of these GDI objects.

All GDI object classes are derived from the class CGdiObject. Figure 24.5 illustrates GDI object classes in MFC.

GDI object classes.

The CGdiObject class provides generic support for GDI objects in the form of a series of member functions. The Attach and Detach member functions can be used to attach a CGdiObject-derived MFC object to a GDI object or detach it from the GDI object. The handle of the object, stored in the m_hObject member variable, can be retrieved through the "safe" function GetSafeHandle. (This function can also be used with null CGdiObject pointers.) A pointer to a CGdiObject that corresponds to a Windows GDI object handle can be obtained by calling the static member function FromHandle. To obtain a GDI object's type, use the GetObjectType member function.

The CreateStockObject member function can be used to crate a stock pen, brush, font, or palette. Note that this function should be called with a CGdiObject-derived object that is of the appropriate class (CPen, CBrush, CFont, or CPalette).

The UnrealizeObject member function can be used to reset the origin of a brush or reset a palette. Do not use this member function for objects of any other type.

The DeleteObject member function deletes the GDI object that the CGdiObject-derived MFC object is attached to. The DeleteTempMap function, called usually from the idle-time handler of your application's CWinApp object, is used to delete any temporary CGdiObject objects that were created by the FromHandle member function.

Support for GDI pen objects in MFC is provided through the CPen class. A pen can be created either in a single step or in two steps. If you wish to create a pen in a single step, you can utilize overloaded versions the CPen constructor for this purpose. For example, to create a dashed black pen, you can declare the pen object as follows:

CPen myPen(PS_DASH, 0, RGB(0, 0, 0));

Alternatively, pens can be created in a two-step operation, by creating first the MFC CPen object using a parameterless constructor, and then calling the CreatePen or CreatePenIndirect member functions to create a corresponding GDI pen object. For example:

CPen *pPen;

pPen = new CPen;

pPen->CreatePen(PS_SOLID, 3, RGB(255, 0, 0));

To obtain a LOGPEN structure from a CPen object, use the GetLogPen function. You can also use a CPen object in any GDI function calls that require a pen handle of type HPEN because the CPen class defines the operator HPEN operator function.hes

The MFC supports GDI brushes through the CBrush class. Like pens, brushes can also be created in either a single step or a two-step process.

To create a GDI brush while constructing the CBrush object, use one of the overloaded versions of the CBrush constructor. For example, to create a solid yellow brush, you could construct the CBrush object as follows:

CBrush *pBrush;

pBrush = new CBrush(RGB(255, 255, 0));

Alternatively, you can first create the CBrush MFC object through the parameterless constructor and then create the GDI brush object by calling the CreateSolidBrush, CreateHatchBrush, CreatePatternBrush, CreateDIBPatternBrush, CreateSysColorBrush, or CreateBrushIndirect member functions. For example:

CBrush cyanBrush;

cyanBrush.CreateSolidBrush(RGB(0, 255, 255));

To obtain a LOGBRUSH structure from a CBrush object, use the GetLogBrush member function. You can also use CBrush objects in place of handles of type HBRUSH in GDI function calls because the CBrush class defines the operator HBRUSH operator function.

MFC Example