Advanced Preprocessor Statements

	Chapter 22 - The Microsoft Foundation Class Library: Fundamentals
	Visual C++ 6: The Complete Reference
	Chris H. Pappas and William H. Murray, III

Copyright 1998 The McGraw-Hill Companies

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	It All Begins with CObject

	Libraries such as the MFC library often start with a few parent classes. Additional classes are then derived from the parent classes. CObject is one parent class used extensively in developing Windows applications. The MFC library header files located in the MFC/INCLUDE subdirectory provide a wealth of information on defined classes.

	Let’s take a brief look at an edited version of Cobject that is defined in the AFX.H header file:

	///////////////////////////////////////////////////////////// // class CObject is the root of all compliant objects class CObject { public: // Object model (types, destruction, allocation)  virtual CRuntimeClass* GetRuntimeClass( ) const;  virtual ~CObject( );  // virtual destructors are necessary  // Diagnostic allocations  void* PASCAL operator new(size_t nSize);  void* PASCAL operator new(size_t, void* p);  void PASCAL operator delete(void* p);

	#if defined(_DEBUG) && !defined(_AFX_NO_DEBUG_CRT)  // for file name/line number tracking using DEBUG_NEW  void* PASCAL operator new(size_t nSize,                            LPCSTR lpszFileName,                            int nLine); #endif // Disable the copy constructor and assignment by default // so you will get compiler errors instead of unexpected // behavior if you pass objects by value or assign objects. protected:  CObject( ); private:  CObject(const CObject& objectSrc);     //no implementation  void operator=(const CObject& objectSrc); // Attributes public:  BOOL IsSerializable( ) const;  BOOL IsKindOf(const CRuntimeClass* pClass) const;

	// Overridables  virtual void Serialize(CArchive& ar);  // Diagnostic Support  virtual void AssertValid( ) const;  virtual void Dump(CDumpContext& dc) const; // Implementation public:  static const AFX_DATA CRuntimeClass classCObject; #ifdef _AFXDLL  static CRuntimeClass* PASCAL _GetBaseClass( ); #endif };

	This code has been edited slightly for clarity, but is essentially the same code that you will find in the header file.

	Upon inspection of the CObject listing, notice the components that make up this class definition. First, CObject is divided into public, protected, and private parts. CObject also provides normal and dynamic type checking and serialization. Recall that dynamic type checking allows the type of object to be determined at run time. The state of the object can be saved to a storage medium, such as a disk, through a concept called persistence. Object persistence allows object member functions to also be persistent, permitting retrieval of object data.

	Child classes are derived from parent classes. CGdiObject is an example of a class derived from CObject. Here is the CGdiObject definition as found in AFXWIN.H. Again, this listing has been edited for clarity.

	///////////////////////////////////////////////////////////// // CGdiObject abstract class for CDC SelectObject class CGdiObject : public CObject {  DECLARE_DYNCREATE(CGdiObject) public: // Attributes  HGDIOBJ m_hObject;  // must be first data member  operator HGDIOBJ( ) const;  HGDIOBJ GetSafeHandle( ) const;

	static CGdiObject* PASCAL FromHandle(HGDIOBJ hObject);  static void PASCAL DeleteTempMap( );  BOOL Attach(HGDIOBJ hObject);  HGDIOBJ Detach( ); // Constructors  CGdiObject( ); // must create a derived class object  BOOL DeleteObject( ); // Operations  int GetObject(int nCount, LPVOID lpObject) const;  UINT GetObjectType( ) const;  BOOL CreateStockObject(int nIndex);  BOOL UnrealizeObject( );  BOOL operator==(const CGdiObject& obj) const;  BOOL operator!=(const CGdiObject& obj) const;

	// Implementation public:  virtual ~CGdiObject( ); #ifdef _DEBUG  virtual void Dump(CDumpContext& dc) const;  virtual void AssertValid( ) const; #endif };

	CGdiObject and its member functions (methods) allow drawing items such as stock and custom pens, brushes, and fonts to be created and used in a Windows application. Classes such as CPen are further derived from the CGdiObject class.

	Microsoft has provided complete source code for the MFC library in order to allow the utmost in programming flexibility and customization. However, for the beginner, it is not even necessary to know how the various classes are defined in order to use them efficiently.

	For example, in traditional procedure-oriented Windows applications, the DeleteObject( ) function is called with the following syntax:

	DeleteObject(hBRUSH);  /*hBRUSH is the brush handle*/

	In object-oriented applications, using the MFC library, the same results can be achieved by accessing the member function with the following syntax:

	newbrush.DeleteObject( ); //newbrush is current brush

	As you can see, switching between procedure-oriented Windows function calls and class library objects can be intuitive. Microsoft has used this approach in developing all Windows classes, making the transition from traditional function calls to MFC library objects very easy.

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