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Run Time Class Handler: Instantiating Classes by Name

Posted by Ernst Versteeg on December 18th, 1999
Environment: Win16 (VC++ 1.52) and Win9x/NT (VC++ 6.0)

With this class I was able to simply add a file with a new class to my project and to instantiate new objects from this class simply by altering or adding the name of the class in the database or in configuration files.  This way I didn't need to add code for the support of the added class to existing code or to register the class with class factories.

So this is VERY useful if you like to disconnect classes from each other on compile level or to avoid making special class factories.

This class is tested and is in use for both 16/32 bit versions.

Notes:

  1. Use DECLARE_SERIAL and the IMPLEMENT_SERIAL macro for classes which should be used for this function.

  2. As with all classes which use DECLARE_SERIAL a default empty constructor is needed.  If you need initialization, a "Create" function be made and used after a cast of the returned object to the appropriate class.

  3. DECLARE_DYNCREATE does not work with the 32 bit part of this code, perhaps somebody knows why.

  4. As far as I know, this way of coding is part of SmallTalk.

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Comments

  • Solution to make it work with classes in MFC extension DLLs

    Posted by Legacy on 06/03/1999 12:00am

    Originally posted by: Peter Luxem 

    I used this code to (Copy and Paste out of MFC source), but I've copied everything, so it finds classes in
MFC extension DLL's too (even if you load them explicitly).


    CRuntimeClass *FindRuntimeClass( const char *szClass )

    {

    #if _MSC_VER != 1200

    #pragma message( "Check if this code is still correct" )

    #endif

      bool bFound=false;


      //>> Undocumented MFC, can break in next version, copy/paste from arccore.cpp line 315

      void AFXAPI AfxLockGlobals(int nLockType);

      void AFXAPI AfxUnlockGlobals(int nLockType);

      #define CRIT_RUNTIMECLASSLIST 0

    	#define CRIT_DYNLINKLIST    0


      // Alleen voor Declare_serials

      AFX_MODULE_STATE* pModuleState = AfxGetModuleState();

    	AfxLockGlobals(CRIT_RUNTIMECLASSLIST);

    	for ( CRuntimeClass *pClass = pModuleState->m_classList; pClass != NULL;

    		pClass = pClass->m_pNextClass)

    	{

    		if ( lstrcmpA( szClass, pClass->m_lpszClassName) == 0 )

    		{

    	  bFound = true;

    			break;

    		}

    	}

    	AfxUnlockGlobals(CRIT_RUNTIMECLASSLIST);


      if ( !bFound )

      {

    // This code is to check for the class in loaded DLL's

    #ifdef _AFXDLL

    	  // search classes in shared DLLs

    	  AfxLockGlobals(CRIT_DYNLINKLIST);

    	  for (CDynLinkLibrary* pDLL = pModuleState->m_libraryList; pDLL != NULL;

    		  pDLL = pDLL->m_pNextDLL)

    	  {

    		  for (pClass = pDLL->m_classList; pClass != NULL;

    			  pClass = pClass->m_pNextClass)

    		  {

    			  if (lstrcmpA(szClass, pClass->m_lpszClassName) == 0)

    			  {

    	      bFound = true;

    				  break;

    			  }

    		  }


    	 if ( bFound )

    	    break;

    	  }

    	  AfxUnlockGlobals(CRIT_DYNLINKLIST);

      }

    #endif

      //<< Undocumented MFC


      return pClass;

    }

    Reply

  • Reason DECLARE_SERIAL is needed

    Posted by Legacy on 06/02/1999 12:00am

    Originally posted by: ChrisD

    The list you are travering to find the class name is only created for objects that have DECLARE/IMPLEMENT _SERIAL defined. The IMPLEMENT_SERIAL definition has a line that reads
    static const AFX_CLASSINIT _init_##class_name(RUNTIME_CLASS(class_name));

    this line instantiates a static AFX_CLASSINIT object for the class suppllied which add it to the list of RuntimeClass names.

    Unless you are supllying strings at runtime you can use in your code
    COBject* pBoj = RUNTIME_CLASS(CreatableClass)->CreateObject();
    Chris

    Reply
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