Dynamically loading classes from DLLs

Tuesday Oct 1st 2002 by Jürgen Kraus

Dynamically loading classes from DLLs

ProXima Engineering Office -->

Have you ever wondered whether there is a way to have an application extrinsically load a DLL (e.g. via the LoadLibrary() function) which exports classes so that the loader of the DLL can then use objects of the class ?

This is basically no problem, the only thing that makes this a pain in the [censored] are the 'C++ mangled names' of the methods in the DLL's export section (e.g. '?CreateObject@CFrameDoc@@SGPAVCObject@@XZ'). The simple trick get around this (and not using (D)COM) is: If you have (e.g.) a class called CMyClass, declare it in the header file common to both application and DLL:

#include ...

#ifdef _DLL // assume this is defined when we build the DLL
#define _DYNLINK __declspec( dllexport)
#define _DYNLINK __declspec( dllimport)

class _DYNLINK CMyClass
            CMyClass ();
    virtual ~CMyClass();

    void    DoSomethingUseful(); 

then, export a function from the DLL that returns a pointer to an instance of CMyClass, let's call it 'CreateMyClass()' - it basically should do 'return ( new CMyClass());'
#ifndef _DLL
typedef CMyClass* ( *PFNCREATEMYCLASS)();
_DYNLINK CMyClass* CreateMyClass() 
	return ( new CMyClass());

Also provide a function from the DLL that performs a 'delete' on the object. Let's call it 'DeleteMyClass()' - it basically should do 'delete pObj;'. This function is necessary due to the fact that the CRT uses different heaps and therefore different allocators for EXEs/DLLs, so that you're just lucky if there's no crash...

#ifndef _DLL
typedef void ( *PFNDELETEMYCLASS)( CMyClass*);
_DYNLINK void DeleteMyClass ( CMyClass* pObj) 
	delete pObj;

Last but not least, we need a way to get access to the objects methods without 'knowing' about the source code:

typedef void ( CMyClass::*PMYCLASSMETHOD)();
// the type created above ALWAYS points to a memer function of class 'CMyClass'
// which takes no arguments and returns nothing

#ifndef _DLL
	return &CMyClass::DoSomethingUseful;

We also need a module definition file for our DLL. Remember: We use a C interface to our DLL! (the 'PRIVATE' keyword instructs the linker not to list the function in the DLL's import library).

   LIBRARY dynclass.dll
    CreateMyClass               @2 PRIVATE          ; object creation
    DeleteMyClass               @3 PRIVATE          ; object destruction
    GetClassMethod              @4 PRIVATE          ; method access

In your application, load the DLL and get a pointer to 'CreateMyClass()' (error checking omitted for brevity)

PMYCLASSMETHOD		pDoSomethingUseful;
CMyClass* pMyClass;


hDll = LoadLibrary ( ...);

pfnCreateMyClass  = (PFNCREATEMYCLASS)  GetProcAddress ( hDll, "CreateMyClass");
pfnDeleteMyClass  = (PFNDELETEMYCLASS)  GetProcAddress ( hDll, "DeleteMyClass");
pfnGetClassMethod = (PFNGETCLASSMETHOD) GetProcAddress ( hDll, "GetClassMethod");

// et voila - an instance of CMyClass!
  pMyClass = ( pfnCreateMyClass) ();

  pDoSomethingUseful = ( pfnGetClassMethod ());

  ( pMyClass->*pDoSomethingUseful) ();

  ( pfnDeleteMyClass ( pMyClass));

Outlook: This article just intends to demonstrate the technology 'how to do it'. There's much work to do if you want to use it in your applications, e.g. creating just one function in the DLL that fills in a structure which holds all the function pointers you need, or even a proxy class that does this automatically. Well, i'll keep this for a future update of this article.

Download demo project - 7KB

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