?? dyndllclass.shtml
字號:
<!-- Header information-->
<HEAD>
<META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=iso-8859-1">
<!-- add your name in the CONTENT field below -->
<META NAME="Author" CONTENT="Jürgen Kraus">
<TITLE>Dynamically loading classes from DLLs</TITLE>
</HEAD>
<!-- Set background properties -->
<body background="/fancyhome/back.gif" bgcolor="#FFFFFF">
<!-- A word from our sponsors... -->
<table WIDTH="100%">
<tr WIDTH="100%"><td align=center><!--#exec cgi="/cgi/ads.cgi"--><td></tr>
</table>
<CENTER><H3><FONT COLOR="#AOAO99">
<!-- Article Title -->Dynamically loading classes from DLLs
</FONT></H3></CENTER>
<HR>
<!-- Author and contact details -->
This article was contributed by <!-- Author Email --><A HREF="mailto:kraus@proxima.de"><!-- Author Name -->Jürgen Kraus</A>, by courtesy of <A HREF="http://www.proxima.de" >ProXima Engineering Office</A>.
<!-- Sample image - gif or jpg -->
<!--
<P ALIGN=CENTER><A HREF="http://www.proxima.de" ><IMG SRC="dyndllclass_pxm.gif" ALT="ProXima Engineering Office" BORDER=0></A>
-->
<!-- Text / source code -->
<p><!-- The 'p' starts a paragraph of normal text -->
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 ?
<p><!-- The 'p' starts a paragraph of normal text -->
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:
<!-- start a block of source code -->
<PRE><TT><FONT COLOR="#990000">
#include ...
#ifdef _DLL // assume this is defined when we build the DLL
#define _DYNLINK __declspec( dllexport)
#else
#define _DYNLINK __declspec( dllimport)
#endif
class _DYNLINK CMyClass
{
public:
CMyClass ();
virtual ~CMyClass();
void DoSomethingUseful();
};
</FONT></TT></PRE>
<p><!-- The 'p' starts a paragraph of normal text -->
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());'
<PRE><TT><FONT COLOR="#990000">
#ifndef _DLL
typedef CMyClass* ( *PFNCREATEMYCLASS)();
#else
_DYNLINK CMyClass* CreateMyClass()
{
return ( new CMyClass());
}
#endif
</FONT></TT></PRE>
<p><!-- The 'p' starts a paragraph of normal text -->
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...
<PRE><TT><FONT COLOR="#990000">
#ifndef _DLL
typedef void ( *PFNDELETEMYCLASS)( CMyClass*);
#else
_DYNLINK void DeleteMyClass ( CMyClass* pObj)
{
delete pObj;
}
#endif
</FONT></TT></PRE>
<p><!-- The 'p' starts a paragraph of normal text -->
Last but not least, we need a way to get access to the objects methods without
'knowing' about the source code:
<PRE><TT><FONT COLOR="#990000">
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
typedef PMYCLASSMETHOD ( *PFNGETCLASSMETHOD)();
#else
_DYNLINK PMYCLASSMETHOD GetClassMethod ()
{
return &CMyClass::DoSomethingUseful;
}
#endif
</FONT></TT></PRE>
<p><!-- The 'p' starts a paragraph of normal text -->
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).
<PRE><TT><FONT COLOR="#990000">
LIBRARY dynclass.dll
EXPORTS
CreateMyClass @2 PRIVATE ; object creation
DeleteMyClass @3 PRIVATE ; object destruction
GetClassMethod @4 PRIVATE ; method access
</FONT></TT></PRE>
<p><!-- The 'p' starts a paragraph of normal text -->
In your application, load the DLL and get a pointer to 'CreateMyClass()'
(error checking omitted for brevity)
<PRE><TT><FONT COLOR="#990000">
PFNCREATEMYCLASS pfnCreateMyClass;
PFNDELETEMYCLASS pfnDeleteMyClass;
PFNGETCLASSMETHOD pfnGetClassMethod;
PMYCLASSMETHOD pDoSomethingUseful;
CMyClass* pMyClass;
HANDLE hDll;
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));
</FONT></TT></PRE>
<p><!-- The 'p' starts a paragraph of normal text -->
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.
<!-- create more blocks of source code as needed -->
<!-- demo project -->
<p><!-- first the filename (zip files) --><A HREF="dyndllclass.zip">Download demo project - 7KB</A>
<!-- Posted / Update date mm/dd/yy - add to the list -->
<p>Date Posted: 22-Jun-1998<!-- date here -->
<P><HR>
<!-- Codeguru contact details -->
<TABLE BORDER=0 WIDTH="100%">
<TR>
<TD WIDTH="33%"><FONT SIZE=-1><A HREF="http://www.codeguru.com">Goto HomePage</A></FONT></TD>
<TD WIDTH="33%">
<CENTER><FONT SIZE=-2>© 1998 Zafir Anjum</FONT> </CENTER>
</TD>
<TD WIDTH="34%">
<DIV ALIGN=right><FONT SIZE=-1>Contact me: <A HREF="mailto:zafir@home.com">zafir@home.com</A> </FONT></DIV>
</TD>
</TR>
</TABLE>
<!-- Counter -->
<CENTER><FONT SIZE=-2><!--#exec cgi="/cgi/counters/counter.cgi"--></FONT></CENTER>
</BODY>
</HTML>
?? 快捷鍵說明
復制代碼
Ctrl + C
搜索代碼
Ctrl + F
全屏模式
F11
切換主題
Ctrl + Shift + D
顯示快捷鍵
?
增大字號
Ctrl + =
減小字號
Ctrl + -