Hello Everybody,
I am trying to export a class from dll and use it another VC++ code. The code is give below.
/*DllTest.h*/
class __declspec(dllexport) CDllTest
{
public:
__stdcall CDllTest(){}
__stdcall ~CDllTest(){}
public:
void __stdcall SayHello();
};
//
#include "stdafx.h"
#include "DllTest.h"
BOOL APIENTRY DllMain( HANDLE hModule, DWORD ul_reason_for_call, LPVOID lpReserved)
{
switch (ul_reason_for_call)
{
case DLL_PROCESS_ATTACH:
case DLL_THREAD_ATTACH:
case DLL_THREAD_DETACH:
case DLL_PROCESS_DETACH: break;
}
return TRUE;
}
void CDllTest::SayHello()
{
printf("Hello C++!");
}
I compiled the above program and got the DllTest.Dll file. Now i want to use it in another VC++ program for that i used to versions of VC++ code that are
progr 1:
/*DllTestUse.h*/
class __declspec(dllimport) CDllTest
{
public:
__stdcall CDllTest(){}
__stdcall ~CDllTest(){}
public:
void __stdcall SayHello();
};
/* DllTestUse.cpp*/
#include "DllTestUse.h"
#include<windows.h>
#pragma comment(lib,"DllTest.lib")
void main()
{
CDllTest d;
d.SayHello();
}
It is working Ok.
prg 2:
/*DllTestUse.h*/
class __declspec(dllimport) CDllTest
{
public:
__stdcall CDllTest(){}
__stdcall ~CDllTest(){}
public:
void __stdcall SayHello();
};
/* DllTestUse.cpp*/
#include "DllTestUse.h"
#include<windows.h>
void main()
{
HINSTANCE hInstance;
hInstance=LoadLibrary("DllTest.dll");
CDllTest p;
p.SayHello();
FreeLibrary(hInstance);
}
Compiling...
DllTestUse.cpp
Linking...
DllTestUse.obj : error LNK2001: unresolved external symbol "__declspec(dllimport) public: __thiscall CDllTest::~CDllTest(void)" (__imp_ 1CDllTest@@QAE@XZ)
DllTestUse.obj : error LNK2001: unresolved external symbol "__declspec(dllimport) public: void __stdcall CDllTest::SayHello(void)" (__imp_ SayHello@CDllTest@@QAGXXZ)
DllTestUse.obj : error LNK2001: unresolved external symbol "__declspec(dllimport) public: __thiscall CDllTest::CDllTest(void)" (__imp_ 0CDllTest@@QAE@XZ)
Debug/DllTestUse.exe : fatal error LNK1120: 3 unresolved externals
Error executing link.exe.
DllTestUse.exe - 4 error(s), 0 warning(s)
could any body tell me what the problem is going on and how can i solve this. Thanks in Advance.
Help me Exporting classes
Umut Alev - MSFT
You even can not export a variale from a DLL and use it with LoadLibrary.
The reason is simple. The lnker needs all execution points of all functions at link time. But you don't have a binary image were the linker can bind to. This want to give this binary after you call LoadLibrary.
Also. What would happen if you have 3 DLLs all implementing the same class and you call LoadLibrary
Solution:
Build pure virtial interfaces. Implement those in the DLL. Create a function in the DLL that returns such an interface pointer to the EXE.
pogsworth
Hello Martin Richter,
Again ,i am very pleased on your explanation and thank you very much. Could you tell me a book name or web-site that teaches how to programm COM(with out MFC) from scratch with suitable examples. Thanks in Advance.
nijen04
You can not use dynamic loading and exporting of a class from a DLL. The linkage to the function is placved directly inside the EXE you can not make this dynamic.
One way would be to build a wrapper class inside the EXE and redirect the calls to the final DLL class function by function. Its not possible to di this as a whole for the complete class.
What you can do, is to use a pure Interface without the implementation like COM does to solve this problem.
Mexx
For pure simple COM I prefer ATL.
And a good book for it is "ATL Internals" from Rector/Sells!
Arjan Ligthart
Hello Martin Richter,
Thank you very much on explaining my problem. But, can you cleary explain or give reference to a web-site that clears my doubt. i.e, we can export functions and variable from a dll, but why not we export classes from a dll. what is the reason . Thanks in advance.