Thread: Proper DLL class interfaces

I'm having problems getting a DLL interface to work on multiple compilers. The actual DLL is compiled in VC2003 and the client prog using it compiles and works fine in VC2003 also. However if compiled in another compiler (in this case DevCpp) it raises a seg fault exception when calling a class member.

How do you make proper class interfaces that works on multiple compilers?

Dll:

Code:

#include <windows.h>

class C
{
	public:
		virtual VOID Set(INT NewInt) = 0;
		virtual INT Get() CONST = 0;
};

class CC : public C
{
	public:
		CC()  { i = new INT(); }
		~CC() { delete i;      }

		virtual VOID Set(INT NewInt) { (*i) = NewInt; }
		virtual INT Get() CONST      { return (*i);   }

	protected:
		INT* i;
};

C* c = NULL;

extern "C" __declspec(dllexport) C* GetC()
{
	return c;
}

BOOL WINAPI DllMain(HINSTANCE Dll, DWORD Reason, LPVOID Reserved)
{
	switch(Reason)
	{
		case DLL_PROCESS_ATTACH:
		{
			c = new CC();
			break;
		}

		case DLL_PROCESS_DETACH:
		{
			delete c;
			break;
		}
	}

	return TRUE;
}

Client:

Code:

#include <windows.h>

class C
{
	public:
		virtual VOID Set(INT NewInt) = 0;
		virtual INT Get() CONST = 0;
};

typedef C* (*GET_C)();

INT WINAPI WinMain(HINSTANCE, HINSTANCE, LPSTR, INT)
{
	C* c;
	GET_C GetC;

	HINSTANCE Dll = LoadLibrary("..\\Release\\DllMultiCompilerTest.dll");
	if(Dll == NULL)
	{
		MessageBox(NULL, "No dll!", "Message", MB_OK);
		return 0;
	}

	GetC = reinterpret_cast<GET_C>(GetProcAddress(Dll, "GetC"));
	if(GetC == NULL)
	{
		FreeLibrary(Dll);
		MessageBox(NULL, "No func!", "Message", MB_OK);
		return 0;
	}

	c = GetC();
	if(c == NULL)
	{
		FreeLibrary(Dll);
		MessageBox(NULL, "No object!", "Message", MB_OK);
		return 0;
	}

	//*****************************************
	//* This crashes DevCpp (but not VC++)
	//*****************************************
	c->Set(1337);
	INT i = c->Get();

	FreeLibrary(Dll);
	MessageBox(NULL, "Success!", "Message", MB_OK);
	return 0;
}

Does it work on other compilers?

I solved this a little while back, I had to add the keyword __stdcall to all class methods.

Hm, it is not 100% solved apparently. In general it works very well but in some very rare cases the DevCPP code calls the wrong method.

The methods have the same name, but different argument lists:

Code:

class C
{
  virtual void m(const char* Name) = 0;
  virtual void m(const char* Name, int width, int height) = 0;
}

calling c->m("file.ext"); will in some cases call the 2nd method instead of the first for some odd reason (in DevCPP, not VC). There is no default argument values and the stubs (see other DLL post) points to the correct method. All methods have the same order in their declrations, no virtual destructors etc...

I guess it can be hard to guess without code but the project is very large at the moment. I can only hope someone has experienced similar problems before and get an aha!-experience...

It's the randomness that confuses me. If it were some error in the way I implement class interfaces it *should* be errors in all calls I make...

AHA!!!

I found the problem, the ordering of overloaded names (with different aegument lists) gets reversed on some compilers. Just watch this example program, when compiled in VC it gives the sequence 1 2 3 while in DevCPP it gives 3 2 1. So, what causes this and how can I solve it?

The DLL is ALWAYS compiled in VC++2003 (the client in both VC and DevCPP).

Dll:

Code:

#include <windows.h>

interface I
{
	virtual INT __stdcall Get() = 0;
	virtual INT __stdcall Get(INT X) = 0;
	virtual INT __stdcall Get(INT X, INT Y) = 0;
};

class C : public I
{
	public:
		C() { }
		~C() { }

		virtual INT __stdcall Get() { return 1; };
		virtual INT __stdcall Get(INT X) { return 2; };
		virtual INT __stdcall Get(INT X, INT Y) { return 3; };
};

I* c = NULL;

extern "C" __declspec(dllexport) I* GetC()
{
	return c;
}

BOOL WINAPI DllMain(HINSTANCE Dll, DWORD Reason, LPVOID Reserved)
{
	switch(Reason)
	{
		case DLL_PROCESS_ATTACH:
		{
			c = new C();
			break;
		}

		case DLL_PROCESS_DETACH:
		{
			delete c;
			break;
		}
	}

	return TRUE;
}

Client:

Code:

#include <windows.h>
#include <sstream>

interface I
{
	virtual INT __stdcall Get() = 0;
	virtual INT __stdcall Get(INT X) = 0;
	virtual INT __stdcall Get(INT X, INT Y) = 0;
};

typedef I* (*GET_C)();

INT WINAPI WinMain(HINSTANCE, HINSTANCE, LPSTR, INT)
{
	I* c;
	GET_C GetC;
	std::stringstream Stream;

	HINSTANCE Dll = LoadLibrary("DllMultiCompilerTest.dll");
	if(Dll == NULL)
	{
		MessageBox(NULL, "No dll!", "Message", MB_OK);
		return 0;
	}

	GetC = reinterpret_cast<GET_C>(GetProcAddress(Dll, "GetC"));
	if(GetC == NULL)
	{
		FreeLibrary(Dll);
		MessageBox(NULL, "No func!", "Message", MB_OK);
		return 0;
	}

	c = GetC();
	if(c == NULL)
	{
		FreeLibrary(Dll);
		MessageBox(NULL, "No object!", "Message", MB_OK);
		return 0;
	}

	Stream << "Get (should be 1 2 3):\n" << c->Get() << " " << c->Get(0) << " " << c->Get(0, 0);
	MessageBox(NULL, Stream.str().c_str(), "Message", MB_OK);

	FreeLibrary(Dll);

	return 0;
}

Forgot to mention, the DLL is ALWAYS compiled in VC++2003 (the client in both VC and DevCPP).

DirectX and COM can do this just fine, how do they do it? I couldn't find much technical information (implementation techniques) of how it works...

Originally Posted by Magos

DirectX and COM can do this just fine, how do they do it? I couldn't find much technical information (implementation techniques) of how it works...

DirectX is a COM component.

have a read of this article on msdn. It explains how COM evolved from C++. It should help you fix your problem too.

I know that. But as I recently read in this article method overloading is not possible in COM, so all hope is lost (well, not really, but I have to rename each method uniquely, yuck)