Importing an Embarcadero C++ Builder XE3 DLL into Embarcadero C++ Builder XE3 - dll

Question

I try to create a DLL in Embarcadero C++ Builder XE3, and use it in a test-project in the same environment.
I take example on a tutorial which code does not give a good result for me (!) : http://docwiki.embarcadero.com/RADStudio/XE3/en/Tutorial:_Using_Dynamic_Linked_Libraries_in_C%2B%2BBuilder_Applications
Here is the content of my DLL :
BaseAuth.h file :
#ifndef BaseAuthH
#define BaseAuthH
#include <System.hpp>
class TBaseAuth
{
public:
virtual void TestMessage() = 0;
};
#endif // BaseAuthH
Auth.h file :
//---------------------------------------------------------------------------
#ifndef AuthH
#define AuthH
//---------------------------------------------------------------------------
#include "BaseAuth.h"
class TAuth : public TBaseAuth
{
public:
TAuth();
~TAuth();
void TestMessage();
};
#endif
Auth.cpp file :
//---------------------------------------------------------------------------
#pragma hdrstop
#include "Auth.h"
//---------------------------------------------------------------------------
#pragma package(smart_init)
TAuth::TAuth()
{
}
TAuth::~TAuth()
{
}
void TAuth::TestMessage()
{
MessageBox(0, "Test message", "Test", MB_OK);
}
and File1.cpp :
#pragma hdrstop
#pragma argsused
#include "Auth.h"
extern "C" __declspec(dllexport) void* __stdcall GetClassInstance()
{
return static_cast<void*>(new TAuth());
}
extern "C" int _libmain(unsigned long reason)
{
return 1;
}
Now in the test application I have :
the same BaseAuth.h file
a form with a Button :
Test_DLLAuthOrga.h :
#ifndef Test_DLLAuthOrgaH
#define Test_DLLAuthOrgaH
//---------------------------------------------------------------------------
#include <System.Classes.hpp>
#include <Vcl.Controls.hpp>
#include <Vcl.StdCtrls.hpp>
#include <Vcl.Forms.hpp>
//---------------------------------------------------------------------------
#include "BaseAuth.h"
//---------------------------------------------------------------------------
class TForm1 : public TForm
{
__published: // Composants gérés par l'EDI
TButton *Button2;
void __fastcall Button2Click(TObject *Sender);
private: // Déclarations utilisateur
TBaseAuth *mpAuth;
public: // Déclarations utilisateur
__fastcall TForm1(TComponent* Owner);
};
//---------------------------------------------------------------------------
extern PACKAGE TForm1 *Form1;
//---------------------------------------------------------------------------
#endif
Test_DLLAuthOrga.cpp :
//---------------------------------------------------------------------------
#include <vcl.h>
#pragma hdrstop
#include "Test_DLLAuthOrga.h"
#include "BaseAuth.h"
//---------------------------------------------------------------------------
#pragma package(smart_init)
#pragma resource "*.dfm"
TForm1 *Form1;
//---------------------------------------------------------------------------
const wchar_t* library = L"DLLAuthOrga.dll";
extern "C" __declspec(dllimport) void* __stdcall GetClassInstance();
//---------------------------------------------------------------------------
__fastcall TForm1::TForm1(TComponent* Owner) : TForm(Owner)
{
HINSTANCE load;
try
{ load = LoadLibrary(library); }
catch(Exception &e)
{ ShowMessage(e.Message); }
if (load)
{
ShowMessage("Library Loaded!");
void *myFunc;
myFunc = (void *)GetProcAddress(load, "GetClassInstance");
mpAuth = (AuthParent*)myFunc;
}
else { ShowMessage("Library not loaded!"); }
}
//---------------------------------------------------------------------------
void __fastcall TForm1::Button1Click(TObject *Sender)
{
if (mpAuth == NULL) return;
try { pRes = mpAuth->TestMessage(); }
catch(Exception &e) { ShowMessage(e.Message); return; }
}
The result is :
the pointer mpAuth has an adress.
But its methods have no adress, and when I call a simple method such as "void TestMessage()", it raises an access violation.
=> It first seemed to be a question of string compatibility (but between "C++ Builder XE3" and "C++ Builder XE3" I would expect the same string format to be used ?!) : Error calling DLL with Unicode Delphi
=> I found a similar issue but with C++ DLL into Delphi, not C++ DLL into C++ ... : Call C++ DLL in Delphi app
=> I tried using "HMODULE" instead of "HINSTANCE load;" : same result.
=> I tried without success using
mpAuth = static_cast<AuthParent*>(myFunc);
instead of :
mpAuth = (AuthParent*)myFunc;
=> I also tried replacing "__stdcall" by "__cdecl" or "" (removing) : the libray loads but GetProcAdress returns NULL.
=> What am I doing wrong in attempting to call the DLL's method "TestMessage()" ?

Answer 1

To correctly bind function from dll you should give it full definition, including calling convention, arguments, return type and __dllexport/__dllimport modifier. The easiest way to do it - using typedef so instead of typing (in Test_DLLAuthOrga.cpp)
void *myFunc;
myFunc = (void *)GetProcAddress(load, "GetClassInstance");
use
typedef __declspec(dllimport) void (__stdcall *MyFuncPointerType)(void);
MyFuncPointerType myFunc;
myFunc = (MyFuncPointerType)GetProcAddress(load, "GetClassInstance");
If you are using __cdecl convention you should also add underscore to the target function name
typedef __declspec(dllimport) void (__cdecl *MyFuncPointerType)(void);
MyFuncPointerType myFunc;
myFunc = (MyFuncPointerType)GetProcAddress(load, "_GetClassInstance");
you can also explicitly define AuthParent* as a return type for your factory function to get rid of uneccessary casts to void* and back to AuthParent* (in File1.cpp and Test_DLLAuthOrga.cpp) so, the final code snippet would look like this:
typedef __declspec(dllimport) AuthParent* (__cdecl *MyFuncPointerType)(void);
MyFuncPointerType myFunc;
myFunc = (MyFuncPointerType)GetProcAddress(load, "_GetClassInstance");
mpAuth = myFunc();