Re: [Dev-C++] DLL: Howto make an instance of a class defined in a DLL

The easiest way to instantiate classes defined in a DLL is to give the DLL
file a factory function, i.e. a function that returns new DllClass(xxx);

The naming of exported symbols is called name mangling. All C++ compilers
uses name mangling (but not identical) to facilitate type-safe linking.
The symbols are decorated with a number of extra characters that tells
data types, return values, parameters, calling convention, code generation
etc.

In your example code, you are using malloc. C++ program should normally
use the new operator instead of malloc.

/Per W

On Thu, 24 Aug 2006, M=FCnt, Bernd wrote:

> Hi together,
> I made a DLL in the following way (simplified example just to explain wha=
t is the problem):
> dll.h:
> #ifndef _DLL_H_
> #define _DLL_H_
>
> #if BUILDING_DLL
> # define DLLIMPORT __declspec (dllexport)
> #else /* Not BUILDING_DLL */
> # define DLLIMPORT __declspec (dllimport)
> #endif /* Not BUILDING_DLL */
>
> class DLLIMPORT DllClass
> {
>   public:
>     DllClass();
>     virtual ~DllClass(void);
>     void testVoid();
> };
> -------------------------------------------------
> dll.cpp:
>
> #include "dll.h"
> #include <windows.h>
> #include <iostream>
>
> DllClass::DllClass() {}
> DllClass::~DllClass () {}
> void DllClass::test() {
>     printf("testFunction called!\n");
> }
>
> BOOL APIENTRY DllMain (HINSTANCE hInst     /* Library instance handle. */=
 ,
>                        DWORD reason        /* Reason this function is bei=
ng called. */ ,
>                        LPVOID reserved     /* Not used. */ ) {
>     return true;
> }
> -------------------------------------------------
>
> So far, so good, I could compile it with gcc and got a dll.dll as output.
> dumpbin /exports dll.dll gives the following output:
>     ordinal hint RVA      name
>
>           1    0 000011D0 _Z5testFv
>           2    1 000011F0 _ZN8DllClass4testEv
>           4    2 00001250 _ZN8DllClassC1Ev
>           5    3 00001230 _ZN8DllClassC2Ev
>           6    4 00001290 _ZN8DllClassD0Ev
>           7    5 00001280 _ZN8DllClassD1Ev
>           8    6 00001270 _ZN8DllClassD2Ev
>           9    7 00041C9C _ZTV8DllClass
>
> So, the Class and ist included void are exported.
> [Question beside: Why have the symbols such cryptic names. Do I have any =
influence to rename them just to "DllClass" etc.?]
>
> No I made a host application:
> main.cpp:
> -------------------------------
> #include <cstdlib>
> #include <iostream>
> #include <windows.h>
> #include "dll.h"
>
> using namespace std;
>
> int main(int argc, char *argv[]) {
> HMODULE hMod =3D LoadLibrary("dll.dll");
> if (hMod=3D=3DNULL) {printf("DLL NOT LOADED\n"); return 1} else printf("D=
LL LOADED\n");
>
> DllClass *pDllClass =3D (DllClass *) malloc (sizeof (DllClass));
> if (NULL =3D=3D pDllClass) {printf ("Memory allocation failed\n");return =
1;} else printf("Memory allocated\n");
>
> typedef void   (WINAPI * PCTOR) ();
> PCTOR pDLLClass =3D (PCTOR) GetProcAddress (hMod, "_ZTV8DllClass");
> if (NULL =3D=3D pDLLClass) {printf ("GetProcAddress failed\n");} else  pr=
intf ("DLL ProcAddress success\n");
> -------------------------------------
> But now? How to make an instance of this class?
> I found this hint:
> https://secure.codeproject.com/dll/classesexportedusingLL.asp
> __asm { MOV ECX, pDLLClass }
> pDLLClass();
>
> But the assember syntax is for Visual C++, not working with gcc.
> I tried:
> asm("MOV ECX, pDLLClass");
> But this does not compile:
> "too many memory references for `mov'"
>
> Sorry, I've got no experience in programming assembler on a X86 machine, =
the last time I did assembler programming was on a ZX81 von VC 64 some 20 y=
ears ago ;-)
>
> Thank you for any tips how to get an instance of a class which is defined=
 in a dll, Bernd
>
> --
> Bernd M=FCnt                   Durchwahl: 030/69032-509
> euroscript Deutschland GmbH  Zentrale:  030/69032-300
> Abteilung IT-Management      Fax:       030/69032-505
> Alt-Moabit 91                Mail:      Ber...@eu...
> 10559 Berlin                 Web:       http://www.euroscript.de
>
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