Export C++ class from main program to DLL

Question

With C++, i'm trying to use a class declared on my main program instance to be used with on DLL. It's a lot of examples to a class from DLL to use on main program, but I need the other way. For now, a simple declaration is ok, but my next step will be use a static class from main program on DLL.

For knowledge, I'm using MinGW 3.4.5 on Windows 10.

For an example I'm trying to generate 3 files:

tst_class.lib
tst_library.dll
tst.exe

And the source code files are:

tst_class.h

class ON_MAIN tst_class {
public:
     tst_class(char *);
    ~tst_class(void);
};

tst_class.c

#include <stdio.h>

#define ON_MAIN  __declspec(dllexport)
#define ON_DLL   __declspec(dllimport)

#include "tst_class.h"

tst_class::tst_class(char *source)
{
    printf("new tst_class() from %s\n", source);
}
tst_class::~tst_class(void)
{
    printf("delete (tst_class *)\n");
}

tst_library.c

#include <windows.h>

#define ON_MAIN  __declspec(dllimport)
#define ON_DLL   __declspec(dllexport)

#include "tst_class.h"

ON_DLL void tst(void)
{
    tst_class *t = new tst_class("library");
    delete t;
}

tst.c

#include <stdio.h>
#include <windows.h>

#define ON_MAIN  __declspec(dllexport)
#define ON_DLL   __declspec(dllimport)

#include "tst_class.h"

typedef void (__cdecl *f_pointer)(void);

int main(int argc, char **argv)
{
    tst_class *t = new tst_class("main");
    delete t;

    HMODULE module = LoadLibrary("tst_library.dll");

    if(module != NULL) {
        printf("Dll loaded\n");

        void *function_pointer = (void *)GetProcAddress(module, "tst");

        if(function_pointer != NULL) {
            f_pointer function = (f_pointer)function_pointer;

            function();
        }

        FreeLibrary(module);
    } else {
        printf("Can't load dll\n");
    }

    return 0;
}

To compile them:

g++ -c tst_class.c -o tst_class.o OK
ar cr tst_class.lib tst_class.o OK
g++ tst_library.c tst_class.lib -o tst_library.dll -shared Error

And here I've got this error message:

.../ccpg0mO9.o:tst_library.c:(.text+0x5a): undefined reference to '_imp___ZN9tst_classC1EPc' .../ccpg0mO9.o:tst_library.c:(.text+0xb4): undefined reference to '_imp___ZN9tst_classD1Ev'

But following to the last step...

g++ tst.c tst_class.lib -o tst.exe OK

What I'm doing wrong with compiling/linking the library?

The executable works well. Not loading the library of cource, because it not exists.

score 1 · Accepted Answer · edited May 23 '17 at 11:44

1

Your defines are not quite right. The form of the declspec is generally

#if BUILDING_MYDLL
  #define MYDLL_EXPORT  __declspec(dllexport)
#else
  #define MYDLL_EXPORT  __declspec(dllimport)
#endif

And for the definition of the class

class MYDLL_EXPORT tst_class {
public:
     tst_class(char *);
    ~tst_class(void);
};

Then when building and linking the dll code, you use a command line define for BUILDING_MYDLL (e.g. -D BUILDING_MYDLL) or you define BUILDING_MYDLL in a header file that is internal to the dll (e.g. in Visual Studio based solutions that was often stdafx.h).

Here is another answer that is related.

edited May 23 '17 at 11:44

Community

1
1

answered Nov 05 '15 at 12:47

Niall

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Ok. I got this. My problem with `undefined reference` is gone. Now I can do more tests, but what I figured out is: the class code is on the .lib file. What I want is the class code is on the main program, and with new versions, the DLL will use the new class source. – Gangrel Nov 05 '15 at 13:11
Good, I'm glad it works. That sounds like a new question. Write it up with the code samples, expected outcomes etc. and post it as a new question. – Niall Nov 05 '15 at 13:15
I'll make another question. But the solution is only for the `undefined reference`. The main question is: Use class (code) from main program on DLL. – Gangrel Nov 05 '15 at 15:28
Explore pure virtual base classes as an interface. You may need to shift them into a third dll that will be shared with both the current dll and the exe. – Niall Nov 05 '15 at 18:57

Export C++ class from main program to DLL

1 Answers1