Object Oriented Programming c++ dll Code::Blocks

Question

I'm working on oop c++ with code::Blocks.

These are my first steps in oop because I program in C for microprocessors.

I'm having trouble linking a dll.

My the main from the dll project is:

#include "main.h"
#include "xclass.h"

// a sample exported function
void DLL_EXPORT SomeFunction(const LPCSTR sometext)
{
    MessageBoxA(0, sometext, "DLL Message", MB_OK | MB_ICONINFORMATION);
}

BOOL WINAPI DllMain(HINSTANCE hinstDLL, DWORD fdwReason, LPVOID lpvReserved)
{
    switch (fdwReason)
    {
        case DLL_PROCESS_ATTACH:
            // attach to process
            // return FALSE to fail DLL load
            break;

        case DLL_PROCESS_DETACH:
            // detach from process
            break;

        case DLL_THREAD_ATTACH:
            // attach to thread
            break;

        case DLL_THREAD_DETACH:
            // detach from thread
            break;
    }
    return TRUE; // succesful
}

This is the header:

#ifndef __MAIN_H__
#define __MAIN_H__

#include <windows.h>
#include "xclass.h"

/*  To use this exported function of dll, include this header
 *  in your project.
 */
#ifdef BUILD_DLL
    #define DLL_EXPORT __declspec(dllexport)
#else
    #define DLL_EXPORT __declspec(dllimport)
#endif


#ifdef __cplusplus
extern "C"
{
#endif

void DLL_EXPORT SomeFunction(const LPCSTR sometext);

#ifdef __cplusplus
}
#endif

#endif // __MAIN_H__

Basic stuff as you can see.

The problem is that I am including the class xclass with th main:

#include "xclass.h"
xclass::xclass()
{
    //ctor
}

xclass::~xclass()
{
    //dtor
}

and header

#ifndef XCLASS_H
#define XCLASS_H

class xclass
{
    public:
        xclass();
        virtual ~xclass();
        unsigned int GetCounter() { return m_Counter; }
        void SetCounter(unsigned int val) { m_Counter = val; }
    protected:
    private:
        unsigned int m_Counter;
};

#endif // XCLASS_H

I was able to link and use the dll in other project. A can even use the function in the DLL SomeFunction("teste x"); but I can not access and us the class:

#include <iostream>
#include "main.h"
//#include "../cvWrapper/main.h"

using namespace std;

int main()
{
    xclass ClassInDll;// not working

    SomeFunction("teste x"); //using the function in dll

    printf("%d\n", 1);
    return 0;
}

The build error is:

||=== testDLL, Debug ===| obj\Debug\main.o||In function main':| C:\Users\SoftVision\Desktop\PrinterCode\DLL_test\testDLL\main.cpp|9|undefined reference toxclass::xclass()'| C:\Users\SoftVision\Desktop\PrinterCode\DLL_test\testDLL\main.cpp|14|undefined reference to xclass::~xclass()'| C:\Users\SoftVision\Desktop\PrinterCode\DLL_test\testDLL\main.cpp|14|undefined reference toxclass::~xclass()'| ||=== Build finished: 3 errors, 0 warnings ===|

Thank for the help...

Answer 1

Actually you should export the class:

class DLL_EXPORT xclass
{
    public:
        xclass();
        virtual ~xclass();
        unsigned int GetCounter() { return m_Counter; }
        void SetCounter(unsigned int val) { m_Counter = val; }
    protected:
    private:
        unsigned int m_Counter;
};

Be careful when you export a class which is not a pure virtual class because you may meet some problems with a memory alignment. This happens because of different RTL versions in a different compilers. Instead export a pure virtual interface of you class.

class DLL_EXPORT IXClass
{
    public:
        IXClass();
        virtual ~IXClass();
        virtual unsigned int GetCounter()=0;
        virtual void SetCounter(unsigned int val) =0;
};

Also avoid macros... Good luck :).

Answer 2

You need to export the class too:

class DLL_EXPORT xclass {
  //...

You might need to rearrange your headers a little - e.g. put the #define for DLL_EXPORT somewhere that can be included in both 'main.h' and 'xclass.h'.

http://www.codeproject.com/Articles/28969/HowTo-Export-C-classes-from-a-DLL