CODEWITHSUNDEEP
c++visual-studio-2012dll
Daniel Bencik
Daniel Bencik

Reputation: 969

No functions get exported into DLL despite proper dllExport - Visual Studio

I have a base class (QIndicator) and I want to implement derived classes in DLLs. The DLL project in Visual Studio 2012 for a sample derived class has the following code:

header file with base class

#ifndef _DLL_COMMON_INDICATOR_
#define _DLL_COMMON_INDICATOR_

// define the DLL storage specifier macro
#if defined DLL_EXPORT
    #define DECLDIR __declspec(dllexport)
#else
    #define DECLDIR __declspec(dllimport)
#endif

class QIndicator 
{
    private:
        int x;
        int y;
};

extern "C"      
{
    // declare the factory function for exporting a pointer to QIndicator
    DECLDIR QIndicator * __stdcall getIndicatorPtr(void);
}

#endif

source file with derived class

#define DLL_EXPORT

#include "indicator.h"

class QIndicatorDer : public QIndicator
{
    public:
        QIndicatorDer       (void) : QIndicator(){};
        ~QIndicatorDer      (void){};

    private:
        // list of QIndicatorDer parameters
        int x2;
        int y2;
};

extern "C"     
{
    DECLDIR QIndicator * __stdcall getIndicatorPtr(void)
    {
        return new QIndicatorDer();
    };
}

The problem I have is that upon successful build, the produced DLL file does not contain the exported getIndicatorPtr function (as shown by DependencyWalker). I checked whether the dllexport keyword gets propagated properly into the declaration of getIndicatorPtr and it does.

Another interesting problem is that I already have another derived class like this, in another DLL project, that I created some months ago. This older project is basically the same and everything works well there. I checked all properties of both the old and the current projects, and they seem identical. So I ran out of ideas, why I can't get getIndicatorPtr to export.

Any help is much appreciated, Daniel

Upvotes: 1

Views: 4196

Answers (1)

Mateusz Grzejek
Mateusz Grzejek

Reputation: 12058

That's because it's not being exported. Why?

__declspec specifier should only be placed in the declaration of a function, not it's definition. Also, avoid something like #define DLL_EXPORT. Preprocessor definitions should either defined in project properties (MSVC) or command line option (-D in GCC, for example).

Look at you code:

Header

extern "C"      
{
    DECLDIR QIndicator * __stdcall getIndicatorPtr(void);
}

When compiler parses this header, is sees DECLDIR as dllimport (because you define DLL_EXPORT in .cpp). Then in .cpp, it suddenly appears as dllexport. Which one is used? The first one.

So, leave your header (it's fine), but change your source:

//#define DLL_EXPORT -> remove this!

#include "indicator.h"

class QIndicatorDer : public QIndicator
{
    //...
};

extern "C"     
{
    /* DECLDIR -> and this! */ QIndicator * __stdcall getIndicatorPtr(void)
    {
        return new QIndicatorDer();
    };
}

Then, go to project properties (I assume you use Visual Studio) and then C/C++ -> Preprocessor -> Preprocessor Definitions and add there DLL_EXPORT=1.

That should work.

Upvotes: 3

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