Exporting static data in a DLL

Question

I have a DLL which contains a class with static members. I use __declspec(dllexport) in order to make use of this class's methods. But when I link it to another project and try to compile it, I get "unresolved external symbol" errors for the static data.

e.g. In DLL, Test.h

class __declspec(dllexport) Test{
protected:
    static int d;
public:
    static void m(){int x = a;}
}

In DLL, Test.cpp

#include "Test.h"

int Test::d;

In the application which uses Test, I call m().

I also tried using __declspec(dllexport) for each method separately but I still get the same link errors for the static members.

If I check the DLL (the .lib) using dumpbin, I could see that the symbols have been exported.

For instance, the app gives the following error at link time:

1>Main.obj : error LNK2001: unresolved external symbol "protected: static int CalcEngine::i_MatrixRow" (?i_MatrixRow@CalcEngine@@1HA)

But the dumpbin of the .lib contains:

Version      : 0
  Machine      : 14C (x86)
  TimeDateStamp: 4BA3611A Fri Mar 19 17:03:46 2010
  SizeOfData   : 0000002C
  DLL name     : CalcEngine.dll
  Symbol name  : ?i_MatrixRow@CalcEngine@@1HA (protected: static int CalcEngine::i_MatrixRow)
  Type         : data
  Name type    : name
  Hint         : 31
  Name         : ?i_MatrixRow@CalcEngine@@1HA

I can't figure out how to solve this. What am I doing wrong? How can I get over these errors?

P.S. The code was originally developed for Linux and the .so/binary combination works without a problem

EDIT: In the given case, the static variables are not directly referred by the application but the method is inlined since it's in the header. I was able to resolve the link errors by moving the methods to the .cpp file.

Answer 1

In this thread at cprogramming.com it is suggested that a static variable is local to the dll and not exported.

Summary of discussion below

The static member is not accessed directly by code in the calling application, only through member functions of the class in the dll. However there are several inline functions accessing the static member. Those functions will be inline expanded into the calling applications code makeing the calling application access the static member directly. That will violate the finding referenced above that static variables are local to the dll and cannot be referenced from the calling application.

Answer 2

My guess is that the class which uses the DLL should see dllimport instead of dllexport in the header. If I am correct, this can typically be achieved by defining a preprocessor macro like:

#ifdef EXPORTING
#define DECLSPEC __declspec(dllexport)
#else
#define DECLSPEC __declspec(dllimport)
#endif

and then use it in the class declaration:

class DECLSPEC Test{
protected:
    static int d;
public:
    static void m(){}
}

So that in Test.cpp (or wherever it makes sense in your DLL project) you can specify that you are exporting so that it will be exported with dllexport:

#define EXPORTING
#include "Test.h"

int Test::d;

while the other project, which does not define EXPORTING, will see dllimport.

Does it make sense?

Answer 3

With Windows DLLs, there is a specific distinction between __declspec(dllexport) vs __declspec(dllimport), dllexport should be used when compiling the DLL, dllimport should be used when compiling programs that link to this DLL. The standard way of defining this would be with a macro.

The following is the visual studio example:

// The following ifdef block is the standard way of creating macros which make exporting 
// from a DLL simpler. All files within this DLL are compiled with the DLL_EXPORTS
// symbol defined on the command line. this symbol should not be defined on any project
// that uses this DLL. This way any other project whose source files include this file see 
// DLL_API functions as being imported from a DLL, whereas this DLL sees symbols
// defined with this macro as being exported.
#ifdef DLL_EXPORTS
#define DLL_API __declspec(dllexport)
#else
#define DLL_API __declspec(dllimport)
#endif

Answer 4

Despite the summary, one can export static data from a DLL. However, there is a problem that comes up with the standard macros supplied by the Visual Studio DLL project:

#ifdef DLL_EXPORTS
#define DLL_API __declspec(dllexport)
#else
#define DLL_API __declspec(dllimport)
#endif

If you have multiple DLLs calling code from one DLL to another or between EXE and DLL, there is going to be a problem with this macro because every header is going to be exporting. One needs unique macros that handle __declspec. The safest way to handle this problem is as follows:

#ifdef MYPROJECT_DLL_EXPORTS
     #define MYPROJECT_API __declspec(dllexport)
#else
     #define MYPROJECT_API __declspec(dllimport)
#endif

Then only in the compiler preprocessor options for the DLL project define MYPROJECT_API. In your header code:

struct/class MYPROJECT_API myclass {
   static int counter;
};

And in a .cpp file:

int myclass::counter = 0;

Answer 5

use inline for c++17 static inline std::string static_variable