Exporting global variables from DLL

Question

I'm trying to export a global variable from a DLL.

Foo.h

class Foo
{
public:
    Foo()
    {}
};

#ifdef PROJECT_EXPORTS
    #define API __declspec(dllexport)
#else
    #define API __declspec(dllimport)
#endif

API const Foo foo;

Foo.cpp

#include "Foo.h"

const Foo foo;

When I compile the above code, Visual Studio complains:

foo.cpp(3) : error C2370: 'foo' : redefinition; different storage class 1> foo.h(14) : see declaration of 'foo'

If I use:

external const Foo foo;

in Foo.h the compiler is happy but then the DLL does not export the symbol. I've managed to export functions with problems, but variables don't seem to work the same way... Any ideas?

Solution 1

In your header:

API extern const Foo foo;

In your source file:

API const Foo foo;

If you don't have the extern keyword, your C compiler assumes you mean to declare a local variable. (It doesn't care that you happened to have included the definition from a header file.) You also need to tell the compiler that you're planning on exporting the variable when you actually declare it in your source file.

Solution 2

The class Foo most likely will have member functions in reality, calling those from another module would cause linker errors with the OP/accepted answer. The class must be defined as dll export/import as well in order to use the exported instance of it outside this module to eliminate the link errors.

class API Foo
{
public:
    Foo()
    {}
    void DoSomeWork(); // calling this would cause link error if Foo is not defined as import/export class
};

With that said, it might be better to rename #define API with something like DLLEXPORT so it makes sense for both APIs and export class.

Comments

[meolic]

Please, correct me if I am wrong. Using __declspec(dllexport) with C compiler (gcc from MINGW) will automatically add "extern" keyword for variables, while using C++ compiler (g++ from MINGW) this is not the case and you have explicitly use "extern".

[user13947194]

@meolic will you have to be wrong because I did that and the person asking the question clearly stated that he did that.