Exporting const static data members to dll

Question

Hello there,

I am using VC++ 2003. I have a class somewhat like this:

// C.h
class __declspec(dllexport) C {
public:
    const static float CVALUE;
    static void foo();
};

// C.cpp
const static float C::CVALUE = 10.0f;
void C::foo() { }

When I attempt to call C::foo() from an external program using my dll everything is fine. When I attempt to access C::CVALUE I get a linker error like this:

error LNK2001: unresolved external symbol "public: static float const C::CVALUE".

I checked the dll with Depends.exe and it seems that the symbol C::VALUE is being exported. Do you guys have an idea of what I could do to solve this problem?

Thanks,

William

Answer 1

You can not export data fields from a DLL. You can only export functions!

The reason is simple. The binding mechanism for function to a DLL have a jump table where the final reference from the DLL is set into.

Because the load address is unknown it is not possible to unreference a variable in a DLL.

Write a function that returns that static value!

Answer 2

I suspect that your answer is wrong, because:

(1) I have seen and used a DLL that does exactly what I described.
(2) The data fields do appear in the DLL (can be seen with Depends.exe)

Martin, if you have an idea of how the system I told you about sorted this out, please tell me. Writing the function is a solution, but it isn't as elegant as having the data fields exported. :)

If anybody has a hint, please write it.

Thanks
William

Answer 3

Hint: declare your static variable as __declspec(dllimport) on the client side.

Answer 4

We can use data segments to share variable in a dll.

#pragma data_seg("SHARED")

float C::CVALUE= 0;

#pragma data_seg()

http://www.codeproject.com/Articles/240/How-to-share-a-data-segment-in-a-DLL

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Thanks, Renjith V R