Question

I’m currently creating a DLL and the client which goes with it using the stored procedure mentioned at a lot of places on the internet. Basically, create a DLL project which actually defines a PROJECT_EXPORTS in the Project.h file.

Something like this:

// Assume the name of the project is SanProj and the header file is SanProj.h
#ifdef SANPROJ_EXPORTS
    #define SANPROJ_API __declspec(dllexport)
#else
    #define SANPROJ_API __declspec(dllimport)
#endif

Now the normal way of using this header is to include this in all the headers of your API classes and using SANPROJ_EXPORTS for "exporting" declarations when in the DLL and "importing" declarations when used as a client. For e.g. let’s say we have a header file with a currency class:

// currency.hpp
#include "SanProj.h"
#include <ostream>
#include <string>

namespace SanProj {

    class SANPROJ_API Currency {

    public:
        Currency();
        const std::string& name();
        const std::string& code();
        bool empty() const;

    protected:
        std::string name_;
        std::string code_;
    };

    SANPROJ_API bool operator==(const Currency&,
                    const Currency&);

    SANPROJ_API bool operator!=(const Currency&,
                    const Currency&);

    SANPROJ_API std::ostream& operator<<(std::ostream& out, Currency& c);
}

And another header file with specific currencies:

// allccy.hpp
namespace SanProj {

    class SANPROJ_API USDCurrency : public Currency {
    public:
        USDCurrency() {
            name_ = "American Dollar";
            code_ = "USD";
        }
    };


    class SANPROJ_API CADCurrency : public Currency {
    public:
        CADCurrency() {
            name_ = "Canadian Dollar";
            code_ = "CAD";
        }
    };

}

The above classes form the contract of the DLL project. Now let’s look at the client project files, which is a single class with main function:

#include "currency.hpp"
#include "allccy.hpp"

#include <iostream>

using namespace SanProj;

int main(int argc, char* argv[])
{
    USDCurrency uccy;
    std::cout << uccy;
}

Assuming all referencing/settings are already done in the Visual Studio project, I get the following error when trying to compile the client:

1>testdll.obj : error LNK2001: unresolved external symbol "__declspec(dllimport) public: __thiscall SanProj::USDCurrency::~USDCurrency(void)" (__imp_??1USDCurrency@SanProj@@QAE@XZ)
1>testdll.obj : error LNK2001: unresolved external symbol "__declspec(dllimport) public: __thiscall SanProj::USDCurrency::USDCurrency(void)" (__imp_??0USDCurrency@SanProj@@QAE@XZ)

Not surprisingly, this error goes away when I remove the dllimport part from the SanProj.h file and the executable is created.

My question is, what’s the point of the IDE generated dllimport if we can’t compile clients against the header? Is there a way I can continue to use the header with both dllimport and dllexports and remove the linker errors? Also, why is it trying to resolve the symbol which has dllimport from the LIB file?

TIA,
/sasuke

EDIT: Linker Command used by VisualStudio; as you can see, it has the LIB file.

/OUT:"E:\vsprojects\SomeSln\Release\testdll.exe" /INCREMENTAL:NO
/NOLOGO "E:\vsprojects\SomeSln\Release\SanProj.lib" "kernel32.lib"
"user32.lib" "gdi32.lib" "winspool.lib" "comdlg32.lib" "advapi32.lib"
"shell32.lib" "ole32.lib" "oleaut32.lib" "uuid.lib" "odbc32.lib"
"odbccp32.lib" /MANIFEST
/ManifestFile:"Release\testdll.exe.intermediate.manifest"
/ALLOWISOLATION /MANIFESTUAC:"level=’asInvoker’ uiAccess=’false’"
/DEBUG /PDB:"E:\vsprojects\SomeSln\Release\testdll.pdb"
/SUBSYSTEM:CONSOLE /OPT:REF /OPT:ICF
/PGD:"E:\vsprojects\SomeSln\Release\testdll.pgd" /LTCG /TLBID:1
/DYNAMICBASE /NXCOMPAT /MACHINE:X86 /ERRORREPORT:QUEUE

Answer 1

It seems that there is no solution to this problem. I ended up giving up on using dllimport in the client code and take a performance hit. 🙁