Visual Studio dll export issue for class and function template instantiations

Question

I am using VS2008 in win7 and g++ 4.7 in CentOS 18. The issue is only seen on Windows when I used dynamically shared library. When I convert it static library the program links fine.

I understand that in shared library template functions/class either should be defined in header file or template instantiation of the template types(parameters) should be provided through compilation unit. I have chosen the later option. I have done it before, I went through

Why can templates only be implemented in the header file?

C++ Shared Library with Templates: Undefined symbols error

But I cannot figure out why in windows as soon as I convert the library to dll it failed to resolve symbols: error LNK2019: unresolved external symbol "void __cdecl HelpingRegistration(double)" (??$HelpingRegistration@N@@YAXN@Z) referenced in function _main

In Windows it works fine with static library. In Linux both dynamic and shared library works.

//Static library
//Library header
#ifndef _TEMPLATED_STATIC_LIB_
#define _TEMPLATED_STATIC_LIB_

#include <iostream>
#include <string>
#include "Export.h"

template<typename T>
class EXPORT TemplatedStaticLib
{
public:
    TemplatedStaticLib(){};
    ~TemplatedStaticLib(){};

    void print(T t);

};

template<typename T>
EXPORT void HelpingRegistration(T);

#endif

//library .cpp

#include "TemplatedStaticLib.h"
#include <typeinfo>


template<typename T>
void TemplatedStaticLib<T>::print(T t)
{
    std::cout << "Templated Print: "<< t<< " type:: "  << typeid(t).name() << std::endl;
}

//Class Template explicit instantiation
template class TemplatedStaticLib<double>;
template class TemplatedStaticLib<std::string>;

template<typename T>
void  HelpingRegistration(T t)
{
    std::cout << "Function Templated Print: "  << t << " type: " << typeid(t).name() << std::endl;
       //return t;
}

//function template explicit instantiation
template void HelpingRegistration<>( double );
template void HelpingRegistration<>( std::string );

//Windows symbol exporter

//.h

#ifndef STATIC_LIB_EXPORT
#define STATIC_LIB_EXPORT

#if !defined WIN32
    #define EXPORT
#elif defined LIB_EXPORTS
    #define EXPORT __declspec(dllexport)
#else
    #define EXPORT __declspec(dllimport)
#endif

//STATIC_LIB_EXPORT
#endif

//library user .cpp

#include <TemplatedStaticLib/TemplatedStaticLib.h>

#include<string>

int main(int argc, char* argv[])
{
    double aDouble = 3.9;
    TemplatedStaticLib<double> double_test; 
    double_test.print(aDouble);

    std::string aString = "James";
    TemplatedStaticLib<std::string> string_test; 
    string_test.print(aString);

    HelpingRegistration(aDouble);
    HelpingRegistration(aString);


    return 0;
}

Answer 1

I believe you need to export the specializations. Have you tried this in your .cpp file:

template class EXPORT TemplatedStaticLib<double>;
template class EXPORT TemplatedStaticLib<std::string>;

and pretty much the same in your header:

template class EXPORT TemplateStaticLib<double>;
template class EXPORT TemplateStaticLib<std::string>;

I think that will work with your EXPORT macro (assuming the .cpp file see __declspec(dllexport) and the header sees __declspec(dllimport)). I admit I'm not an expert with Windows' __declspec.

I admit that I drew my answer from this other answer on the intarwebs: http://social.msdn.microsoft.com/Forums/vstudio/en-US/4fd49664-e28e-4f23-b1eb-b669d35ad264/function-template-instantation-export-from-dll (scroll all the way to the bottom for Franjo555's final version.)