Importing explicitly instantiated template class from dll.

Question

Being a dll newbie I have to ask the allmighty SO about something.

Say I explicitly instantiate a template class like this:

template class __declspec(dllexport) B<int>;

How do I use import this templated class again?

I've tried the adding the code below in my .cpp file where I want to use B

template class __declspec(dllimport) B<int>;

Answer 1

When you instantiate a template fully -- you have a complete type. It is no different from any other types. You need to include the header for B and also compile-time linking in with a lib file or dynamically load the dll to link to the definition.

Have you read this article: http://support.microsoft.com/kb/168958 ?

Here's a brief summary of what I tested (and it worked):

---

Create a dummy DLL project

• Used the Win32 Console application wizard to generate the dll header/source files called: template_export_test
• Added the following:

file: template_export_test.h

---

#ifndef EXP_STL
#define EXP_STL
#endif 

#ifdef EXP_STL
#    define DECLSPECIFIER __declspec(dllexport)
#    define EXPIMP_TEMPLATE
#else
#    define DECLSPECIFIER __declspec(dllimport)
#    define EXPIMP_TEMPLATE extern
#endif

EXPIMP_TEMPLATE template class DECLSPECIFIER CdllTest<int>;

---

file: template_export_test.cpp

---

template<class T>
CdllTest<T>::CdllTest(T t)
: _t(t)
{
    std::cout << _t << ": init\n";
}

---

Create the test application

• Use the wizard to create a Win32 Console application called: driver
• Edit the Linker project settings of this project:
  • Add to Linker > General > Additional Library Directories: path to template_export_test.lib
  • Add to Linker > Input > Additional Dependencies: template_export_test.lib
• Include the template_export_test.h in the main cpp file

---

#include "c:\Documents and Settings\...\template_export_test.h"
using namespace std;

int main(int argc, char** argv) {
    CdllTest<int> c(12);
}

---

• Compile and go!

Answer 2

It seems, even with explicit instatiation of the template, problems may arise that lead to run-time errors. Take a look at this interesting article to C4251 (especially the "Conclusion").