C++ instantiate template class from DLL

Question

I tried to make a DLL that contains:

base template class, only with a virtual destructor and no attributes (I called it MatrixInterface)

a derived class with constructors, destructor, operator= and attributes (matrix class)

a function that returns a base class pointer to a new derived object:

#ifdef BUILD_DLL
    #define DLL_EXPORT __declspec(dllexport)
#else
    #define DLL_EXPORT __declspec(dllimport)
#endif

template<class T>
MatrixInterface<T> DLL_EXPORT * CreateMatrixInstance(unsigned int n,unsigned int m)
{
    return new matrix<T>(n,m);
}

I wanted to instatiate the matrix class in my program using this function, but I cannot assign a function pointer to this function and I don't understand why. I can load any function that is not a template function this way.

#include <windows.h>
#include <iostream>
using namespace std;

template<class T>
class MatrixInterface
{
public:
    virtual ~MatrixInterface(void);
};


typedef MatrixInterface<int>* (*Fptr)(unsigned int,unsigned int);

int main(int argc, char* argv[])
{
    Fptr p;
    MatrixInterface<int> *x;
    char path[]="basicmatrix.dll";
    HINSTANCE hDll = LoadLibrary(path);
    cout<<(char*)path<<endl;
    if(hDll)
    {
        cout<<"Library opened succesfully!"<<endl;
        p = (Fptr)GetProcAddress(hDll,"CreateMatrixInstance");
        if(p) {
            cout<<"working!\n";
            x=p(7,8);
            cout<<"MatrixCreated"<<endl;
            delete x;

        } else {
            cout<<"Failed loading function CreateMatrixInstance\n";
        }
    }
    else
    {
        cout<<"Failed loading library "<<(char*)path<<endl;
    }
    system("pause");
    FreeLibrary(hDll);
    return 0;
}

the base class is present in both DLL and executable file.

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For some reason, Visual Studio cannot open the DLL (compiled with MSVC or MinGW). I compiled the program with MinGW and it loads the .dll file.

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Can you please tell me what is wrong with my code?

Answer 1

Templates are resolved at compile time only ! And they are going to be different types in two different compile units. (This is the reason why it's really dangerous to export functions with std::string as parameters).

As a consquence you should explicitely instanciate the templates to the types that you are going to use / allow to be used.

In you exportimport.h file, there should be template instanciation of all the types you are going to expose in your dll. Namely MatrixInterface<int>.

You should write:

template class MatrixInterface<int>;

so as to expose one and only one type. cf. What does `class template Example<int>;` statement mean with C++11?

see documentation reference here: https://en.cppreference.com/w/cpp/language/class_template#Class_template_instantiation